Muhammad Rakib Mansur Page i
Deposition and Characterization of a Coating from Calcium Phosphate and Titanium Alloy on Austenitic Stainless Steel
A Thesis Submitted For the Degree of Doctor of Philosophy
By
Muhammad Rakib Mansur
Faculty of Engineering and Industrial Science
Swinburne University of Technology
Melbourne, Australia
.
Muhammad Rakib Mansur Page ii
Declaration
The candidate hereby declares that the work in this thesis is that of the candidate
alone. It contains no material which has been accepted for the award of any other
degree or diploma at any university and to the best of my knowledge and belief
contains no material previously published or written by another person or persons,
except where due reference is made.
Muhammad Rakib Mansur
Muhammad Rakib Mansur Page iii
Dedicated to
My parents, teachers, mentors
And my family members
Muhammad Rakib Mansur Page iv
ABSTRACT
The necessity for artificial implant materials is increasing with the growing rate of the
aged population. In the year 2000, approximately twelve billion US dollars were spent
for orthopaedic applications, mainly for on-joint replacements. Most common
orthopaedic biomaterials include stainless steel and titanium alloy for load-bearing
applications. Austenitic stainless steel is an economic solution for biomedical
application due to its resistance to pitting and crevice corrosion from the body plasma
but titanium alloy (Ti-6Al-4V) has superior biocompatibility and enhanced corrosion
resistance compared to stainless steel. Hydroxyapatite (HA), a form of calcium
phosphate, is a unique material because of its similarity to hard tissue and, therefore,
has a unique ability to bond directly to bone. It is bioactive and Osseo conductive.
Composite coatings made of titanium alloy and HA can enhance the corrosion
resistance of austenitic stainless steel used for prosthetic applications. Furthermore,
the inclusion of HA and Ti on the top, will form an apatite layer and functional groups
such as Ti-OH. It is therefore expected that the composite coating made from HA and
Ti-6Al-4V will make austenitic stainless steel more biocompatible, bio-friendly and
Osseo conductive.
Different techniques have already been adopted to produce pure HA or composite
coatings on titanium or stainless steel substrates for biomedical applications. These
techniques include plasma spraying, HVOF, cold spraying, sputtering, pulsed laser
deposition, sol-gel method, electro deposition, chemical vapour deposition and plasma-
enhanced chemical vapour deposition etc.
The literature search revealed that the laser-assisted deposition technique is yet to be
adopted for producing composite coating of HA and Ti-6Al-4V. In this research, single-
layer composite coatings and a multilayer layer composite material has been prepared
from HA and Ti-6Al-4V, on stainless steel substrate using a laser-assisted direct
material deposition (DMD) method. Among these, the composite coating is the main
focus of this research. For the purposes of comparison, pure HA coating has also been
developed by laser on stainless steel. Two types of lasers have been used for the
deposition of the composite coating. One is CO2 laser-operated direct material
deposition (DMD) technique; the other one is Nd:YAG laser-assisted deposition. In
both cases, the continuous mode of the laser has been used.
Muhammad Rakib Mansur Page v
The developed composite has been characterized in order to glean information about
the microstructural, morphological, mechanical and chemical properties of the material.
Feedstocks have been characterized (to understand the morphology and chemical
constituents) for better understanding of the coating. Scanning electron microscopy
(SEM) is used to observe the microstructure, morphology and particle size of the
feedstocks. Raman microscopy and X-Ray diffraction has been used to identify the
chemical phases present in the raw material. As the deposition process involves high
energy, transformation is expected in between different phases of calcium phosphates.
That is why; a comprehensive study has been made of different types of calcium
phosphates using Raman microscopy and X-Ray diffraction, as well as analysis of raw
materials.
The microstructure of a material reveals significant information related to the structural,
mechanical and thermal behaviour of the material. Knowledge about microstructure
also helps to find out the causes of material failure and the mechanism behind fracture
formation. Surface topography and roughness are other important factors that
determine the interaction between the implant material and the surrounding host tissue.
The microstructures and surface topography of the coatings were characterized using
optical microscopy and scanning electron microscopy (SEM). Roughness was
assessed by profilometry, while X-Ray diffraction (XRD) was employed to determine
the chemical nature of HA. The chemical composition and mechanical properties, of
the single layer coatings (hardness) and multilayer composites (hardness, fracture
toughness and elastic modulus) were investigated using energy-dispersive X-ray
spectroscopy (EDS) and Vickers microhardness tests.
The results obtained after the analysis of HA and Ti-6Al-4V composite coatings indicate
that average roughness increases with traverse speed and depends significantly on the
power level. The crack orientation was found to be sensitive to traverse speed, while
the number of cracks was related to the power level. Porosity of the coating decreased
as the power level increased. The results show that the microstructure, chemistry and
mechanical properties of the coatings are influenced by laser power and traverse
speed. The aspect ratio of the coating, the ratio of calcium and phosphorous (i.e.,
Ca/P) in the coating, and the rate of diffusion of titanium into the substrate vary with
power and traverse speed. The diffusion coefficient of titanium into iron was
determined and correlated with specific energy.
This study reveals traverse speed has more influence on surface morphology and Ca/P
ratio than power. The variation in microhardness along the cross-section of the heat-
Muhammad Rakib Mansur Page vi
affected zone was investigated. The relationship between microstructural, chemical
and mechanical parameters with the specific energy was established.
Temperature and cooling rate are two important parameters which depend upon laser-
processing parameters. They influence the evolved microstructure and chemistry of the
composite. The experimental and theoretical investigation of temperature and cooling
rate can help to establish the correlation between different process parameters with the
surface, microstructural and mechanical properties of the produced coating.
Therefore research was carried out to experimentally determine temperature and
cooling rate. Temperature and cooling rate were determined by using thermocouples
for CO2-assisted DMD, and for Nd:YAG-assisted coating, temperature and cooling rate
was determined using a two-colour infrared pyrometer. Temperature and cooling rate
was also calculated using mathematical models and evaluated against the obtained
experimental values. The research also yielded correlation between microstructural,
chemical and mechanical parameters with temperature and cooling rate, along with
principal process parameters.
Muhammad Rakib Mansur Page vii
ACKNOWLEDGEMENTS
The search for knowledge is a never-ending endeavour for mankind, which started
from the earliest times and which will continue until the end of time. A doctoral
dissertation constitutes a systematic approach to learning, which ends with some new
scientific invention or findings and philosophical realizations. The journey increases the
thirst for knowledge. I feel greatly indebted to my principal supervisor Professor
Christopher C. Berndt, for his support and philosophical guidance throughout my PhD
at Swinburne University of Technology. This work would not have been possible
without his encouragement, helpful discussion and keen interest. He helped me not
only to learn some new things, but to experience the guidance that only a truly
knowledgeable mentor can bestow. In addition to receiving the benefit of his expertise,
I have, thanks to him, realized that efficiency relies on time management and smart
dealing with the assigned problem. As a mechanical engineer my approach to the
material world was slightly different to his, and it’s to his full credit in selecting me as
his student that I was given the opportunity to open a new horizon of knowledge.
My deep gratitude also goes to Dr. James Wang, my associate supervisor, under
whom I have learnt to deal with some exquisite and fascinating instruments including
XRD, SEM and EDS. It is he who always inspired me to focus on the fine-tuning of the
laboratory instruments and to keep the instruments tidy. He also helped me to refine
some of my initial ideas.
I am very grateful to my parents who have given me my life, and provided my
education and all manner of support and motivation from my birth. Without their loving
care and support, I would never have been able to reach the level I have at present.
Thanks to my younger brother, who is also my best like-minded friend; he feels for me
and has helped me in many ways. My loving wife has always tried to understand my
quest for knowledge and has supported me throughout the whole PhD timeframe; I
offer her my heartfelt thanks.
I would like to thank, with deep gratitude, Mr Girish Thipperudrappa, Andrew Moore,
Brian Dempster, Ms Meredith Jewson, Michael Culton, Dr Daniel White, and Ms
Jennifer Hartley of Swinburne University of Technology, for their technical assistance in
different phases of my experiment.
Muhammad Rakib Mansur Page viii
In many ways I am indebted to my lab mates - Jo Ann Gan, Tanveer Choudhury,
Fahad Hasan, Kun Mediaswanti, Noppakun Sanpo, Mitchell Sesso and Andrew Ang. I
am also grateful to the colleagues and friends with whom I have shared a big office
room for a long time, namely Mohammad Khalid Imran, A B M Saifullah, Tariqul Islam
Majumder, Rezwanul Haque, Remya Matthew, Indah Vidyasuti, Mostafa Nikzad, Azizur
Rahman, Martina Abrigo, Hanna Askew, Duke and Sayem. I will always remember
their friendship.
I greatly appreciate the kind and generous assistance of Swinburne University of
Technology by giving me the scholarship (SUPRA) and thus allowing me to fulfil my
long-cherished dream. I am very proud, grateful and delighted to study and become an
alumni of Swinbune University of Technology. I will always remember the generosity,
along with the warm and friendly environment of the Swinburne University of
Technology. My thanks go to all the staffs and members of the higher degree research
office who helped me in administrative matters.
Overall, I have to pay gratitude to God Almighty who has given me the opportunity and
ability to study and to conduct research and perform experiments in the quest for
knowledge.
Muhammad Rakib Mansur Page ix
TABLE OF CONTENTS
1. INTRODUCTION ............................................................................................................... 1
1.1 Background ........................................................................................................................... 2
1.2 Aim and scope of this study .................................................................................................. 5
1.3 Thesis overview ..................................................................................................................... 6
1.4 Publications from this research ............................................................................................ 8
2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION
TECHNIQUES ................................................................................................................. 10
2.1 Introduction ........................................................................................................................ 11
2.2 Properties and classification of calcium phosphate ........................................................... 12
2.2.1 Crystalline and amorphous calcium phosphate ............................................................ 13
2.2.2 Thermal change of calcium phosphate ......................................................................... 16
2.3 Titanium and its alloys ........................................................................................................ 19
2.4 Stainless steel ...................................................................................................................... 21
2.5 Deposition techniques ........................................................................................................ 23
2.5.1 Plasma spraying ............................................................................................................ 24
2.5.2 Laser-assisted deposition .............................................................................................. 25
2.6 Characterization .................................................................................................................. 32
2.6.1 Feedstock or powder characterization ......................................................................... 33
2.6.2 Surface Characterization ............................................................................................... 35
2.6.3 Chemical characterization ............................................................................................. 37
2.6.4 Diffusion ........................................................................................................................ 50
2.6.5 Mechanical characterization ......................................................................................... 52
2.6.6 Microstructural characterization .................................................................................. 59
2.7 Conclusion ........................................................................................................................... 63
3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM
PHOSPHATES ................................................................................................................ 64
3.1 Introduction ........................................................................................................................ 65
Muhammad Rakib Mansur Page x
3.2 Microstructure and morphology......................................................................................... 65
3.3 Chemical analysis ................................................................................................................ 68
3.3.1 Raman microscopy of feedstock ................................................................................... 68
3.3.2 X-Ray Diffraction (XRD) analysis of feedstock ............................................................... 73
3.4 Conclusion ........................................................................................................................... 81
4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS
STEEL ............................................................................................................................. 82
4.1 Introduction ........................................................................................................................ 83
4.2 Experimental details ........................................................................................................... 84
4.2.1 Deposition of CaP and Ti-6Al4V composite coating developed by DMD ...................... 85
4.2.2 CaP and Ti-6Al-4V multilayer composite developed by DMD ....................................... 88
4.2.3 Deposition of CaP and Ti-6Al-4V composite coating developed by Nd:YAG ................ 88
4.2.4 After-deposition characterization ................................................................................. 89
4.3 Conclusion ........................................................................................................................... 90
5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE
COATING ........................................................................................................................ 91
5.1 Introduction ........................................................................................................................ 92
5.2 Experimental details ........................................................................................................... 93
5.3 Results and discussion ........................................................................................................ 93
5.3.1 Surface roughness and morphology ............................................................................. 93
5.3.2 Crack orientation ........................................................................................................... 97
5.3.3 Microstructural observation ......................................................................................... 98
5.3.4 Diffusion between iron and titanium .......................................................................... 104
5.3.5 Chemical Analysis ........................................................................................................ 106
5.3.6 Microhardness ............................................................................................................ 109
5.4 Conclusion ......................................................................................................................... 110
6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE ........................ 112
6.1 Introduction ...................................................................................................................... 113
Muhammad Rakib Mansur Page xi
6.2 Experimental details ......................................................................................................... 114
6.3 Results and discussion ...................................................................................................... 115
6.3.1 Microstructural characterization ................................................................................ 115
6.3.2 Chemical characterization ........................................................................................... 118
6.3.3 Mechanical characterization ....................................................................................... 120
6.3.4 Comparison between single-layer coating and multilayer composite ....................... 122
6.3.5 Comparison between pure HA and composite coating on SS .................................... 123
6.4 Conclusion ......................................................................................................................... 125
7. TEMPERATURE AND COOLING RATE ....................................................................... 127
7.1 Introduction ...................................................................................................................... 128
7.2 Experimental plan for configuration one .......................................................................... 130
7.3 Results and discussion for configuration one ................................................................... 131
7.4 Experimental set-up for configuration two ...................................................................... 142
7.5 Results and discussion for configuration two ................................................................... 143
7.6 Conclusion ......................................................................................................................... 151
8. EFFECT OF TEMPERATURE AND COOLING RATE .................................................. 152
8.1 Introduction ...................................................................................................................... 153
8.2 Relationship for CO2 laser-assisted coating ...................................................................... 153
8.3 Relationship for Nd:YAG laser ........................................................................................... 157
8.4 Conclusion ......................................................................................................................... 159
9. CONCLUSION AND FUTURE SCOPE ......................................................................... 160
9.1 Conclusion ......................................................................................................................... 161
9.2 Contribution to new knowledge ....................................................................................... 164
9.3 Recommendations for further study ................................................................................ 164
10. REFERENCES .............................................................................................................. 165
11. APPENDIX .................................................................................................................... 175
11.1 Appendix 1 ........................................................................................................................ 176
Muhammad Rakib Mansur Page xii
LIST OF FIGURES
Figure 2-1: Various forms of calcium phosphate and their calcium phosphate ratios (Ca/P). ............... 13
Figure 2-2: Theoretical positions of the ionic species within the unit cell of HA [12]. ............................ 14
Figure 2-3: Phase diagram (reproduced) of CaO-P2O5 under 500mmHg water vapour pressure
[57]. ...................................................................................................................................... 18
Figure 2-4: Different deposition techniques in terms of coating thickness and substrate
temperature [68]. ................................................................................................................ 24
Figure 2-5: Schematic set-up of a dc arc plasma system [68]. ............................................................... 25
Figure 2-6 : Schematic of a pulsed-laser deposition system, PLD [3] ...................................................... 30
Figure 2-7 : Schematic of a Nd:YAG system [3]. ...................................................................................... 31
Figure 2-8 : Schematic of (a) DMD process (b) DMD system [35]. .......................................................... 32
Figure 2-9 : Roughness average and waviness [82]. ............................................................................... 36
Figure 2-10: Bragg-Brentano configuration and significance of X-ray diffraction graph. ....................... 38
Figure 2-11: XRD of HA [12]. .................................................................................................................... 42
Figure 2-12 : The schematic of a Raman microscope showing the laser source and important
optics associated with it. ..................................................................................................... 45
Figure 2-13 : Raman spectral profile as a function of ϴ (O0 to 900) from 180 to 3600 cm-1 [103] of
HA crystal (at 100X magnification and 40 mW power). ...................................................... 47
Figure 2-14 : Raman spectra in the (A) ν 2 and ν 4 region, (B) ν 1 region and (C) ν 3 phosphate
region. Shown spectra of each region were acquired from (a) commercial α-TCP, (b)
commercial TTCP, (c) HA precursor powder and from the cross-section of the laser-
processed samples: (d) irregular grains, (e) matrix, (f) elongated grains [28]. .................... 49
Figure 2-15 : (a) FTIR reflection spectra of the sample surface and the HA precursor powder; (b)
representative Raman spectra of the surface, the HA precursor powder and the
precipitated HA after 7 days in cell culture [28] .................................................................. 49
Figure 2-16: The FTIR spectrum of HA [12]. ............................................................................................. 50
Figure 2-17 : The impression of surface profile after nano-indentation [108]. ......................................... 53
Figure 2-18 : Knoop indentation and the elastic plastic zone after indentation [109]. ............................. 55
Figure 3-1 : Scanning electron microscopic (SEM) image of HA powder from Sigma Aldrich. ................ 66
Figure 3-2 : Scanning electron micrograph (SEM) of (a) HA powder (Plasma biotal). and (b) Ti-
6Al-4V powder ..................................................................................................................... 67
Figure 3-3 : Raman spectroscopic image of the HA procured from Sigma Aldrich showing the
presence of crystalline HA. ................................................................................................... 69
Figure 3-4 : Comparison between the Raman spectrum of HA and ACP (at standard mode)
acquired from Plasma Biotal. .............................................................................................. 70
Figure 3-5 : Raman spectrum of different types of calcium phosphates (ACP, BTCP, ATCP and
HA). ...................................................................................................................................... 71
Muhammad Rakib Mansur Page xiii
Figure 3-6 : Raman micrograph of HA powder showing the effect of standard mode and
confocal mode. .................................................................................................................... 72
Figure 3-7 : Different forms of CaP studied under standard mode and confocal mode a) HA, b)
ACP, c) ATCP, d) BTCP .......................................................................................................... 73
Figure 3-8 : XRD graph for Ti-6Al-4V powder. ......................................................................................... 74
Figure 3-9 : XRD profile for SS AISI 316L substrate. ................................................................................ 75
Figure 3-10 : The XRD profile of HA (Sigma Aldrich). ................................................................................ 76
Figure 3-11 : XRD pattern of HA (procured from Plasma Biotal). ............................................................. 77
Figure 3-12 : XRD of three different types (HA, ATCP and BTCP) of calcium phosphate. .......................... 79
Figure 3-13 : Rietveld analysis of HA powder procured from Plasma biotal. ............................................ 80
Figure 3-14 : Rietveld analysis of HA powder procured from Sigma Aldrich. ............................................ 80
Figure 4-1: Different routes used for deposition of calcium phosphates. .............................................. 84
Figure 4-2: (a) A schematic diagram of the DMD process (b) CaP and Ti-6Al-4V composite
coatings deposited onto austenitic stainless steel. The run numbers are indicated. ........... 87
Figure 5-1: (a) Average roughness (Ra) of experimental runs showing the surface profile (b)
Average roughness of the 10 coating trials, n = number of roughness measurements
carried on each trial/run. ..................................................................................................... 94
Figure 5-2: Variation of average roughness with power, n = number of roughness
measurements carried. ........................................................................................................ 95
Figure 5-3: Surface morphology of the coatings deposited using different parameters (a) Run
01, (b) Run 02, (c) Run 03, (d) Run 04, (e) Run 08, (f) Run 09, (g) Run 10 and (h) cross
sectional view of run 04 showing the top ceramic layer . .................................................... 96
Figure 5-4: Cracks formed in longitudinal and transverse orientation with respect to traverse
direction (a) Run 1, and (b) Run 2. ....................................................................................... 97
Figure 5-5 : Cross-sectional view of experimental run 01 (a) and the schematic model (b) of the
cross section. ........................................................................................................................ 98
Figure 5-6: Variation of depth of HAZ and crust height with change in power at a constant
traverse speed 300 mm/min. ............................................................................................. 100
Figure 5-7: Etched microstructure of the (a) stainless steel substrate and (b) diffused heat-
affected zone (HAZ). .......................................................................................................... 101
Figure 5-8: Microstructure of the crust of CaP and Ti-6Al-4V composite coatings (a)
experimental run 01, (b) experimental run 08, (c) experimental run 09, and (d)
experimental run 10. .......................................................................................................... 102
Figure 5-9: Variation of porosity with power (a) and specific energy (b). ............................................ 103
Figure 5-10: Change of Ti concentration with the depth, for different runs. ......................................... 104
Figure 5-11: Dependence of diffusion coefficient and concentration gradient of titanium on
specific energy. .................................................................................................................. 106
Muhammad Rakib Mansur Page xiv
Figure 5-12: Elemental mapping of the coated composite sample, (a) SEM image, and elemental
map of (b) Calcium, (c) Phosphorous, (d) Titanium, (e) Oxygen, (f) Aluminium (Run
08). ..................................................................................................................................... 107
Figure 5-13: Change of concentration (wt.%) calcium, phosphorous, oxygen and aluminium
along with the change of specific energy of laser. ............................................................. 108
Figure 5-14: Variation of hardness in the heat-affected diffusion zone of the coatings. ....................... 109
Figure 6-1 : SEM micrograph of HA and Ti-6Al-4V composite (a) Top (b) bottom. ............................... 116
Figure 6-2 : Maximum feret diameter, minimum ferret diameter and aspect ratio of voids for HA
and Ti-6Al-4V composite coating top (a, c and e) and bottom (b, d and f). No of
particles in y direction actually indicates the no of pores. ................................................. 117
Figure 6-3 : Roundness, circularity and solidity of voids for HA and Ti-6Al-4V composite coating
top (a, c and e) and bottom (b, d and f). No of particles in y direction actually
indicates the no of pores. ................................................................................................... 118
Figure 6-4 : Change of concentration along the cross-section of the composite. ................................. 119
Figure 6-5 : XRD of the top area of the composite. ............................................................................... 120
Figure 6-6 : Vickers micro-hardness and fracture toughness of composite made of HA and Ti-
6Al-4V with respect to depth. ............................................................................................ 121
Figure 6-7 : X-ray diffraction pattern of single-layer pure HA coating on stainless steel substrate. .... 124
Figure 6-8 : XRD graph of single-layer and multi-layer pure HA coating on stainless steel
substrate. ........................................................................................................................... 125
Figure 7-1 : (a) Schematic of the experimental set-up and (b) Experimental arrangement for
measuring temperature using pyrometer. ......................................................................... 131
Figure 7-2 : Variation of temperature with power at different traverse speed. ................................... 134
Figure 7-3 : Variation of temperature with power for different traverse speed. .................................. 135
Figure 7-4 : Temperature versus time to obtain the heating and cooling curve. .................................. 136
Figure 7-5 : Cooling rate V power for experimental and analytical results. ......................................... 139
Figure 7-6 : Variation of cooling rate with power calculated by different models for different
traverse speeds. ................................................................................................................. 140
Figure 7-7 : (a) Variation of temperature with power and traverse speed (b) variation of cooling
rate with power and traverse speed. ................................................................................. 141
Figure 7-8 : Picture of the (a) experimental setup (b) schematic diagram of substrate disk fitted
with thermocouples (c) substrate disk inserted with thermocouples and k-type
connectors. ........................................................................................................................ 142
Figure 7-9 : Temperature plotted against time measured by thermocouple 1 and 2 for run 1
placed underneath the substrate. ...................................................................................... 145
Figure 7-10 : Temperature measured by thermocouples plotted against time for two different
traverse speeds at 1500 W power. .................................................................................... 146
Muhammad Rakib Mansur Page xv
Figure 7-11 : Relationship between temperature and power evaluated by transient Ashby and
relationship along with experimental results for traverse speed 120 mm/min (top
three lines) and 300 mm/min (bottom three lines) determined at 8 mm beneath the
surface. .............................................................................................................................. 148
Figure 7-12 : Change of cooling rate with power at different traverse speed. ....................................... 149
Figure 7-13 : (a) Variation of temperature with power and traverse speed (b) Change of cooling
rate with power and traverse speed .................................................................................. 150
Figure 8-1 : The change of aspect ratio and angle of the coating with temperature. .......................... 155
Figure 8-2 : Roughness average V temperature. Here n = number of roughness measurements
carried on each trial/run. ................................................................................................... 156
Figure 8-3 : Variation of micro-hardness with cooling rate. ................................................................. 156
Figure 8-4 : Variation of coating height and width with temperature. ................................................. 158
Figure 8-5 : Vickers micro-hardness and cooling rate. .......................................................................... 159
Figure 11-1 : Histograms presenting different microstructural observations for HA procured from
Sigma Aldrich. .................................................................................................................... 176
Figure 11-2 : Histograms presenting different microstructural observations for HA procured from
Plasma Biotal. .................................................................................................................... 177
Figure 11-3 : Histograms presenting different microstructural observations for Ti-6Al-4V powders
procured from TLS Technik. ............................................................................................... 178
Figure 11-4 : XRD spectrum of ATCP. ...................................................................................................... 179
Figure 11-5 : XRD Spectrum of BTCP. ...................................................................................................... 180
Figure 11-6 : Optical micrograph of composite made from HA and Ti-6Al-4V (a) Top section (b)
Bottom section ................................................................................................................... 181
Figure 11-7 : EDS spectrum of HA and Ti-6Al-4V composite coating on SS (AISI 316L). .......................... 182
Figure 11-8 : Linear variation of HAZ area with the specific energy. ...................................................... 183
Muhammad Rakib Mansur Page xvi
LIST OF TABLES
Table 2-1 : Different forms of calcium phosphates [45, 52]. ...................................................................... 15
Table 2-2: Reaction at different temperatures. .......................................................................................... 17
Table 2-3 : Comparison between enamel, cortical bone and HA in terms of different constituents
and characteristics [58]. ......................................................................................................... 19
Table 2-4 : Diffusion coefficients of different phases of titanium (for self-diffusion) [5]. ........................... 20
Table 2-5: Physical properties of high-purity polycrystalline alpha Ti at 25 C0 [5]. .................................... 20
Table 2-6 : Important mechanical properties of Ti-6Al-4V annealed bar [59]. ........................................... 21
Table 2-7 : Crystal data of Fe – Cr – Ni alloy [60, 61]. ................................................................................ 22
Table 2-8 : Mechanical properties of austenitic stainless steel [65, 66]. .................................................... 22
Table 2-9 : Different phases present in Cr-Fe-Ti ternary phase diagrams [67]. .......................................... 23
Table 2-10: Comparison of different thermal spray process coating characteristics of ceramic
(rearranged and reproduced) [68]. ......................................................................................... 25
Table 2-11 : Typical lasers with their wavelength, frequency, energy, cavity information and mode. ...... 26
Table 2-12 : Reflectivity (R) of different materials (for wavelength λ = 1.06 µm). ..................................... 27
Table 2-13: Laser-processing parameters used for laser cladding of CaP coatings. .................................. 28
Table 2-14 : Important parameters for microstructural analysis of a powder. .......................................... 33
Table 2-15 : Different ratios for particle measurement [80, 81]. ............................................................... 34
Table 2-16 : Crystal data of HA and ATCP. ................................................................................................. 42
Table 2-17 : Raman active bands of in single crystals of HA [103]. ............................................................ 48
Table 2-18 : Mechanical properties of some important biomaterials, which includes bone, HA, SS
and Ti-6Al-4V. ......................................................................................................................... 52
Table 3-1 : Microstructural parameters collected after image analysis of powder samples. .................... 68
Table 3-2 : Peaks observed under Raman microscopy for different types of CaP collected from
Plasma Biotal. ......................................................................................................................... 71
Table 3-3 : Prominent peak positions for Ti-6Al-4V indicated in Figure 3-8 and substrate SS 316L
indicated in Figure 3-9. ........................................................................................................... 75
Table 3-4 : Peaks observed in the XRD profile for two types of HA feedstock. ........................................... 76
Table 3-5 : Peak positions for HA, ATCP and BTCP collected from PDF-2 database [93]. ........................... 78
Table 3-6 : Lattice parameters yielded after Rietveld analysis for HA PB and HA SA. ................................ 79
Table 4-1 : Experimental runs and corresponding DMD parameters (variable parameters). .................... 86
Table 4-2 : Variable parameters for each experimental run. ..................................................................... 89
Table 5-1 : Different microstructural parameters (height, width and aspect ratio) for run 01, 02,
08, 09 and 10 varying with power, traverse speed and specific energy. ................................ 99
Table 5-2 : Different microstructural parameters (crust height, angle of coating, height of HAZ and
HAZ area) for run 01, 02, 08, 09 and 10 varying with power, traverse speed and
specific energy. ..................................................................................................................... 100
Muhammad Rakib Mansur Page xvii
Table 5-3: Concentration gradient and diffusion coefficient of titanium in austenitic stainless steel. .... 105
Table 6-1 : The microstructural parameters of pores. .............................................................................. 115
Table 6-2 : Table contains Knoop micro-hardness and Modulus of Elasiticity data for composite
made of HA and Ti-6Al-4V. ................................................................................................... 122
Table 7-1: Material data used for calculation (for AISI 304L austenitic stainless steel) [65] ................... 133
Table 7-2 : Temperature presented in tabular form. ................................................................................ 134
Table 7-3 : Cooling rate in tabular form. .................................................................................................. 138
Table 7-4 : Parameters used for temperature determination. ................................................................. 143
Table 7-5 : Material data used for calculation (for AISI 316L austenitic stainless steel)[66]. .................. 144
Table 7-6 : Experimental and estimated temperatures at 8 mm depth and on the surface for 120
mm/min. ............................................................................................................................... 147
Table 7-7 : Experimental and estimated temperatures at 8 mm depth and on the surface for 300
mm/min. ............................................................................................................................... 147
Table 7-8 : Table representing surface temperature and cooling rate calculated using Ashby
model. ................................................................................................................................... 149
Table 8-1 : Height, width and aspect ratio of the coatings. ..................................................................... 154
Table 8-2 : Crust height, angle of the coating and height of the HAZ. ..................................................... 154
Table 8-3 : Temperature and cooling rate for the experimental runs. ..................................................... 157
Table 8-4 : Microstructural parameters for Nd:YAG laser-deposited composite coating. ....................... 158
Table 11-1 : Table containing roughness average data for composite coating (from run 01 to run
10) along with mean, standard deviation and standard error. ............................................ 182
Table 11-2 : Table containing RMS roughness (Rq) data for composite coating (from run 01 to run
10) along with mean, standard deviation and standard error. ............................................ 183
Table 11-3 : EDS data of HA and Ti-6Al-4V multilayer composite cross section (from Top to
bottom). ................................................................................................................................ 184
Table 11-4 : Table contains Vickers micro-hardness and fracture toughness data for composite
produced from HA and Ti-6Al-4V. ......................................................................................... 185
Table 11-5 : Table contains Vickers micro-hardness and fracture toughness data for pure HA
coating. ................................................................................................................................. 186
Table 11-6 : Elements (at%) present on the top section of the single layer composite coating and
multilayer composite. ........................................................................................................... 187
Muhammad Rakib Mansur Page xviii
NOMENCLATURE
ACP = Amorphous calcium phosphate
ATCP = Alpha tri-calcium phosphate
BCC = Body centred cubic lattice
BTCP = Beta tri-calcium phosphate
CaP = Calcium phosphate
CW = Continuous wave
DMD = Direct material/metal deposition
EDS = Energy dispersive X-ray spectroscopy
EDM = Electro discharge machining
FCC = Face centred cubic lattice
FESEM = Field emission scanning electron microscope
FEM = Finite element method
FTIR = Fourier transformation infrared spectroscopy
HA = Hydroxyapatite
HCP = Hexagonal closed packed
HVOF = High velocity oxy-fuel
ISO = International organization for standardization
LENS = Laser engineered net shaping
Nd:YAG = Neodymium yttrium garnet
NMR = Nuclear magnetic resonance
PLD = Pulsed laser deposition
PSZ = Partially stabilized zirconia
Muhammad Rakib Mansur Page xix
SBF = Simulated body fluid
SEM = Scanning electron microscope
SS = Stainless steel
TTCP = Tetra-calcium phosphate
XRD = X-Ray diffraction
Chapter 1. INTRODUCTION
Muhammad Rakib Mansur Page 1
1. INTRODUCTION
Chapter 1. INTRODUCTION
Muhammad Rakib Mansur Page 2
1.1 Background Demand for artificial implants is increasing every year with the growing rate of the
aging population. The global market for orthopaedic and dental implants was valued at
about 8.8 billion euro in 2004 with, for example, annual market growth rates of 15-18%
for dental implants [1]. This increasing market demand is motivating researchers to
develop implants which will be superior in terms of functionality, durability and
biological response. The functionality and durability of an implant depend on the
microstructural, mechanical and chemical properties of the implant material; and the
biological response of the implant relies on its surface chemistry, topography,
roughness, wettability, surface charge, and surface energy [2, 3].
Researchers have engineered different composite materials and coatings using various
techniques and materials. Materials used to produce artificial implants range from
metals to ceramics and polymers [3, 4]. Hydroxyapatite (HA), bio-glass and glass-
ceramic apatite-wollastonite are the three most popular bioactive ceramics used for
implant applications [4]. Among metals, single phase austenitic stainless steel (that is,
316L having 18Cr-14Ni-2.4Mo) is a cost-effective material for implant applications due
to its resistance to pitting and crevice corrosion from the body plasma [3]. Compared to
austenitic stainless steel, titanium is costly but has proven best in terms of corrosion
resistance, biocompatibility and Osseo-integration [3, 5].
Ti-6Al-4V is an alloy comparable to commercially pure titanium: it demonstrates
superior mechanical properties such as higher fatigue limit, yield strength and ultimate
tensile strength [6]. Ti-6Al-4V alloy also exhibit excellent corrosion resistance
properties. The corrosion potential of Ti-6Al-4V is similar to commercially pure titanium
but the passive current density is lower, which indicates better corrosion resistance [7].
The cytotoxicity of aluminium and vanadium present in Ti-6Al-4V has been studied by
many investigators. For example, Rae et al. tested the toxicity of metals used in
orthopaedic prostheses using cultured human synovial fibroblast; they concluded that
particulate titanium and aluminium performed well under specific conditions [8]. This
study also demonstrated that the vanadium component of this titanium alloy does not
present a toxic hazard under the adopted experimental condition.
Ortega et al. [9] evaluated the cytotoxicity and geno-toxicity of a commercial sample of
Ti-6Al-4V that was prepared by sand-blasting and nitric acid processing. The relevant
ISO testing procedures were followed; i.e., 7405:2008 [7, 9] for cytotoxicity and 10993-
Chapter 1. INTRODUCTION
Muhammad Rakib Mansur Page 3
5:2009 [7, 9] for geno-toxicity. It was concluded that Ti–6Al–4V alloy, after aluminium-
oxide sand blasting and nitric-acid passivation, exhibited high biocompatibility with no
cytotoxic effects on mouse and human fibroblasts, and did not induce geno-toxic
responses in bacterial and cell-mutation assays [9]. An in-vitro study performed by
Faria et al. also concluded a positive outcome in terms of cytotoxicity for Ti-6Al-4V [10].
A histological study was performed [11] on 20 pedicle screw and rod systems, made of
Ti-6Al-4V, which were removed from the human body to assess biocompatibility. No
adverse tissue reaction was observed around the screws and rods, which signals in
vivo biocompatibility of Ti-6Al-4V.
Calcium phosphate (CaP) has been synthesized and used for manufacturing various
forms of implants, for example dental, hip, knee, shoulder, and elbow implants due to
their closeness with hard tissue. The application ranges from orthopaedics, dental
implants, periodontal treatment, alveolar ridge augmentation, maxillofacial surgery and
otolaryngology.
According to Ratner [12], there are four different means of attaching the prostheses to
the skeletal system. The first is morphological fixation; in this case dense, nonporous,
nearly inert ceramics are (Al2O3 single crystal and polycrystalline) press-fitted into a
defect or cemented into the tissues. The second one is biological fixation; in this case,
bone in growth occurs on the porous inert (polycrystalline Al2O3 and HA coated porous
metals) metals, which mechanically attaches the bone to the material. The third one is
bioactive fixation; that is, the method of fixing dense, nonporous surface reactive
ceramics, glasses, and glass ceramics directly by chemical bonding with the bone. The
fourth means is use of resorbable (dense, nonporous or porous) ceramics, which are
designed to degrade gradually with time and be replaced by bone (host tissue).
HA is non-toxic, bioactive and forms an interfacial bond between the material and
tissue [13]. HA coatings promote bone ingrowth and provide enhanced fixation. The
current work employs HA with Ti-6Al-4V to promote bone ingrowth by creating a
protective ceramic layer on the top of Ti-6Al-4V. In this fashion the coating system is
more bioactive whilst retaining biocompatibility. Another reason of using Ti-6Al-4V is to
increase the fracture toughness of the composite. As a ceramic material HA is brittle
and has poor fracture toughness. Addition of Ti-6Al-4V will increase the fracture
toughness of the composite acting as a matrix material in the composite coating.
A frequently used method to apply a biocompatible coating to an implant material is the
thermal spray technique, which includes plasma spray [14-19], cold spray [20], flame
Chapter 1. INTRODUCTION
Muhammad Rakib Mansur Page 4
spray [21] and high velocity oxy-fuel spray (HVOF) [22] techniques. Other methods
such as hot pressing [23], sputtering [3], chemical vapour deposition (CVD) [24],
plasma enhanced chemical vapour deposition (PECVD) [25], the sol gel method [26]
and powder metallurgy [27] are also deployed for biomaterial production. The laser-
assisted deposition is a new technique that encompasses laser cladding [28-30], laser
surface alloying [31, 32], and pulsed laser deposition [3, 33, 34].
Direct metal deposition (DMD) is a laser-assisted rapid prototyping technique that uses
a closed loop optical feedback system to create a product from a computer-aided
design (CAD). In this technique, a laser is used to create a melt pool on the substrate
and the powders are melted so that they form a coating onto the substrate [35, 36].
DMD was used for producing grafts [28] from HA to treat large bone defects (for
osseous reconstruction), but composite coatings have not been manufactured using
the DMD process.
Laser processing techniques involve complex thermal, chemical, mechanical and
metallurgical processes and the interactions between them. Laser surface processing
involves rapid heating and cooling that provide an opportunity to produce novel
materials without them being constrained by an equilibrium phase diagram [37]. The
surface chemistry of a material can be significantly improved by rapid solidification
during the laser cladding process. The increased solubility of the solute atoms because
of high cooling rate can produce metastable materials. So, for laser processing,
temperature and cooling rate are the two vital pieces of information requiring
investigation and correlation between microstructural, chemical, mechanical and
metallurgical parameters. Additionally, correlation of the temperature and cooling rate
with after-effects and process parameters can pave the way to more understanding
about the interaction.
Various techniques have been adopted by researchers to determine the temperature
and cooling rate of laser material processing. A thermal imaging technique was used
by Hofmeister et al. to measure the temperatures and cooling rates around the melt
pool of AISI 316L stainless steel and H13 tool steel processed by a laser-engineered
net shaping (LENS) technique [38]. They used Rosenthal’s solution [39, 40], scaled
with traverse velocity. Ueda et al. have used fused fiber-coupled infrared radiation
pyrometer to measure the temperature of a work piece made of partially stabilized
zirconia (PSZ) and Al2O3 irradiated by CO2 laser [41]. They compared between the
experimental results with the numerically determined results using a finite element
method (FEM). Smurov et al. have used two pyrometers (one being 2D and the
Chapter 1. INTRODUCTION
Muhammad Rakib Mansur Page 5
another monochromatic pyrometer) placed on the same plane at an angle of 450, in
conjunction with an infrared camera set on a plane perpendicular to the pyrometer
plane at 600 angle, to monitor brightness temperature [42].
1.2 Aim and scope of this study The current work develops a composite coating made from HA and Ti-6Al-4V powders
on austenitic stainless steel substrates using CO2 laser-aided direct metal deposition
(DMD) and an Nd:YAG laser-cladding technique. The wavelength of CO2 and Nd:YAG
is different (10.6 µm for CO2 and 1.06 µm for Nd:YAG). Because of this big difference
in wavelength both the lasers interact with material in a different way. Thus CO2 and
Nd:YAG lasers have been employed to find the effect of these two types of laser on the
deposition of composite coatings on to the stainless steel substrate.
The microstructure of the deposited coating was intensively studied to collect
qualitative and quantitative information concerning the physical, structural and chemical
properties of the coating. The microstructure of the heat-affected zone (HAZ), where
diffusion was dominant was inspected. Correlation was established between
microstructural parameters such as (i) crust height, (ii) aspect ratio (i.e., the ratio
between the width and height of deposited tracks), and (iii) the angle of the coating in
relation to the substrate surface; all with respect to the power and specific energy. The
characterization also included the study of surface morphology and hardness of the
coating.
Chemically, bone is an organic-ceramic composite of complex chemistry having
collagen (20 wt.%), calcium phosphate (69 wt.%) and water (9 wt. %) [43]. The ratio of
calcium and phosphorous (Ca/P) has a significant impact on biocompatibility [13]. The
calcium phosphate (CaP) present in bone is a modified form of hydroxyapatite (a
variant of CaP), having a Ca/P ratio of about 1.65, which is close to that of pure
hydroxyapatite (HA), oxyapatite (OA) and oxyhydroxyapatite (OHA), having Ca/P ratio
of 1.67 [44, 45]. Present studies have revealed that the Ca/P ratio of bone and dentine
is different from the 1.67 value for geologic hydroxyapatite [46]. The research
performed by Liu et al. concluded that Ca/P ratio indicated clearly the dominant form of
calcium phosphate [47]. The aim of the experiment is to determine the Ca/P ratio of
different experimental runs and to find out if there exists any kind of relationship
between the Ca/P ratios and the energy or power levels.
Chapter 1. INTRODUCTION
Muhammad Rakib Mansur Page 6
The diffusion coefficient is practically an important parameter for the calculation of
diffusion between solute and solvent. The diffusion coefficient depends on temperature
and cooling rate. The diffusion coefficient of titanium gives an indication of the solubility
of titanium in stainless steel (316L), which influences the bond and adhesion of the
coating with the substrate. In this study, the diffusion of titanium in stainless steel was
analysed and the diffusion coefficient of titanium in iron was estimated.
Bone defects can be produced by severe trauma, tumour resection or congenital
deformity [28]. Bone graft materials are required to repair bone defects larger than
critical size – that is, the size over which the self-healing of bone is not possible.
Multilayer composites could be the building blocks to produce a bone graft material for
morphological fixation. A composite material made from Ti-6Al-4V and HA could be a
potential graft material.
The current work employs HA with Ti-6Al-4V to promote bone ingrowth by creating a
protective ceramic layer on the top of Ti-6Al-4V, using a DMD (rapid prototyping)
technique. Because of the inherent property of the used materials, this is expected to
promote bone ingrowth by creating a protective ceramic layer on the top of Ti-6Al-4V.
In this fashion the composite will be more bioactive whilst retaining biocompatibility.
The composite has been examined to find out its microstructural (porosity, pore size,
shape and morphology), chemical (phases present, Ca/P ratio etc.) and mechanical
properties (Vickers micro-hardness, fracture toughness and modulus of elasticity).
1.3 Thesis overview The literature review in this thesis is focused on the properties of the materials used for
producing coating, suitable methods of deposition and the characterization of deposited
material. The materials used in the experiment are hydroxyapatite (a variant of calcium
phosphate), titanium alloy powder (Ti-6Al-4V) and austenitic stainless steel (AISI 316L
& AISI 304L) as a substrate. The first section of the review discusses on the properties,
different forms and chemical changes of calcium phosphate. Titanium alloy is
discussed in the second section and stainless steel in the third. The fourth section is
dedicated to a discussion of different coating techniques and frequently used methods
applied to produce biocompatible coatings on an implant material. The fifth part of the
literature review focuses on various chemical, mechanical and microstructural
characterization techniques applied for powder and coating.
Chapter 1. INTRODUCTION
Muhammad Rakib Mansur Page 7
The review discusses briefly the most popular thermal spray technique, which includes
plasma spray [14-19]; but cold spray [20], flame spray [21] and high velocity oxy-fuel
spray (HVOF) [22] techniques are not being discussed. Other methods such as hot
pressing [23], sputtering [3], chemical vapour deposition (CVD) [24], plasma-enhanced
chemical vapour deposition (PECVD) [25], the sol gel method [26] and powder
metallurgy [27] are also deployed for biomaterial production [48] is not discussed in the
review considering them out of the scope of the current research.
The laser-assisted deposition technique is a technique that encompasses laser
cladding[28-30], laser surface alloying [31, 32], and pulsed laser deposition [3, 33, 34].
Direct metal deposition (DMD) is a laser-assisted rapid prototyping technique that uses
a closed-loop optical feedback system to create a product from a computer-aided
design (CAD). In this technique, a laser is used to create a melt pool on the substrate
and the powders are melted so that they form a coating onto the substrate [35, 36].
DMD was used for producing grafts [28] from pure HA to treat large bone defects (for
osseous reconstruction), but so far, composite coatings have not been manufactured
using the DMD process.
The third chapter is dedicated to the analysis of feedstock material. Morphological and
microstructural analyses of powders used in the experiment are covered in this section.
The shape and size of the raw material was analysed in this section. Chemical phases
present in the feedstock were analysed using Raman microscopy and X-Ray
diffraction. The elemental analysis of the powder has been carried out using energy
dispersive X-ray spectroscopy (EDS). Elemental analysis yielded calcium phosphate
ratio of the HA powders.
The fourth chapter describes the experimental methods used for the deposition of the
coating and the characterization techniques.
The characterization of the deposited composite coating made of HA and Ti-6Al-4V has
been formed in chapter five. The microstructure of the deposited coating was studied to
collect qualitative and quantitative information concerning the physical and structural
properties of the coating. The microstructure of the heat-affected zone (HAZ) where
diffusion was dominant was inspected. The roughness of the surface and the
topography of the coating were also studied. Diffusion coefficient and concentration
gradient of titanium into iron was determined for different experimental runs and
correlated with specific energy.
Chapter 1. INTRODUCTION
Muhammad Rakib Mansur Page 8
Chapter six contains the morphological, microstructural and chemical characterization
of multilayer composite coatings prepared from Ti-6Al-4V and HA. Besides this, effort
has been devoted to making a comparative study of single layer coating and multilayer
composite material made from Ti-6Al-V and HA. A comparison between composite and
pure HA coatings on stainless steel is presented in this chapter. The microstructure of
the deposited material was studied to collect qualitative and quantitative information
concerning the physical and structural properties of the composite.
Chapter seven is dedicated to the determination of the maximum temperature evolved
during laser-assisted deposition process. The cooling rate was also determined
experimentally. Two configurations have been used to perform temperature
measurement. In one configuration, temperature and cooling rate were determined
using a two-colour infrared pyrometer for Nd:YAG laser-treated AISI 304L austenitic
stainless steel. In another configuration, temperature and cooling rate were determined
using contact pyrometers fixed at the bottom of the AISI 316L austenitic stainless steel
substrate. In the experiment, the temperature profile was recorded and analysed to find
out peak surface temperature and cooling rate. Two analytical models were used to
determine temperature and cooling rate numerically for prediction and evaluation
against the obtained experimental values.
Chapter eight discusses the relationship between different properties and temperature
and cooling rate. Different properties like the surface average roughness, micro-
hardness and the diffusion coefficient were correlated either with temperature or with
cooling rate.
Chapter nine discusses the conclusion and future scopes of the conducted research.
1.4 Publications from this research Two journal papers (first paper is published and the second paper is accepted) have
been produced so far from the conducted research. They are:
Muhammad Rakib Mansur, James Wang, Christopher C. Berndt,
“Microstructure, composition and hardness of laser-assisted hydroxyapatite and
Ti-6Al-4V composite coatings”, Journal of Surface and Coatings Technology,
vol. 232, (2013), pp. 482 – 488
Chapter 1. INTRODUCTION
Muhammad Rakib Mansur Page 9
Muhammad Rakib Mansur, James Wang, Christopher C. Berndt,
“Hydroxyapatite and titanium composite coatings on austenitic stainless steel
substrates using direct material deposition”, Periodical of Material Science
Forum, with title “Advances in Materials and Processing Technologies XV”, vol.
773-774, (2014), accepted and will be published in 2014.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 10
2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 11
2.1 Introduction Bone is composed of 3 major components; living cells comprise the first part, including
osteoblasts (bone growing cells), osteoclasts (bone resorbing cells) and osteocytes
(mature bone cells surrounded by hydroxy carbonate apatite, HCA); the second part is
made up of non-living organic crystals (collagen, muco-polysaccharides), and non-
living inorganic crystals (hydroxycarbonate apatite, HCA) constitute the third part. The
mineral part of the bone contains 50 to 70 wt.%; whereas 20 to 40 wt.% is comprised of
organic matrix, 5 to 10% water and less than 3 wt.% lipid [49].
Among the types of bio-ceramics, the most clinically used ceramics are bioglass,
sintered HA, sintered β–tricalcium phosphate (TCP), HA / TCP bi-phase ceramic, and
glass ceramic A-W containing crystalline oxyfluoroapatite and β-wollastonite
(CaO.SiO2) in MgO-CaO-SiO2 glassy matrix [4].
Hydroxyapatite, a form of calcium phosphate is a unique material because of its
similarity to hard tissue, which means that can directly bond to the bone [43]. Bioactive
ceramics manufactured from HA form an apatite layer on it after implantation, which
helps the ceramic to integrate with bone matrix. It is also non-toxic, bioactive and forms
an interfacial bond between the material and tissue [13]. In vivo formation of an apatite
layer on the surface of a bioactive material is important in terms of bone and material
integration [4]. Besides this, some functional groups such as Ti-OH are also useful for
the nucleation of apatites [4]. Titanium and its alloys have found acceptability because
of their unique corrosion resistance properties, load bearing capacity and ability to form
functional groups that are useful for apatite formation.
This chapter, choosing from the many prospective bio-friendly and bio-active materials,
is dedicated to different types of CaP, titanium alloy (which includes Ti-6Al-4V) and
stainless steel. The first section of this chapter describes the ceramic materials suitable
for prosthesis application, which includes different types of CaP. In the second section,
load-bearing materials are discussed with a focus on titanium alloy and stainless steel
because of their suitability for prosthesis applications. The chapter contains a
discussion of deposition techniques adopted for the deposition of CaP coatings; the
latter discussion is mainly focused on plasma spraying and different laser-based
deposition techniques.
The chapter also encompasses a comprehensive literature review of different
characterization techniques, which includes surface, microstructural, mechanical and
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 12
chemical characterization of ceramic-based coatings. The characterization part of this
chapter also discusses the feedstock material.
2.2 Properties and classification of calcium phosphate Calcium phosphate falls within the group of ceramic materials. Ceramics are usually
refractory, polycrystalline and inorganic compounds that include various forms of
metallic oxide and can contain metallic and non-metallic elements. Ceramics are hard
in nature and the hardness of apatite is in the range of 5 on the Mohs scale. Ceramics
are difficult to shear if compared against metals and polymers, and this fact is attributed
to the ionic bonds prevailing among the ions inside the ceramic; they are more
polycrystalline in nature and may be a mixture of two or more crystalline phases.
‘Apatite’ denotes a group of phosphate minerals that includes hydroxyapatite (HA),
fluoro apatite, chloro-apatite, etc. The general formula for the apatite group mineral is,
M10(ZO4)6X2, where M = Ca, Sr, Pb, Na …, Z = P, As, Si, V…, and X = F, OH, Cl….[50].
Of all the members in this group, HA is the most important, because of its similarity to
human bones and teeth. The HA present in bones and teeth also contains many other
impurities, for example, magnesium (Mg2+), carbonate (CO3
2-), sodium (Na+), chloride
(Cl-), potassium (K+), fluoride (F-), and acid phosphate (HPO4). Trace elements include
strontium (Sr2+), barium (Ba2+) and lead (Pb2+), and exhibit a complex structure.
Important forms of calcium phosphate, along with their Ca/P ratio, are presented in
Figure 2-1.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 13
Figure 2-1: Various forms of calcium phosphate and their calcium phosphate ratios
(Ca/P).
2.2.1 Crystalline and amorphous calcium phosphate Depending on the following variables - Ca/P ratio, presence of water, impurities, and
temperature - calcium phosphate can be crystallized into salts such as hydroxyapatite
and beta tri-calcium phosphate (β-TCP). At temperatures lower than 900°C and in the
presence of water, it is more likely to form hydroxyl- or hydroxyapatite, while in a dry
atmosphere and at a higher temperature, β-TCP will be formed [51]. Hydroxyapatite
and β-TCP forms are tissue-compatible, and constitute a good bone substitute in either
granular or solid block form. The apatite form of calcium phosphate is considered to be
the closest to the mineral phase of bones and teeth. The unit cell of HA contains
calcium (Ca), phosphate (PO4) and hydroxyl (OH) ions, closely packed together to
represent the apatite structure [45]. The position of the different atoms in HA is
presented in Figure 2-2. Most researchers suggest that HA has a hexagonal crystal
structure.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 14
Figure 2-2: Theoretical positions of the ionic species within the unit cell of HA [12].
Another important form of calcium phosphate is amorphous calcium phosphate (ACP).
ACP can be formed by a low temperature wet process or a high temperature dry
process. It can occur in high temperature processing techniques, for example plasma
spray deposition, electrostatic spray deposition (ESD) and pulsed laser deposition
(PLD). In plasma spray coatings, ACP plays an important role in the mechanical
properties of CaP coatings. The amounts of different phases can be attributed to spray
conditions such as gas flow, (which controls how long the HA particles spend in the
plasma flame), plasma temperature, nature of the gas, the cooling conditions, the
distance between substrate and the flame, and the quality of the powder, density,
absorbed water and size of the HA particles [52]. Different forms of calcium phosphate
have been described in a tabular format, in Table 2-1.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 15
Table 2-1 : Different forms of calcium phosphates [45, 52].
According to K. A. Gross et al., a higher cooling rate leads to an amorphous phase, but
a lower cooling rate leads to the formation of oxyapatites [53], even though the water
vapor in air can modify the transformation by the inclusion of hydroxyl ions. At the
metal coating interface, amorphous regions are more prominent, and an increase in
crystallinity alongside the decrease in coating thickness [54] is the general trend. This
phenomenon is attributed to the heat transfer mechanism. The crystallinity of HA
coatings can be increased by heat treatment. Tri and tetra calcium phosphates are
more stable than HA in a dry atmosphere, so the addition of water molecules enhances
the transformation and the recrystallization of HA.
Name of the form Symbol Chemical Formula Phase’s Name
Ca/P
Dicalcium Phosphate
Anhydrous
DCPA CaHPO4 Monetite 1.00
Dicalcium Phosphate
Dihydrate
DCPD CaHPO.2H2O Brushite 1.00
Amorphous Dicalcium
Phosphate
CaHPO4 - 1
α-Tricalcium Phosphate α-TCP α-Ca3(PO4)2 - 1.50
β-Tricalcium Phosphate β-TCP β-Ca3(PO4)2 Whitlockite 1.50
Amorphous Tricalcium
Phosphate
ACP Ca3(PO4)2.nH2O - 1.5
Tetracalcium Phosphate TTCP Ca4(PO4)2O - 2.00
Octocalcium Phosphate OCP Ca8H2(PO4)6.5H2O - 1.33
Amorphous Octacalcium
Phosphate
Ca8 H2 (PO4)4.n H2O - 2
Oxyhydroxyapatite OHA Ca10(PO4)6(OH)2-2xOx - 1.67
Oxyapatite OA Ca10(PO4)6O - 1.67
Hydroxyapatite HA Ca10(PO4)6(OH)2 - 1.67
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 16
Chemically, bone is an organic-ceramic composite of complex chemistry, having
collagen (20 wt.%), calcium phosphate (69 wt.%) and water (9 wt.%) [43]. The ratio of
calcium and phosphorous (Ca/P) has a significant impact on biocompatibility [13]. The
calcium phosphate (CaP) present in bone is a modified form of hydroxyapatite having a
Ca/P ratio of about 1.65, which is close to that of pure hydroxyapatite (HA), oxyapatite
(OA) and oxyhydroxyapatite (OHA) having Ca/P ratio of 1.67 [44, 45]. Present studies
revealed the Ca/P ratio of bone and dentine is different from the 1.67 value for geologic
hydroxyapatite [46]. The research performed by Liu et al. concluded that the Ca/P ratio
indicates the dominant form of calcium phosphate [47].
The main concerns about the HA coating are dissolution or delamination and in vivo
durability. The coatings that have a higher degree of crystallinity exhibit low dissolution
rates in in-vitro tests and less resorption and more direct bone contact in in-vivo tests
[55]. Higher amorphous contents lead towards rapid weakening and disintegration of
the coating and often promote an inflammatory response in the surrounding tissue [55].
Although a high degree of crystallinity is desired in the coating, the presence of a small
amount of amorphous HA at the coating surface may promote beneficial physiological
activity [48, 55].
2.2.2 Thermal change of calcium phosphate Almost every deposition technique involves thermal decomposition. Three phenomena
could occur, if HA is heated. They are evaporation of water, dehydroxylation and
decomposition. HA can absorb water that may be present on the surface of the powder
and trapped within pores [56]. Water can also be present as a part of the lattice
structure. When first heated, the water present on the surface and pores begins to
evaporate. At higher temperatures dehydroxylation takes place, and the crystal
gradually loses the hydroxyl (OH-) group. The decomposition of HA occurs in 4 steps
[19]:
Step 1: Ca10(PO4)6(OH)2 → Ca10(PO4)6(OH)2-2xOx [ ]x + xH2O
(hydroxyapatite) → (oxyhydroxyapatite)
Step 2: Ca10(PO4)6(OH)2-2xOx [ ]x → Ca10(PO4)6Ox [ ]x + (1-x)H2O
(oxyhydroxyapatite) → (oxyapatite)
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 17
Step 3: Ca10(PO4)6Ox [ ]x → 2Ca3(PO4)2 + Ca4O(PO4)2
Step 4a: 2Ca3(PO4)2 →3CaO +P2O5
Step 4a: Ca4O(PO4)2 →4CaO +P2O5
First it becomes OHA (oxyhydroapatite), which has a large number of vacancies in its
structure, then further dehydroxylation forms OA (oxyapatite). OHA and OA has the
tendency to retransform to HA in the presence of water [19]. The effect of temperature
on HA is presented in Table 2-2 [45].
Table 2-2: Reaction at different temperatures.
Temperature Reactions
25 – 6000 C Evaporation of absorbed water
600 – 8000 C Decarbonation
800 – 9000 C Dehydroxylation of HA forming partially dehydroxylated
(OHA) or completely dehydroxylated oxyapatite (OA)
050 – 14000 C HA decomposes to form β-TCP and TTCP
< 11200 C β-TCP is stable
1120 -14700 C β-TCP is converted to α-TCP
15500 C Melting temperature of HA
16300 C Melting temperature of TTCP, leaving behind CaO
17300 C Melting of TCP
The phase diagram of CaO-P2O5 at 500 Hg vapour pressure is presented in Figure 2-3.
The phase diagram shows the transformation of calcium phosphate at different
temperatures.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 18
Figure 2-3: Phase diagram (reproduced) of CaO-P2O5 under 500mmHg water vapour
pressure [57].
The chemical constituents, crystallographic and mechanical properties of HA, enamel
and cortical bone is presented and compared in Table 2-3. The table shows enamel
and cortical bone contains Sodium (Na+), Potassium (K+), Magnesium (Mg+),
Carbonate (CO32+), Fluoride (F-), Chloride (Cl-) ions, however HA contains a trace
amount of those ions. Cortical bone is less crystalline and contain large amount of
water compared to dentin and HA.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 19
Table 2-3 : Comparison between enamel, cortical bone and HA in terms of different
constituents and characteristics [58].
Enamel Cortical bone HA
Constituents
Calcium, Ca2+ 36 24.5 39.6
Phosphorus, P 17.7 11.5 18.5
(Ca/P) molar 1.62 1.65 1.67
Sodium, Na+ 0.5 0.7 Trace
Potassium, K+ 0.08 0.03 Trace
Magnesium, Mg+ 0.44 0.55 Trace
Carbonate, CO32+ 3.2 5.8 -
Fluoride, F- 0.01 0.02 -
Chloride, Cl- 0.3 0.1 -
Total inorganic 97.0 65.0 100
Total organic 1.0 25.0 -
Absorbed H2O 1.5 9.7 -
Crystallographic properties
(Lattice parameters, +0.03 nm)
a 0.9441 0.9419 0.9422
c 0.6882 0.6880 0.6880
Crystallinity index 70-75 33-37 100
Crystallite Size, nm 130 X 30 25 X 2.5 - 5
Products after sintering (>800 C0) HA + TCP HA + CaO HA
Mechanical Properties
E (GPa) 14 20 10
Tensile Strength (MPa) 70 150 100
2.3 Titanium and its alloys Titanium and its alloys are gaining popularity in the field of aerospace, chemical
engineering and biomedical applications because of their low density, high strength,
good corrosion, and erosion and oxidation resistance. The two main allotropic forms of
titanium alloy are alpha and beta alloys. Alpha alloys have a hexagonal closed packed
(HCP) crystal structure and high temperature beta alloys have a body centred cubic
lattice (BCC) crystal structure. Some alloying elements affect the alpha and beta forms
of Ti at the time of the formation of the solid solution. Aluminium, gallium, oxygen,
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 20
nitrogen and carbon acts to stabilize the alpha forms of titanium and are called alpha
stabilizers. On the other hand, vanadium, molybdenum, niobium, iron, chromium and
nickel helps to stabilize beta forms and are classed as beta stabilizers. Ti-6Al-4V is an
alpha-beta alloy because of 6% Al (alpha stabilizer) and 4% V (beta stabilizer). At room
temperature in Ti-6Al-4V, the alpha phase dominates (90 vol%) and determines the
physical and mechanical properties of the alloy and the beta phase is affected by heat
treatment. Diffusion in alpha Ti (hcp) is lower than beta titanium (bcc) because of their
densely packed atoms. The diffusion coefficients for alpha and beta titanium at different
temperatures are given in Table 2-4 [5].
Table 2-4 : Diffusion coefficients of different phases of titanium (for self-diffusion) [5].
Phase of
titanium
Temperature
(0C)
Time
(h)
Depth
(µm)
Diffusion coefficient, D
(m2/s)
α-Ti 500 50 0.8 10-19
β-Ti 500 50 0.9 10-18
α-Ti 1000 50 4 10-15
β-Ti 1000 50 40 10-13
Mechanical and crystalline properties of pure polycrystalline alpha phase of titanium
are presented in Table 2-5. The table represents space group, lattice parameters and
different thermal and mechanical properties of pure polycrystalline alpha titanium.
Table 2-5: Physical properties of high-purity polycrystalline alpha Ti at 25 C0 [5].
Structure Prototype Mg
Pearson symbol hP2
Space Group P63/mmc (194)
Βeta –transus temperature 882 0C
Lattice parameters a = 0.295 nm
b = 0.468 nm
c/a = 1.587
Thermal expansion coefficient ( 10-6 K-1 ) 8.36
Thermal conductivity ( W/mK ) 14.99
Specific heat capacity ( J/kgK ) 523
Electrical resistance ( 10-9 Ωm ) 564.9
Elastic Modulus ( GPa ) 115
Shear Modulus ( GPa ) 44
Poisson’s Ratio 0.33
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 21
The density of Ti-6Al-4V in the annealed condition is 4.43 g/cc with melting point 1604 oC to 1660 oC. The beta transition temperature for Ti-6Al-4V alloy is 980 0C with a
specific heat capacity of 0.5263 J/g-oC and thermal conductivity of 6.70 W/moK. The
major mechanical properties of Ti-6Al-4V are presented in Table 2-6. Data mentioned
in this paragraph and in the table has been collected from MatWeb [59].
Table 2-6 : Important mechanical properties of Ti-6Al-4V annealed bar [59].
Elastic Modulus ( GPa ) 113.8
Shear Modulus ( GPa ) 44
Poisson’s Ratio 0.342
Fracture Toughness ( MPa √m ) 74.6
Charpy Impact ( J ) 17.0
Elongation at break ( % ) 14
Hardness, Vickers ( Hv ) 349
2.4 Stainless steel Stainless steel belongs to a special grade of iron-based alloys that contain chromium
and exhibit resistance to atmospheric corrosion. According to their microstructure, they
are classified mainly as austenitic, martensitic and ferritic. Single phase austenitic
stainless steel (SS 316L having 18Cr-14Ni-2.4Mo) is a popular material for implant
applications due to its resistance to pitting and crevice corrosion from the body plasma
[3] and for its cost effectiveness. The alphabet ‘L’ after 316 indicate the carbon content
of the stainless steel alloy is lower than 0.03%. Lower percentage of carbon reduces
the chance of chromium carbide precipitation along the grain boundaries. Pure iron has
two crystal forms; body-centred cubic lattice (BCC) and face-centred cubic (FCC)
lattice. At relatively low temperatures (up to 910oC), iron exists in the BCC structure
(which is called α iron) then it transforms into a FCC crystal structure and remains
stable up to 1,390oC. The γ phase of iron is called austenite, which is a non-magnetic
allotrope of iron. The crystal data of Fe-Cr-Ni alloy system is presented in Table 2-7.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 22
Table 2-7 : Crystal data of Fe – Cr – Ni alloy [60, 61].
Phase
label
Formula Prototype Space
group
Density
(Mg/m3)
Volume
(nm3)
Cell
parameter
(nm)
Angle
(0)
α Cr0.05
Fe0.90
Ni0.05
W Im-3m
7.84
0.0236
a=0.287
b=0.287
c=0.287
α=90
β=90
γ=90
γ
Cr0.08
Fe0.65
Ni0.27
Cu Fm-3m
8.17
0.0458
a=0.3577
b=0.3577
c=0.3577
α=90
β=90
γ=90
The AISI 316L grade of stainless steel comprises 18% Cr, 14% Ni and 2.4% Mo
besides iron and 304L contains 19% Cr and 10% Ni [62]. The chromium forms a
passive film of chromium oxide that prevents surface corrosion. The addition of nickel
and molybdenum enhances the corrosion resistance [63]. Different passivation
processes have been adopted to improve the in-vitro and in-vivo corrosion resistance
of 316L stainless steel by creating an oxide layer [64] on the surface. Passivation can
be performed thermally, electrochemically, and by using nitric acid. The major
mechanical properties of the two grades of stainless steel used as a substrate for
deposition were collected from the matweb website [65, 66] and are compiled in Table
2-8.
Table 2-8 : Mechanical properties of austenitic stainless steel [65, 66].
Stainless
steel type
Hardness,
HV
Ultimate
Tensile
Strength
(MPa)
Yield
strength
(MPa)
Modulus
of
Elasticity
(GPa)
Fracture
toughness,
Charpy (J)
Elongation
at break
(%)
AISI 316 L 155 515 205 193 103 60
AISI 304 L 159 564 210 193-200 216 58
The different phases in a Cr-Fe-Ti ternary phase alloy are presented in Table 2-9,
which contains crystallographic information of different compounds formed in the alloy
system.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 23
Table 2-9 : Different phases present in Cr-Fe-Ti ternary phase diagrams [67].
Phase
label
Formula Prototype Space
group
Density
(Mg/m3)
Volume
(nm3)
Cell
parameter
(nm)
Angle
(o)
γ Fe Fe Cu Fm-3m
7.64
0.04854
a=0.36477 b=0.36477 c=0.36477
α=90 β=90 γ=90
TiCr2 TiCr2 MgCu2
Fd-3m
6.04
0.33411
a=0.6939 b=0.6939 c=0.6939
α=90 β=90 γ=90
TiFe TiFe CsCl Pm-3m
6.52
0.02643
a=0.29789 b=0.29789 c=0.29789
α=90 β=90 γ=90
Cr,Fe Cr0.03
Fe0.97
W cI2 Im-3m
7.84
0.02362
a=0.28692 b=0.28692 c=0.28692
α=90 β=90 γ=90
TiCr2 TiCr1.56
Fe0.44
MgZn2 P63/mmc
6.23
0.1636
a=0.4868 b=0.4868 c=0.7973
α=90 β=90 γ=120
Ti5Cr8Fe16
Ti5Cr8Fe16 Ti5Re24 I-43m
7.24
0.7102
a=0.8922 b=0.8922 c=0.8922
α=90 β=90 γ=90
2.5 Deposition techniques Different techniques have been adopted to produce calcium phosphate coatings; these
include thermal spraying techniques; namely plasma spraying, flame spraying, high-
velocity oxy fuel, cold spraying and laser spraying. Other deposition techniques
adopted are laser-assisted deposition, pulsed laser deposition, sputtering, electron
beam evaporation, chemical vapour deposition, plasma-enhanced chemical vapour
deposition, electro deposition, micro arc / plasma electrolytic oxidation, sol-gel, etc. Of
all the techniques used for biomedical applications, the plasma spray method is
popular one and laser-assisted deposition techniques are flourishing. Both of these
techniques have advantages and disadvantages. Different coating techniques
correlating substrate temperature and coating thickness are presented in Figure 2-4.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 24
Figure 2-4: Different deposition techniques in terms of coating thickness and substrate
temperature [68].
2.5.1 Plasma spraying Plasma spraying falls in to the group of thermal-spray processes that use a
concentrated heat source (plasma, ionised gas) to melt feedstock materials and then
use process jets (carrier gas and plasma) to carry the hot, molten and energetic
particles toward a prepared surface [69]. After the impact, the molten (completely or
partially) particles become solidified and the surface becomes coated. The schematic
set up of a dc arc plasma system is presented in Figure 2-5.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 25
Figure 2-5: Schematic set-up of a dc arc plasma system [68].
The plasma can be generated by using a direct current arc, radiofrequency, microwave
and electromagnetic induction. The main advantage of plasma spray is that, as a heat
source, the thermal energy of plasma is at the highest level and is almost clean.
Different thermal spray processes for ceramics are presented along with the
characteristic properties in Table 2-10.
Table 2-10: Comparison of different thermal spray process coating characteristics of
ceramic (rearranged and reproduced) [68].
Characteristics Flame Spray
HVOF Electric arc wire spray
Plasma Spray
Gas temperature [°C] 3000 2600 - 3000 4000 (arc) 12000- 16000
Spray rate [Kg/h] 2-6 1-9 10-25 2-10
Bond strength [MPa] 14 - 34 - - 21 - 41
Coating thickness [mm] 0.25 – 2.0 - - 0.1 – 2.0
Hardness [HRC] 40 - 65 - - 45 - 65
Porosity [%] 5 - 15 - - 1 - 2
2.5.2 Laser-assisted deposition Light amplification by stimulated emission of radiation (i.e., “laser”) is a phenomenon
achieved by the interaction of the atoms and molecules of the active medium with the
electromagnetic field of the pumping source. Lasers have three basic components,
which include active medium, pumping source and optical resonator. The active
medium may be a solid, liquid, gas or plasma. The common laser mediums are: carbon
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 26
dioxide (CO2) gas and neodymium-doped yttrium aluminium garnet (Nd:YAG) as a
solid. Pumping sources include flash lamps, electron beams, ion beams, chemical
reactions and X-ray sources. A simple laser consists of two mirrors placed in parallel,
acting as an optical oscillator. The active medium (gas, solid etc.) is placed inside the
optical oscillator and amplifies the light oscillations by the mechanism of stimulated
emission. One of the two mirrors is partially transparent to allow some amplified
oscillating beam to be used.
For material processing applications CO2, and Nd:YAG, excimer (KrF, ArF, XeCl) and
diode (GaAs, GaAlAs, InGaAs, GaN) are the most used lasers. Typical lasers are
presented in Table 2-11 in terms of wavelength, frequency, energy, mode and cavity
specification.
Table 2-11 : Typical lasers with their wavelength, frequency, energy, cavity information
and mode.
Type of laser
Wavelength λ (µm)
Frequency ν (Hz)
Energy Ea (ev)
Cavity radius (mm)
Cavity length (mm)
Fresnel No Mode*
CO2 fast
axial flow
10.6 2.8 X 1014 0.12 11 5.2 2.2 TEM00/low
Nd:YAG 1.06 2.8 X 1013 1.16 2.4 0.55 9.8 Multimode
*Mode: Peaks resulted due to the constructive interference of many wavelengths within
the laser cavity. TEM: Transverse electromagnetic mode.
Laser-assisted deposition is a relatively new technique that is flourishing at a
considerable pace. Laser can be used for deposition using different arrangements, but
the basic principle is that the laser acts as a heat source to provide thermal energy to
the intended deposition material.
When a laser beam falls onto a material surface, the phenomena that take place are
reflection, refraction, absorption, scattering and transmission. The effects of laser
material interaction are heating, surface melting, surface vaporization, plasma
formation and ablation. These effects solely depend on laser parameters along with the
thermal and physical properties of a material. Metal is considered as an opaque
material and its absorptivity (A) is:
2-1
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 27
Here R = reflectivity of the metal. It is also possible to calculate absorptivity and
reflectivity from the refractive index, n, and extinction coefficient, k. The expression of
absorptivity in terms of n and k is:
[( ) ] 2-2
Extinction coefficient, refractive index and reflectivity of different materials (pure metals)
for 1.06 µm (Nd:YAG) and 10.6 µm (CO2) laser wavelength are presented on Table
2-12. The absorptivity of a material depends on the wavelength of the laser beam,
material and temperature. The lower the wavelength, then the more absorptivity there
is for a particular material. At the same wavelength, reflectivity decreases with
temperature for some materials such as copper and aluminium, but, for steel,
reflectivity increases slightly with an increase in temperature [70]. Absorptivity also
depends upon the thickness of the surface films (for example oxide films) and the
roughness of the surface.
Table 2-12 : Reflectivity (R) of different materials (for wavelength λ = 1.06 µm).
Material Extinction coefficient, k
Refractive index, n
Reflectivity, R
Fe 4.44 3.81 0.64
Ni 5.26 2.62 0.74
Ti 4.0 3.8 0.63
Al 8.50 1.75 0.91
Different types of lasers have been used for processing different materials. Among all
the lasers CO2 and Nd:YAG (neodymium-doped yttrium aluminium garnet) is most
commonly and widely used.
The most common coating methods using lasers are pulsed-laser deposition (PLD) and
direct melting using a pulsed or continuous wave laser. PLD has some unique
advantages. Films produced by PLD are superior to conventional evaporation or
electron beam evaporation. PLD can fabricate nano-crystalline and composite films
and can be used to synthesize metastable materials [30]. Laser-processing parameters
used for developing CaP coatings on different substrates are presented in Table 2-13.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 28
Table 2-13: Laser-processing parameters used for laser cladding of CaP coatings.
Process Feedstock material used Process parameter substrate Ref
Laser
(Nd:YAG) and
induction
plasma
HA (45 to 150 µm) Power: 400 to 500 W Ti substrate,
Grade 2*
[71]
In situ laser
cladding (CO2)
continuous
wave laser)
CaCO3 20%,
CaHPO4.2H2O 80% &
Transitional layer by Ti 50
wt% used by mixing with
Na2SiO3
Power: 1500W , Scanning
speed: 3.5 to 11.2 mm/s,
Beam diameter 4 mm,
Commercially
Pure Ti, TA2 of
dia 15 mm and 10
mm
[30]
Laser
(Nd:YAG)
surface
alloying
HA 30 -50 µm, Carbon
nanotube CNT 20 - 40nm
length 5 to 15µm (5%, 10%
and 20%)
400 W,
Beam diameter 4 mm,
scanning speed 4 mm/s
Ti-6Al-4V
Substrate
preheated
[32]
Laser
(Nd:YVO4)
treated sol gel
coating
Solgel precursor contains
calcium nitrate and
phosphoric acid.
Scanning velocity (mm/s)
100 to 500, Fluency (J/cm2)
280 to 56
Ti Grade 2*, [72]
Pulsed laser
(Nd:YAG)
deposition
HA (CAPTAL 90), 120 µm, Power 80 to 200W,
scanning speeds
between 0.8 and 6.7 mm/s,
powder mass flow bt 3.7 and
20.4 mg/s, Argon as
conveying (8 l/min) &
protective (16 l/min) gas,
Ti-6Al-4V [28]
Laser
(Nd:YAG)
cladding
HA and SiO2 spherical and
unimodal distribution at 1:3
weight ratio.
Average power 80 W, spot
shape rectangular, pulse
width 1 ms, spot diameter
900 µm, laser scan speed
25, 75 21 cm/min, line
spacing 0.1 mm.
Ti-6Al-4V, coupon
size 100 X 50 X 3
mm3
[73]
Continued on next page
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 29
Process Feedstock material used Process parameter substrate Ref
Pulsed
(Nd:YAG)
laser
HA powder (LAH – 97)
from Fin – ceramic, particle
size 50 to 500 µm, Ca / P
ratio 1.68, 95% crystalline.
Power density 2000
W / cm2, frequency
40 Hz, pulse duration
4 ms, traverse speed
1 mm / s.
Ti-6Al-4V, coupon
size 50 X 50 X 6
mm3
[29]
Laser
(Nd:YAG)
induced
hierarchical
calcium
phosphate
structures
CaP Tribasic, Ca3(PO4)2 ,
Solution is air-sprayed on the
coupon.
Power 850W,
Powder flow rate 10
to 30 mg/s, Scanning
speed 21 to 41 mm/s,
Beam spot elliptical
5mm X 1.5 mm
50 mm · 100 mm ·
3 mm of Ti-6A-l4
V
[74]
Laser (CO2)
cladding
wt% of CaHPO4
2H2O and CaCO3 was 81.1 wt.%
and 18.9 wt.%,
laser system Power
2.5KW, Scanning
speed 140mm/min,
laser beam size 15
mm X 1 mm.
Ti-6Al-4V [75]
Pulsed
(Nd:YAG)
laser
CaP (tribasic) mixed with a water
based organic solvent, and
sprayed with an air spray gun on
preheated substrate (50 0C).
Pulse width 0.5 ms,
pulse energy 4 J,
pulse repetition rate
20 Hz, Average
power 80 W, laser
scan speed 36, 48,
66, 78 and 17
cm/min, spot
diameter 0.9 mm,
pulse shape
rectangular
Ti-6Al-4V, coupon
size 100 X 50 X 3
mm3
[76]
Continuous
Wave (CW),
(Nd:YAG)
laser
CaP (tribasic) mixed with a water
based organic solvent, and
sprayed with an air spray gun on
preheated substrate (50 0C).
Beam diameter 3.8
mm, traverse speed
33 cm/min, Power =
500, 600, 700, 800,
900 and 1000W.
Ti-6Al-4V, coupon
size 100 X 50 X 3
mm3 , polished
with 600 grit SiC
paper
[77]
* Grade 2 titanium alloy is unalloyed titanium with standard oxygen content.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 30
In a pulsed-laser deposition (PLD) process, the laser interacts with the bulk target
material. This interaction forms a plasma plume and then, sequentially, the plasma
becomes heated further and 3D isothermal expansion of the plasma occurs, which
results in adiabatic expansion and deposition of thin films on the substrate [78]. The
whole process happens inside a closed chamber at a high vacuum. The schematic of
PLD is presented in Figure 2-6. The laser parameters affecting PLD are: energy
density, pulse duration, wavelength, polarisation and pulse repetition rate [78].
Figure 2-6 : Schematic of a pulsed-laser deposition system, PLD [3].
For PLD, the initial substrate temperature has a strong effect on the nature of the film,
whether it is amorphous, polycrystalline or single crystalline. Higher substrate
temperatures generally produce a crystalline phase. For PLD, the most frequently used
lasers are excimer, Nd:YAG and Nd:glass [78]. Crystallinity and cell attachment is
directly related to laser energy density (fluence) [78].
A continuous wave laser produces laser beams that exhibit constant power over time.
So, in this case, the beam produces more uniform thermal conditions within the beam
substrate interaction region. Thus strong bonding is achieved at the interface in the
case of continuous wave laser. A schematic of a continuous wave laser is presented in
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 31
Figure 2-7. A continuous wave laser is able to produce a systematic and uniform
coating on the substrate.
A pulsed laser has a unique advantage in terms of producing a textured surface. The
3D textured surface has more surface area for the interaction with proteins, which can
support cell and tissue growth [76]. Compared to a continuous wave laser, a pulsed
wave laser produces different thermal conditions, which produce different
morphological, microstructural and phase features within the laser substrate interaction
region.
Figure 2-7 : Schematic of a Nd:YAG system [3].
Direct material deposition (DMD) is a rapid prototyping technique featuring a closed-
loop optical feedback system (Figure 2-8) where lasers have been used to create a
melt pool on the substrate. Continuous wave lasers are usually used for deposition
processes.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 32
Figure 2-8 : Schematic of (a) DMD process (b) DMD system [35].
In the DMD process, the powders are usually supplied using an axial powder delivery
system. The powder is heated by the laser during flight and becomes partially or
completely molten depending upon the particular process parameters such as power,
traverse speed and beam diameter. The partial or completely molten particles then hit
the molten substrate and become attached to the substrate, and thus a coating forms.
It is possible to create a micro and macrostructure using DMD [36]. Other advantages
of DMD are that it creates a small heat-affected zone, can be used on almost any
surface and can mix metals to create a variety of properties including graded structures
[36]. The deposition efficiency of DMD is high because of its feed-back control system
and unique powder delivery system. Another important aspect of DMD is its efficient
protective (inert) gas atmosphere, which reduces the chance of oxidation and
effectively protects the clad layer.
2.6 Characterization In order to characterize a material, the analysis of its chemical composition and
structural and mechanical properties is important. A material can be characterized in
terms of chemical composition and structure, mechanical properties and
microstructural properties. It is also important to characterize the feedstock material as
well as substrates to obtain a preliminary idea of the chemistry, microstructure and
morphology; all of which will be invaluable in analysing coatings. Different
characterization approaches discussed in this section range from feedstock
characterization to mechanical, microstructural, surface and chemical characterization.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 33
2.6.1 Feedstock or powder characterization Analysing particles is not straightforward because of the complex size, shape and other
properties of the particles. The particles may also be anisotropic as well as follow a
particular size distribution. For the proper characterization of a powder, an appropriate
parameter (size, shape, density etc.) should be selected, which will describe the
powder appropriately and which can be linked with its physical, mechanical and
chemical properties. Image analysis of the micrographs of feedstock material can
reveal powder characteristics. The important parameters for image analysis are
represented in Table 2-14.
Table 2-14 : Important parameters for microstructural analysis of a powder.
Parameters Definition
1 Feret diameter (DF) Distance between two parallel tangents.
2 Minimum feret diameter
(DFmin)
Breadth of a particle projection at rest.
3 Martin diameter (DM) Diameter of the particle which divides the particle into
two equal projected areas.
4 Breadth (B) Minimum feret diameter of the projection of the
particle at stability.
5 Length (L) Feret diameter perpendicular to length or the
maximum feret diameter.
6 Geodesic length (LG) Length of a curved particle
7 Chord Length (CL) Distance of an intersection of a particle at a random
point at the perimeter.
8 Thickness (T) Height of the particle when the particle is resting with
stability.
9 Equivalent projected
area diameter (DA)
Diameter of a circle which has the same area as a
projected particle.
10 Equivalent surface area
diameter (DS)
Diameter of a sphere which has the same surface
area as the particle.
11 Equivalent volume
diameter (DV)
Diameters of a sphere which have same volume as
the particle.
12 Stokes diameter (DST) Diameter of a sphere which has the same settling rate
as the particle under conditions of Stokes law.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 34
If the particle is convex then the relation between the perimeter (P) and mean Feret
diameter (DF) is (according to second Cauchy theorem) [79] given by the equation
below.
2-3
The relation between mean projected area A (in different particle orientations) and
particle surface area S can be expressed by (according to the first Cauchy theorem)
[79] the equation below.
2-4
The breadth and thickness are related to the minimum aperture size of a sieve. The
equivalent sphere concept is popular for particle size distribution measurements. If the
particle is flake-like or fibrous, then their shape plays an important role in powder
characterization.
The factors that play an important role in shape characterization are elongation,
flakiness, aspect ratio, chunkiness, and roundness. The ratio between two different
equivalent diameters could be a shape factor for example, the Waddell sphericity
factor. Different p can be used for particle measurement are described in Table 2-15.
Table 2-15 : Different ratios for particle measurement [80, 81].
Name of the ratio Explanation
1 Aspect ratio Maxm feret diameter / Min feret
diameter (Length / Breadth)
DFmax /DFmin
2 Chunkiness 1 / Aspect ratio DFmin/ DFmax
3 Compactness Equivalent area diameter / Length DA / L
4 Elongation ratio Geodesic length / Breadth Lcurve/B
5 Flakiness ratio Breadth / Thickness B / T
6 Roundness 1 (4A/πL2)/ (DA/L)2
7 Roundness 2 P2/A
8 Roundness 3 or
Circularity
1/ Roundness 2 A/P2
9 Solidity Cross-sectional Area / Convex Area A / Convex Area
10 Sphericity Radius of the inscribed circle / Radius
of the circumscribed circle
RI / RC
11 Convexity Convex perimeter / Perimeter Convex P / P
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 35
Another approach used in shape characterization is fractal analysis of the contours of
particle projection, where different length scales (step wise λ) are used to evaluate the
contour length (Perimeter P(λ)) of a 2-dimensional projection of a particle. According to
NIST, mono-dispersed particles should contain 90% of the particles within 5% of their
median size [79].
In this section, the morphology and the microstructural analysis of powder feedstock
material has been discussed. The chemical analysis of the powder is similar to the
chemical analysis of the coatings, and this will be covered in section 2.63.
2.6.2 Surface Characterization The surface of a material is defined as the geometric boundary of an object.
Roughness indicates the fine irregularities of the surface texture. The normal or
perpendicular contour of the surface in a plane, termed ‘profile’, and ‘waviness’, is the
widely spaced component of surface texture. Roughness is superimposed on
waviness. Roughness average (Ra) is a parameter for measuring surface roughness,
expressed in micrometers (µm) or in microinches, and is the arithmetic average of the
measured profile height deviations divided by the evaluation length. There is another
term which is called root mean square average (RMS, Rq), which is the square root of
the average value squared, within the evaluation length, and measured from the mean
line.
Texture and roughness influence the chemical and physical properties of thermal spray
deposits. Surface parameters and statistical functions are used to define the geometric
characteristics of a surface. Height parameters (Ra and Rq roughness), wavelength
parameters (Sm), and shape parameters (Rsk) are classified as surface parameters and
hybrid parameters result from the combinations of the above-mentioned parameters.
The roughness average and waviness is presented in Figure 2-9.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 36
Figure 2-9 : Roughness average and waviness [82].
There are 6 types of instruments and associated methods used for measuring
roughness. Those are described below.
Type I, Profiling Contact Skidless Instruments.
(a) Skidless stylus-type with LVDT (linear variable differential
transformer) vertical transducers.
(b) Skidless type using an interferometric transducer.
(c) Skidless, stylus type using a capacitance transducer.
Type II, Profiling Non-contact Instruments.
(a) Interferometric microscope.
(b) Optical focus sending.
(c) Nomarski differential profiling.
(d) Laser triangulation.
(e) Scanning electron microscope (SEM) stereoscopy.
(f) Confocal optical microscope.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 37
Type III, Scanned Probe Microscope.
(a) Scanning tunnelling microscope (STM).
(b) Atomic force microscope (AFM).
Type IV, Profiling Contact Skidded Instruments.
(a) Skidded, stylus type with LVDT vertical measuring transducer.
(b) Fringe-field capacitance (FFC) transducer.
Type V, Skidded Instruments with Parameters Only
(a) Skidded, stylus type with piezoelectric measuring transducer
(b) Skidded, stylus type with moving coil measuring transducer.
Type VI, Area Averaging Methods
(a) Parallel plate capacitance (PPC) method.
(b) Total integrated scatter (TIS).
(c) Angle-resolved scatter (ARS)/bi-directional reflectance distribution
function.
2.6.3 Chemical characterization The chemical composition of a material can be characterized using different
techniques, namely: Energy dispersive X-ray spectroscopy (EDS); X-ray Photoelectron
Spectroscopy (XPS); Auger electron Spectroscopy (AES); Fourier Transformation
Infrared Spectroscopy (FTIR); X-ray Diffraction Spectroscopy (XRD); and Neutron
diffraction; Secondary Ion Mass Spectroscopy (SIMS). Each technique has unique
advantages and drawbacks.
EDS is a rapid qualitative method for the determination of elements in the imaged
region. XRD is usually used for phase recognition, crystal structure determination and
for estimating the stress inside the material. It is possible to analyse the stress within
coatings of less than 0.1 μm thickness using XRD. Transmission electron microscopy
(TEM) is a powerful technique by which imaging of individual atoms is possible. It can
study the crystalline structure of coatings. TEM is usually used to distinguish
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 38
octacalcium phosphate from HA and other forms of calcium phosphate. It can also be
used to study the interface layers between the coating and the substrate. The
preparation of the TEM specimen is time-consuming.
Raman spectroscopy is a useful tool to reveal phases, material quality, composition,
strain, effects of external perturbations (temperature, pressure and stress), and be
used for the determination of thermodynamic and polarization properties. The
disadvantage of this technique is its weak signal and the presence of fluorescence and
resonance.
The phases present inside calcium phosphate coatings can be measured using X-ray
Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR).
X-Rays are electromagnetic waves having no mass or magnetic dipole moment and
the photons of X-ray interact with electrons present in the inspection material. The X-
rays produced in laboratory scale instruments are created by the firing of electrons onto
a metal target, commonly copper or molybdenum. Two types of diffraction geometries
are used for the experiment; one is classic reflection geometry, which is called the
Bragg-Brentano geometry, and the other is classic transmission geometry, called
Debye-Scherrer geometry. Bragg-Brentano is suitable for strongly absorbing materials
and requires a flat sample surface. Brag-Brentano configuration and the significance of
a XRD graph is shown in Figure 2-10.
Figure 2-10: Bragg-Brentano configuration and significance of X-ray diffraction graph.
Data obtained from an X-ray diffraction experiment comprises the intensity of the
diffraction signal plotted against either the diffraction angle 2θ (o) or d (nm) spacing.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 39
The information obtained from the diffraction pattern is used for verifying the peak
position for d spacing or lattice parameters, the peak area for crystal structure, the
amount of phase in a mixture, and the peak shape for the size of the crystallite or
defects such as strain and atomic disorder.
XRD can be used for the following purposes:
1. To determine crystal structure
2. Analysis of the chemistry of the material
3. Measurement of stress
4. Study of phase equilibrium
5. Measurement of particle or grain size
6. Determination of the orientation of one crystal or the ensemble of orientations in
polycrystalline aggregate.
Many solid materials are composed of either crystalline or amorphous phases. The
presence of both phases is also common in materials, so it is important to determine
the crystallinity of the coating. Usually three methods are applied for the determination
of crystallinity; they are:
1. Rutland Method
2. Relative Intensity Method
3. Rietveld Method
The Rutland Method is the most used and accurate technique. In this method the total
area under the diffraction pattern is compared with the area under the amorphous
region of the diffraction pattern. The equation [45, 83] for determining percentage
crystallinity is:
( )
∑ ∑ ∑
2-5
Where ∑Ac = Sum of the areas under crystalline peaks, ∑Aa = Sum of the area under
amorphous halo region. In the case of the Relative Intensity Method, the intensity of the
maximum HA peak is compared for different XRD patterns.
The purity of the HA coating indicates the substance purity compared with crystalline
HA [45]. It can be calculated using the equation below:
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 40
∑ ∑ ∑
2-6
Where ∑Ac = Sum of the areas of all HA crystalline peak, ∑Ai = Sum of the areas of
impurity peaks. The tallest impurity peaks present in the coating needed to be
considered in the analysis. The amorphous region could be considered as an impurity
and thus incorporated in the equation.
Two methods can be used for the analysis of powder diffraction data, they are:
1. Rietveld method or full pattern analysis
2. Two stage method
The approach suitable for the powder diffraction pattern analysis is:
1. Collection of XRD data
2. Indexing of the collected data
3. Intensity determination
4. Using specific methods or suitable programs
The Rietveld analysis is a popular technique among crystallographers when performing
quantitative phase analysis. In the Rietveld method, the weight fraction is calculated
from the refined scale parameter. The equation [84] to calculate weight fraction is:
∑
2-7
Here S = Scale parameter, M = Mass of the unit cell, V = Volume of the unit cell, and
subscript i indicates ith crystalline component in the mixture. GSAS (General Structure
and Analysis System) is freeware [85] that can be used for the refinement of structural
models from XRD or neutron diffraction data. EXPGUI [86] is the graphical user
interphase of GSAS, and is also freeware. For the complete analysis, crystallographic
information files (CIFs) are necessary along with the software and X-ray diffraction
data. It is possible to find suitable CIF files from open or paid databases.
In Rietveld analysis two parameters are important to measure the accuracy of the
analysis. One parameter is the conventional least square residual R.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 41
∑ | ( )
( )| 2-8
Where ( ) and ( ) are the intensity observed and standard/calculated, respectively at
the jth step in the data, and is the weight [87]. Another parameter is reduced chi-
square (Χ2), which is the ratio between the estimated variance of the fit to the variance
of the parent distribution. It measures the deviations between the data and the mean of
the parent distribution that occur because there are less than an infinite number of
samples/observations. No literature has been found regarding the minimum acceptable
limit of R and Χ2 values. J. C. Knowles et al. have studied the crystallographic
parameters of sprayed powders and coatings using Rietveld analysis [88]. Their
research yielded crystallographic parameters with an R value of 17.56% and Χ2 value
of 13.29 for powder 1 and R value of 12.59% and Χ2 value of 7.265 for powder 2 [88].
Twenty-nine records of hydroxyapatite or hydroxylapatite have been found in the
American Mineralogist Crystal structure database [89]. All the data recorded ranges
from natural to synthetic hydroxyapatites having space group P6_3/m with slight
variations in unit cell parameters (a,b and c) [90]. The existence of monoclinic
hydroxyapatite was proved by Elliott et al., having the space group P21/b [91]. Yashima
et al. studied sintered β-TCP (BTCP) using a neutron diffraction method, which
revealed that β-TCP has a rhombohedral structure, having space group R3c, Z=21.
The unit cell parameters determined by the latter are a=b=10.4352 A0, c = 37.4029 A0,
α=β=900, γ = 1200 [92]. An X-ray diffraction graph of HA is presented in Figure 2-11.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 42
Figure 2-11: XRD of HA [12].
The data in Table 2-16 was collected from the PDF-2 database prepared by the
international centre for diffraction data (ICDD) [93].
Table 2-16 : Crystal data of HA and ATCP.
Name Lattice S.G. a (A0)
b (A0)
c (A0)
Mol. Weight
Hydroxyapaptite Hexagonal P6_3/m 9.418 - 6.884 502.32
Alpha Tri-calcium phosphate Monoclinic P21/a 12.86 9.11 15.23 310.18
The properties of amorphous calcium phosphate (ACP) have been discussed in detail
by Combes et al. [52]. Different forms of ACP’s, their synthesis routes, structures and
characterization are also discussed in the paper. Gross et al. performed a study on
amorphous phase formation in a plasma sprayed HA coating [94]. Three important
factors were identified that influence the formation of the amorphous phase in the
plasma spray process: these being (I) de-hydroxylation of molten particles during flight;
(II) cooling rate; and (III) surface temperature. Unmelted particle regions are more
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 43
crystalline and amorphous regions are more likely in the interface between the coating
and the substrate.
Limin Sun et al. studied the phase and crystallinity of plasma sprayed HA coating using
XRD [95]. Their study revealed that crystalliniy decreases with an increase in spray
power and stand-off distance. They have found that for 160 mm stand-off distance, the
crystallinity of coating decreased from 46% to 19% with an increase in power from
27.5 KW to 42 KW. And for 80 mm stand-off distance, crystallinity of coating decreased
from 88% to 77% for the same increase in power. Additionally the amorphous content
of the coating increases with an increase in stand-off distance. XRD analysis has
confirmed the presence of alpha TCP, BTCP and TTCP and CaO as well as that of HA
in the coating. They found that crystallinity of the coating surface was greater
compared to the crystallinity at the interface between the coating and substrate. FTIR
and NMR (Nuclear Magnetic Resonance) were also used by the latter researchers to
characterize the feedstock and coating.
Chou et al. characterized a zirconium dioxide reinforced HA coating on titanium alloy
using XRD. Besides HA and ZrO2, they have found α-TCP and CaO in the coating [16].
According to them, although HA was transformed to α-TCP and CaO, no phase change
occurred for ZrO2. They also performed TEM and EDS to analyse the structure, the
Ca/P ratio and the elemental distribution.
Khor et al. used XRD for the chemical characterization of the HA/YSZ/Ti-6Al-4V
composite powder (prepared by a slurry mixing method) and coating [96]. A small
amount of CaO and α-TCP was present in the powder, along with HA, YSZ and
titanium. The coating analysis also showed the presence of CaO and alpha TCP, but
no β-TCP or TTCP were found and this phenomenon was attributed to the different
degrees of decomposition of HA and the interdiffusion of HA and YSZ. No oxidation of
titanium occured during the process, which indicates that the composite powder (HA /
YSZ / Ti-6Al-4V) was properly shielded by HA and YSZ. The as-sprayed composite
coating revealed a lower crystallinity than the composite powder. Sintering increased
the crystallinity of the HA present in the composite coating. These researchers reported
that the crystallinity of HA increases up to 12 kW power and decreases with a further
increase in net energy. According to them, higher power with net energy of 12–14 kW
created high temperature at the plasma flame. And a shorter standoff distance of 7–8.5
cm created high-temperature gradient within the substrate and coating. Both
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 44
phenomenon accelerated the transformation of crystalline HA to amorphous calcium
phosphate; thus decreasing the relative crystallinity of HA.
Calcium phosphate coatings deposited by sputtering usually yield amorphous calcium
phosphate (Yang et al.) but annealing can increase the crystallinity of the coating [97].
A laser-treated sol gel derived apatite coating on grade 2 titanium has been chemically
characterized by Bini et al. [72] by using XRD and FTIR. Their research revealed
higher energy density lead towards the formation of more HA, the other hand, lower
energy density yielded a greater amount of β-TCP phase. Roy et al. deposited
commercial grade calcium phosphate powder on titanium using a Nd:YAG laser. XRD
analysis of the coated sample revealed TCP phase [71]. TCP in the coating was
crystalline in nature and depended on laser parameters but the titanate phase found on
the coating was independent of laser parameters.
Calcium phosphate grafts were produced using a rapid prototyping technique based on
laser ablation, and were chemically analysed by Comesana et al. [28] using XRD, FTIR
and Raman spectroscopy. Their XRD analysis concluded the generation of α-TCP and
TTCP along with HA. They also confirmed the presence of amorphous phase (40%
weight). Raman spectra (a peak at 962 cm-1 corresponds to the ν1 PO43- mode)
confirmed the presence of HA in the graft.
Raman and Krishnan described Raman scattering in 1928. It was found that when a
sample was irradiated by monochromatic light, some of the light scattered by
molecules was wavelength-shifted relative to the incident radiation, and this frequency
shift encodes information about the vibrational frequencies of the scattering molecules.
Raman spectroscopy is a quick tool employed to reveal both qualitative and
quantitative chemical information about the calcium phosphate powder or coating. It is
usually non-destructive and requires little or no sample preparation. Raman spectra
can be achieved for solids, liquids and gases. The barrier to achieving good Raman
spectra is fluorescence. Selection of a laser close to the IR spectra is vital to overcome
the problem. The other two technique used to overcome the problem of fluorescence
are photo bleaching and base-line correction [98].
A Raman spectroscope coupled with a microscope is called a Raman microscope. The
advantage of using a microscope is that a low-powered laser can be used since it can
be focussed on a small spot, giving high power density at the sample and allowing a
large collection angle. The small excitation volume can be efficiently imaged into a
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 45
small spectrometer and onto the detector. Raman microscopy has the advantage of
focussing scattered Raman radiation efficiently through the glass lenses and any
sample aperture optically can also have the Raman spectra recorded [99]. Raman
microscopes permits Raman spectroscopy with very high lateral spatial resolution,
minimal depth of-field and high laser energy density for a given laser power.
The schematic of a Raman microscope showing the laser source and the important
optics is presented in Figure 2-12.
Figure 2-12 : The schematic of a Raman microscope showing the laser source and
important optics associated with it.
In a Raman microscope, the excitation and scattering occurs at 1800. This mode is
called backscattering, and the angle between the actual excitation and the collection
direction is called the range of angle. The range of angle increases with the
magnification. Backscattered Raman collectors require optics that can act as a
Rayleigh filter and as a laser mirror. Holographic notch filters and dielectric mirrors are
used for this purpose.
The peaks in the Raman spectrum at a particular wavenumber are the characteristic
finger-print of a particular material or phase. The shift in frequency occurs because of
strain, while the width of the Raman peak is related to the quality of the crystal, and the
intensity is associated with the amount of material or phase present in the inspection
area.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 46
Raman spectral profile of HA crystal is dependent on angle ϴ, is presented in Figure
2-13. From the figure it is evident that the intensity and the peak shape changes with
the angle.
Raman spectroscopy provides molecular structural information. The Raman spectrum
of HA single crystals is dominated by a sharp peak at 962 cm–1, which corresponds to
the stretching mode (ν1) of phosphate groups, and three weak and broad bands
around 1070 cm–1 (ν3, PO4 3–stretching mode), 590 cm–1 (ν4, PO4 3–bending mode),
and 430 cm–1 (ν2, PO4 3–stretching mode) [9].
These Raman emissions were also found for other CaP bulk materials and inorganic
components of bone tissue, but with different relative line intensities [10, 11]. Broad
phosphate bands indicate the amorphous nature. TCP exhibits a peak at around
950cm-1 [100]. A shoulder at 952 cm-1 [101] indicates α-TCP. Near the surface of the
coating, the band of OH stretch vibration was detected by Jing Wen et al. [102]
Nevertheless, useful information on the structure of multiphase CaP coatings [11–13],
ranging from an amorphous CaP to a pure crystalline HA, can be obtained from careful
analyses of the dominant Raman signal in the range between 900 and 1000 cm–1. The
amorphous phase affects coating longevity and produces the peak broadening in
infrared wavelengths [100]. In the case of Raman spectroscopy, the amorphous phase
produces an individual peak at wave number 950 cm -1 [100].
H. Tsuda and J. Arends have performed a comprehensive study on hydroxyapatite
single crystals and human enamel crystallites using orientational micro-Raman
spectroscopy [103]. The Raman spectral profile studied by them is presented in Figure
2-13.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 47
Figure 2-13 : Raman spectral profile as a function of ϴ (O0 to 900) from 180 to 3600
cm-1 [103] of HA crystal (at 100X magnification and 40 mW power).
Their study shows that the intensity of Raman bands of a single crystal depends on the
orientation of c axis. They also assigned the observed Raman bands of those samples
to the Raman active symmetry tensors. Their observation of predicted and observed
bands of a HA single crystal is presented in Table 2-17.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 48
Table 2-17 : Raman active bands of in single crystals of HA [103].
Mode
Band Positions (cm-1) Fluorapatite Chlorapatite Hydroxyapatite
PO43- ν2 427 425 432
440 441 449
446 447 454
PO43- ν4 582 577 581
590 586 593
592 593 609
605 613 617
617 623 -
PO43- ν1 966 959 962
PO43- ν3 1034 1012 1028
1042 1032 1034
1054 1038 1043
1060 1055 1048
1082 1075 1055
- - 1077
The Raman spectrum of different CaP’s is shown in Figure 2-14. The FTIR spectrum of
HA along with a Raman spectrum is shown in Figure 2-15 for comparison. The FTIR
spectrum of HA is presented in Figure 2-16.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 49
Figure 2-14 : Raman spectra in the (A) ν 2 and ν 4 region, (B) ν 1 region and (C) ν 3
phosphate region. Shown spectra of each region were acquired from (a) commercial α-
TCP, (b) commercial TTCP, (c) HA precursor powder and from the cross-section of the
laser-processed samples: (d) irregular grains, (e) matrix, (f) elongated grains [28].
Figure 2-15 : (a) FTIR reflection spectra of the sample surface and the HA precursor
powder; (b) representative Raman spectra of the surface, the HA precursor powder
and the precipitated HA after 7 days in cell culture [28]
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 50
Figure 2-16: The FTIR spectrum of HA [12].
V. Guipont et al. adopted FTIR along with XRD for the determination of amorphous and
other phases [18].
Three techniques have been used for chemical characterization of feedstock and the
coating. They are XRD, Raman microscopy and EDS. XRD has been utilized in
qualitative chemical analysis of the feedstock and the coating. Raman spectroscopy is
used to analyse the feedstock material and different types of CaP. EDS is used to
determine the chemical elements and their distribution in the coating.
2.6.4 Diffusion Diffusion is a phenomenon of material transport by atomic motion. For coatings
generated using laser and treatment of materials, diffusion plays an important role.
Diffusion is the stepwise migration of atoms from lattice site to lattice site [104].
Vacancy diffusion and interstitial diffusion are the two mechanisms for metallic
diffusion. The interchange of an atom from a normal lattice position to an adjacent
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 51
vacant lattice site is called vacancy diffusion. On the other hand, atoms that migrate
from an interstitial position to a neighbouring empty position are classified as
undergoing interstitial diffusion and arises for the inter-diffusion of atoms that are small
enough to fit into interstitial positions.
Two types of diffusion process are evident; one is steady-state diffusion and the other
non-steady state diffusion. For steady-state, the diffusion flux does not change with
time and is related to the concentration gradient. The slope of a concentration profile is
called the concentration gradient. The equation for concentration gradient is:
2-9
Here C is the concentration and x is the position or distance. The concentration
gradient is obtained by plotting concentration of diffusion species versus position.
The steady state diffusion is expressed by Fick’s first law. The law describes:
2-10
Where J is diffusion flux (kg/m2.s), D is diffusion coefficient, C is the concentration and
x is the position or distance.
Practically, diffusion is non-steady state in nature, which means the concentration
varies with time. This relation is presented by Fick’s second law:
2-11
Here D is the diffusion coefficient, C is the concentration, x is the position or distance
and t is time.
The solution of Fick’s second law considering surface concentration as constant for
semi-infinite solid yields:
(
√ )
2-12
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 52
In the above equation Cx represents the concentration at depth x after time t, Co is the
concentration before diffusion, Cs is the concentration of solute after time t and D is the
diffusion coefficient.
2.6.5 Mechanical characterization Mechanical properties of a coating are crucial for the reliability and performance of an
implant. Important mechanical properties are: elastic modulus; yield strength; ultimate
tensile strength; ductility; hardness; residual stress; coating fracture toughness; and,
interfacial fracture toughness (or adhesion energy). The mechanical properties of
calcium phosphate, bone and other ceramics have been obtained from “Biomaterials”,
(ed. by Joon Park and R. S. Lakes) and from “Biomaterials artificial organs and tissue
engineering” (ed. by Larry L. Hench and Julian R. Jones), and are compiled in the table
Table 2-18 [51, 105-107].
Table 2-18 : Mechanical properties of some important biomaterials, which includes
bone, HA, SS and Ti-6Al-4V.
Material Elastic Modulus
(GPa)
Fracture toughness (MPa√m)
Elongation (%)
Tensile strength
(MPa)
Fatigue strength
(MPa)
Poly crystalline HA 40-117 1 - 40-100 -
ACP
Compact Bone 12-18 2-12 - 50-150 -
Austenitic stainless
steel
200 100 40 200-110 200-100
Ti-6Al-4V 105-110 80 12 750-105 350-650
Cobalt Chromium 230 100 10-30 450-100 200-250
Glass fibre 70 1-4 2 200 -
A tensile test of a material can reveal properties such as elastic modulus (E), yield
strength (σyield), ultimate tensile strength (σuts), ductility (%) and toughness (J/m3). For
thick coatings, sometimes it is possible to detach the coating from the substrate and
measure the properties; however for thin coatings, this method is difficult and
impractical. Adhesion is another important property of a coating. It can be measured by
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 53
methods such as pressure sensitive tape tests; indentation tests; scratch tests;
acoustic imaging; acceleration tests and, tensile and shear tests.
Nano-indentation is a versatile test that can determine a wide variety of mechanical
properties, for example, elastic modulus, hardness, fracture toughness and residual
stress. Usually nano-indentation has been performed using a diamond Berkovich
indenter. The determination of elastic modulus by a nano-indentation method is based
upon the analysis of the unloading curve. All materials exhibit either plastic or elastic
deformation during loading. The initial slope of the unloading curve is directly related to
the elastic modulus. The formula used [108] for the determination of the reduced elastic
modulus from the unloading curve is:
√
√
2-13
Here Smax is the slope of the unloading curve and A is the projected area of contact
between the indenter and the material at that point.
The impression profile after nano-indentation is presented in Figure 2-17. Micro-
indentation is similar to nano-indentation and can be employed to determine similar
properties.
Figure 2-17 : The impression of surface profile after nano-indentation [108].
The Hertz solution can be used in the case of spherical indentation during which no
plastic deformation or cracking occurs and it is valid when the ratio between contact
radius and indenter radius R is less than 0.3 [108]. The solution is:
⁄
⁄ 2-14
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 54
Here P is load, Er is the reduced elastic modulus, R is the indenter radius and h is
displacement. Then elastic modulus can be determined using the relation given below
[106, 108]:
2-15
Here Ei and E are the elastic modulus of the indenter and indented material, νi and ν
are the poisson’s ratio of the indenter and indented material, respectively. For
Berkovich or Vickers indenters, the load P can be determined using the relation below:
[( √ )√ ⁄⁄ √ ⁄ √ ⁄ ]
2-16
Leigh et al. studied the elastic response behaviour of a thermal spray deposit using the
Knoop indentation technique [109]. They determined the ratio of hardness to elastic
modulus with an aim to determining fracture toughness. Kweh et al. used the same
relation [96, 109, 110] for the determination of elastic modulus by Knoop micro-
indentations and the relation is:
2-17
Where a and b are major and minor diagonals (Figure 2-18) of the Knoop indenter, a’
and b’ are the reduced major and minor diagonal length, respectively, after elastic
recovery. They considered α as 0.45 for plasma-sprayed HA coatings.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 55
Figure 2-18 : Knoop indentation and the elastic plastic zone after indentation [109].
If sufficient load is applied to a material, then localized plastic deformation occurs and
hardness is the measure of a material’s resistance to that plastic deformation. The
hardness test has importance due to its simplicity, low cost, non-destructive nature
except for a small indentation on the sample and its relationship with other important
mechanical properties, for example tensile strength.
Hardness is measured by performing an indentation at a certain load. The surface of a
material is optically examined after removing the load to determine the area of the
plastically deformed residual imprint [108]. The ratio of the maximum indentation load
and the measured area indicates the hardness of the material. In the case of nano-
indentation, the area is actually the projected contact area at maximum load, and may
not be equal to the area of the final residual imprint. The relation between the hardness
(H), load (Pmax) and the area (A) is [108]:
2-18
Hardness, instead of being a fundamental property of a material, is related to yield
strength, and this relation depends upon the geometry of the indenter. For metals, the
relationship is [108]:
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 56
2-19
In the above equation, H is hardness and Y is yield strength.
Materials contain cracks that are related to fracture process. Fracture toughness is the
property of a material that enables it to resist fracture, often denoted by KIc with
engineering dimensions of MPa√m. It is a quantitative way to express a material’s
resistance to brittle fracture. The higher value of fracture toughness indicates that the
material has the ability to absorb energy and plastically deform before failure.
In the case of brittle material, if the indentation load is sufficiently large, then cracking
arises at the time of indentation. Cracks might be radial/median, palmqvist or lateral in
nature. For sharp pyramidal indenters, radial/median cracks originate from the edges of
the indenter tip. Lateral cracks could initiate under the indenter tip at the edge of the
plastically deformed zone [108]. The initiation and subsequent growth of the cracks is
determined by the elastic and plastic properties of the indented material, and also by
the fracture toughness KIc of the material, as well as the residual stress present in the
indented surface [108]. So it is possible to estimate fracture toughness and residual
stresses from crack features. Median cracking for monolithic materials has been
analysed by Lawn and Evans [111] using a Vickers indenter. The formula derived by
them can correlate between critical load for the crack initiation P*, fracture toughness
(KIC) and hardness (H):
2-20
Kweh et al. [110] measured fracture toughness using the relationship presented below,
which is related to radial/median cracks.
√
(
⁄
⁄ )
2-21
Where KIC is fracture toughness, E is modulus of elasticity, P is load, Hv is Vickers
hardness and c is the crack length.
A study has been carried out by C. B. Ponton and R. D. Rawlings for the determination
of fracture toughness using Vickers hardness testing [112]. The equations devised by
different researchers are compiled in the aforementioned paper.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 57
Some researchers have considered radial/median cracks to investigate toughness and
others have investigated the palmqvist crack made. Aliasghar Behnamghader et al.
studied the cracking behaviour of dense hydroxyapatite using Vickers micro-indentation
within a load range of 25 gf to 2000 gf [113]. They calculated and experimentally
verified the palmqvist to radial/median crack transition load for compact HA. The
calculated transition load is 170 gf; however, the experimental transition load range laid
between 150 gf and 200 gf.
The non-linear relationship between stress and strain in a stress-strain curve is better
described by the Ramberg-Osgood equation. In this equation, the first part ( ) is related
to elastic behaviour while and the second part ( ) is related to plasticity. The
Ramberg–Osgood equation can describe the elastic/plastic deformation of HA coatings
on a metal substrate. According to C. Zhang et al., the relationship [114] is given below
where the value of K is 3.125 X 10-17 Pa-2 and E = 70.8 X 109 Pa for 10 wt%
crystallinity. This relationship is estimated by an indentation test and finite element
modelling.
(
)
2-22
Saeed Saber-Samandari and Karlis A. Gross performed a study on micromechanical
properties of a single HA crystal using the nano-indentation method [115], by which
they measured hardness, elastic modulus and fracture toughness of a single HA
crystal, both at the side and at the base. Another study carried out by them reveals that
hardness and elastic modulus decreases as the powder size increase [116]. They
found that smaller particles led to higher hardness and elastic modulus, compared to
powder containing larger particles [117]. The effect of indentation at different spray
angles (10, 20, 30, 40, and 50) on mechanical properties has also been studied by
them in another paper [118]. Saeed Saber-Samandari, Christopher C. Berndt and
Karlis A. Gross carried out research on uniformly sprayed single HA splat [119]. Their
study revealed that splats deposited on different metals (pure Ti, Ti-6Al-4V, Co-Cr alloy
and stainless steel) have no effect on the values of hardness and elastic modulus as
determined by using a nano-indentation method.
Research carried out by Kweh et al. also reveals that increasing particle size and
standoff distance has a negative impact on mechanical properties such as micro
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 58
hardness, modulus, fracture toughness, and bond strength [120]. This ultimately affects
the structural stability of the coating. According to Kweh et al., 800 0C is the optimum
temperature for heat treatment of plasma-sprayed HA coatings in air. C. C. Berndt and
C. K. Lin published a review paper where they discussed the methods used to measure
the adhesion of coatings or deposits formed by thermal spraying [121]. Thermal spray
coatings are anisotropic in nature and the adhesion of the coating depends on
interfaces of different lamella, the integrity of interfaces between the coating and
substrate, residual stress, crack population, pore size and distribution. The authors
discussed the approach of fracture mechanics for evaluating the coating adhesion. An
experimental relationship was established between elastic energy provided by the
external force and the propagation of a stable crack. This approach considers the
energy required to initiate or propagate cracks, and signifies coating adhesion in terms
of a stress-intensity factor K or strain-energy release rate G (J/m2).
( )
2-23
Here W is the work done by the external forces (J), U is the elastic energy stored in the
system (J), A is the crack area (m2), and the relation between G and fracture toughness
(K) is
√
2-24
Crack propagation occurs when G exceeds a critical value Gc and the coating
ultimately fails. G can also be expressed as the relation of force F (N) required to
extend a crack, crack length L (m), thickness B (m) and compliance C (m/N).
2-25
The compliance values are determined from the displacement at the loading points and
dC/dL can be obtained from an experimentally determined calibration curve. The
above-mentioned paper also discussed four-point bending methods; double cantilever
beam tests; a double torsion test; indentation techniques; and an acoustic emission
approach.
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 59
J. L. Arias et al. carried out micro scratch testing to evaluate coating substrate
adhesion and nano-indentation to determine the hardness and elastic modulus of
calcium phosphate coatings deposited by PLD technique [33]. According to this
research, a PLD-deposited crystalline HA coating is better in terms of internal
cohesion, than an amorphous coating, but the amorphous coatings exhibit a lower
elastic modulus, which is mechanically compatible with natural bone. For their
experiment, Arias et al. used a sintered carbonated HA target and the vacuum
chamber was filled with water vapour, having a vapour pressure of 45 Pa.
Garcia-Sanz et al. compared plasma spray (PS) and pulsed laser-deposited (PLD) HA
apatite coatings [122]. They compared the structural, morphological and mechanical
strength of HA coatings.. The tensile strength of the deposited material was determined
by a pull test. The XRD spectra of two types of coatings were of good quality. For PS,
the splat size was about 50 μm, pore size 10 to 20 µm and the cracks were about 1µm
thick along the coated surface. The cracks were attributed to the thermal shock that
occurred due to the large temperature difference between the molten splats and the
low substrate temperature. Conversely, a PLD-deposited HA coating was smooth and
the coating was formed by a network of superimposed fine polygonal HA crystallites
varying in size from 100 to 500 nm [122]. For the PLD coatings, tensile strength values
were higher than 58 MPa, on titanium substrates that were not grit-blasted. Finally,
Garcia-Sanz et al. concluded that PLD can produce well-adhered thin and
homogeneous coatings without the brittleness of those produced by PS [122].
2.6.6 Microstructural characterization The microstructure of a ceramic depends on the initial fabrication techniques, raw
materials used, phase change, chemical reactions and grain growth during the high
temperature processing. Characterization of a microstructure using image analysis
reveals structural characteristics. G. Montavon et al. carried out image analysis by
means of a metallographic index, which is based on the several stereological and
morphological parameters related to the size-shape distribution of the features, fractal
dimension of the upper surface of the deposit and the Euclidean distance map [123].
Although the above-mentioned work is not related to calcium phosphate deposition,
this method could be applied to the general case of microstructural quantification.
While some researchers used transmission electron microscopy (TEM) to reveal the
microstructure of the CaP coatings, most researchers used either optical microscopy or
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 60
scanning electron microscopy (SEM). Chang et al. inspected surface morphologies and
cross sectional microstructures of plasma-sprayed zirconia-reinforced hydroxyapatite
composite coatings on titanium substrate [124].
The cross-sectional microstructure and surface morphology of a hydroxyapatite coating
deposited on titanium substrate was studied by Demnati et al. [125]. The coatings
showed a typical lamellar structure with isolated volume defects (large pores) with no
cracks and a small amount of partially melted particles. Their observation concluded
that the porosity was due to small inter-lamellar voids. Balbinotti et al. studied the
microstructure of the powder metallurgy derived bio-composite produced from HA and
Ti [126]. They have found the nanometric HA composite produced better results in
terms of microstructural and mechanical strength (compressive strength) compared to
micrometric HA composites produced by powder metallurgy. The compressive strength
of nanometric HA was found to be 40% higher than the micrometric HA. All of the
above researchers used SEM for capturing micrographs, while Ji et al., besides using
SEM, also used TEM to characterize the microstructure [127]. They have observed
crystal grain structures along with diffraction patterns to find
Another important factor is the degree of clustering of the pores that has a direct
influence on mechanical and thermal properties such as tensile strength and thermal
conductivity.
Z. Wang et al. measured porosity volume fraction by using an image analysis
thresholding and precision density method [128].
I. Sevostianov et al. [129] detailed the quantitative characterization of microstructures
of plasma sprayed coatings for analysing their conductive and elastic properties.
Porosity or the volume fraction of pores is not sufficient to describe physical properties.
The orientation of pores could be parallel or normal to the substrate and could have
spherical or irregular geometries.
Spherical pores could be characterized by the relative volume of pores contained in
volume V:
∑ ( )
2-26
Where V(k) represents the volume of individual pores.
Circular cracks are characterized by crack density parameter
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 61
∑ ( )
2-27
Where l(k) is the radius of k-th microcrack.
For anisotropic cases crack orientations are non-random, and the parameter used to
describe them is a crack density tensor:
∑( )( )
2-28
Practically, plasma sprayed coatings can have horizontal cracks that are parallel to the
coating plane and vertical cracks that are normal to the plane. Families of both of these
cracks exhibit 3-D scatter. The distribution contains a scatter parameter, λ, that is
expressed as:
( )
[( )
]
2-29
The scatter parameter λ is zero (λ = 0) for fully random orientation of cracks and the
parameter is infinitive (λ = ∞) for ideally parallel orientations. For transverse isotropy,
the crack density tensor reduced to α11 = α22 and α33.
( ) ( )
2-30
( ) ( )
2-31
f1 and f2 are functions of the scatter parameter in the above equation.
( )
( )
2-32
( )(
)
( )
2-33
In this paper, cracks and pores have been categorized depending upon aspect ratio.
Randomly oriented moderate non-spherical pores (aspect ratio 0.7 – 1.5) can be
replaced by the spherical ones. Strongly non-spherical pores that have an aspect ratio
greater than 1.5 and that are not strongly oblate are not able to be characterized by
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 62
porosity. They are also functions of the average eccentricity. There are various
irregularities in the microstructure; for example:
1. Jagged boundaries of cracks,
2. Spherical pores,
3. Oblate pores
4. Pores with complex shapes.
Leigh and Berndt [130] performed modelling of elastic constants of plasma spray
deposits, considering ellipsoid shape voids. Splats (flattened particles) or voids can be
approximated to spheroids and void shapes are considered as oblate ellipsoids.
Leigh and Berndt also published another article on the quantitative evaluation of void
distributions in plasma-sprayed ceramic [131]. In this article, they used a stereological
analysis to extract quantitative information about voids in terms of shape and size. The
shape factor indicates the eccentricity of a spheroid and the relationship is given below.
(
)
2-34
A shape factor close to zero indicates a spheroidal or circular shape and a value close
to 1 indicates the shape of an ellipse or ellipsoid.
Jadhav et al. [132] developed an analytical model for studying the thermal conductivity
of solution precursor plasma spray (SPPS), deposited thermal barrier coatings
(ZrO2 – 7 wt% Y2O3) and layered SPPS. They compared the theoretical value obtained
from the analytical model with the experimental results. They also performed
simulations using object-oriented finite element (OOF2) software developed by NIST
and compared the results with the experimental data.
The thickness of the coating has a strong influence on the mechanical as well as the
chemical properties of the coating. Shear strength decreases with the increase in
coating thickness but shear stress increases with the increase of coating thickness
[133-135]. A thickness of coating from 50 to 100 μm can be considered as optimum
taking into account of shear strength, stress and fatigue failures [136]. Another point is
the increase of coating thickness up to 100 μm promotes bone ingrowth; this is why 50
Chapter 2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES
Muhammad Rakib Mansur Page 63
to 75 μm for orthopaedic implants and 100 μm for dental implants are considered
optimum [136].
2.7 Conclusion The literature review consists mainly of five sections. In the first section, different types
of CaP are discussed. The section contains a discussion concerning the structure of
HA and a comparison has been made in terms of calcium-to-phosphorous ratio
between different types of CaP. The thermal change of CaP with different temperatures
is also discussed in this section.
Ti-6Al-4V is treated in the next section. The crystal structure, diffusion coefficient and
physical properties of titanium are discussed in the second section. The third section is
dedicated to stainless steel substrates. The crystallographic properties, along with
stainless steel’s mechanical properties are discussed.
The fourth section is dedicated to different deposition techniques that have been
adopted for the deposition of pure and composite coatings made from HA. Of all the
techniques included in the review, plasma spraying and laser-assisted deposition
techniques have been discussed in the most detail because these techniques are the
most popular.
Characterization techniques are discussed in the fifth section, which contains a
discussion of feedstock characterization, as well as the chemical, mechanical, surface
and microstructural characterization of coating. Diffusion is also discussed in this
section because, for laser-assisted deposition, diffusion plays an important role
compared to plasma spray deposition.
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 64
3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 65
3.1 Introduction It is important to investigate in depth the microstructure, morphology and chemical
composition of the feedstock material. The knowledge regarding feedstock material can
pave the way for better understanding of the deposited coating. In particular, knowing
the chemical composition and phases present in the powder is important to interpret
phases that are likely to evolve in the coating. In our study, SEM was used to reveal
the microstructure and morphology of the feedstock. ImageJ, (a public domain java-
based image-processing software) software was used to perform image analysis of the
SEM micrographs.
Raman microscopy and XRD were used to identify the chemical phases present in the
powder. Besides the feedstock, three forms of CaP (ACP, ATCP, BTCP) were
analysed because these phases were expected to evolve as a result of the high
temperature processing. Study of the chemistry of these phases would assist in
understanding the product more comprehensively. In this chapter, analyses of the
microstructure, morphology and chemistry of the feedstock material have been
provided to allow correlation to the properties of the coating.
3.2 Microstructure and morphology Three types of powders were used for the experiment. Two HA powders were procured
from two separate sources. One HA was procured from Sigma Aldrich (based in St.
Louis, MO, USA). HA procured from Sigma Aldrich exhibited a molecular weight of
502.31 g/mol. The SEM (Figure 3-1) reveals the morphology of the powder. The figure
indicates that the powder is partially agglomerated and is mostly spherical and
elliptical. The average maximum feret diameter was 73 µm with an aspect ratio 1.502
and a roundness 0.697.
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 66
Figure 3-1 : Scanning electron microscopic (SEM) image of HA powder from Sigma
Aldrich.
The other type of HA (Figure 3-2a) was procured from Plasma biotal, based in
Tideswell, UK. The trade name of HA powder supplied by plasma biotal is Captal 60,
which has a mean particle size of 60 µm and a tolerance of +10 µm according to data
supplied by the manufacturer. This powder is not agglomerated would be expected to
exhibit good flow ability.
The third powder is Ti-6Al-4V (Figure 3-2b), procured from TLS Technik GmBH & Co.
Spezialpulver KG- headquartered in Bitterfeld, Germany. This company manufacturers
gas-atomized, fine, spherical and high-purity metal powders. The Ti-6Al-4V powder
supplied by TLS Technik is spherical in shape and is 75 µm (mean diameter, data
supplied by the manufacturer) in size.
The HA powder procured from Sigma Aldrich was used for deposition of HA and Ti-6Al-
4V on SS using the CO2 laser-assisted DMD technique. The powder procured from
Plasma Biotal was used for the Nd:YAG laser-assisted deposition of composite
coating. Ti-6Al-4V from TLS Technik was used in both cases to produce a composite
coating.
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 67
Figure 3-2 : Scanning electron micrograph (SEM) of (a) HA powder (Plasma biotal).
and (b) Ti-6Al-4V powder
SEM images of the HA and Ti-6Al-4V feedstock’s are presented in Figure 3-2. The
feret diameter is the distance between two parallel tangents touching the particle at an
arbitrary angle. The feret diameter of the HA particles was 116 µm and the circularity
was 0.743. The average feret diameter of the Ti-6Al-4V is 73 µm.
The microstructural information regarding the particle size and shape were collected
using an image-analysis technique. The information collected after image analysis
using imageJ [137, 138] software is presented in Table 3-1.
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 68
Table 3-1 : Microstructural parameters collected after image analysis of powder
samples.
Avg Max
Feret Ø (µm)
Avg Min
Feret Ø (µm)
Avg Major
Ø (µm)
Avg Minor
Ø (µm)
Avg Aspect Ratio
Avg Circularity
Avg Roundness
HA Sigma Aldrich 72.656 49.366 65.601 45.155 1.502 0.696 0.697
HA Plasma Biotal 116.42 73.998 106.83 68.669 1.581 0.743 0.685
Ti-6Al-4V 72.927 58.838 68.252 57.216 1.26 0.814 0.832
Histograms have been plotted for better representation of the frequency distribution of
the parameters for all three powders. The histograms are shown in Appendix (Figure
11-1, Figure 11-2 and Figure 11-3).
3.3 Chemical analysis The chemical analysis of feedstock powder was performed using three techniques.
They were EDS, Raman microscopy and XRD. EDS was carried out to analyse the
chemical elements present in the powder. Raman microscopy and XRD were used to
analyse the chemical phases present in the powder. The chemical elemental
composition obtained from EDS was used to calculate the Ca/P ratio. The Ca/P ratio
yielded for HA (Sigma Aldrich) was 1.47.
3.3.1 Raman microscopy of feedstock Raman microscopy of the feedstock was carried out using a Horiba Jobin-Yvon
Modular Raman Microscope. HA from Sigma Aldrich and Plasma Biotal were inspected
using the Raman microscope. The laser used for irradiating the sample had a
wavelength of 514 nm and a 2400 g/mm diffraction grating was used for optimum
resolution. These devices were operated in both standard and confocal modes. The
power used for the experiment was 0.305 mW (which is 5% of the 6.1 mW power
available).
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 69
Figure 3-3 : Raman spectroscopic image of the HA procured from Sigma Aldrich
showing the presence of crystalline HA.
The Raman spectrum of the Sigma Aldrich feedstock exhibited a sharp peak at a wave
number 962 cm-1 (Figure 3-3). This corresponds to the ν1 mode of PO43- and indicates
the presence of crystalline HA [1, 28]. The peak could shift due to effects of strain.
The Raman spectrum of the Plasma Biotal feedstock also exhibits a sharp peak at
wave number 961 cm-1 (Figure 3-4), which corresponds to the ν1 mode of PO43- and
indicates the presence of crystalline HA [1, 28].
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 70
Figure 3-4 : Comparison between the Raman spectrum of HA and ACP (at standard
mode) acquired from Plasma Biotal.
As well as HA, three other forms of CaP powder were studied using Raman
microscopy. The powders included amorphous calcium phosphate (ACP), alpha tri-
calcium phosphate (ATCP), beta tri-calcium phosphate (BTCP). These powders were
collected from Plasma Biotal and used in a comparative study. In Figure 3-4 HA is
compared with ACP. HA has more distinctive peaks than ACP. For HA the intensity of
the peak at 961 cm-1 wavenumber is greater and the spectrum more narrower,
compared to ACP. For ACP, the peaks other than wavenumber 961 cm-1 are not very
prominent.
BTCP results possess a doublet at 948 cm-1 and 970 cm-1, which is a more distinctive
range than other forms of CaP. The peaks observed in different forms of CaP are listed
in Table 3-2 and presented in Figure 3-5.
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 71
Table 3-2 : Peaks observed under Raman microscopy for different types of CaP
collected from Plasma Biotal.
CaP type Peaks observed at wavenumber (cm-1)
HA 331, 430, 447, 579, 591, 607, 614, 961, 1027, 1046,
1075, 1121
ACP 428, 578, 590, 961, 1046, 1074, 1121
ATCP 453, 624, 968, 1122
BTCP 406, 610, 948, 969
Figure 3-5 : Raman spectrum of different types of calcium phosphates (ACP, BTCP,
ATCP and HA).
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 72
The Raman micrograph of HA procured from Plasma Biotal illustrates little difference in
terms of peak positions for the standard and confocal modes. The Raman microscopy
graph of HA powder in confocal and standard mode is presented in Figure 3-6. The
intensity of each peak is higher for the standard mode, so all peaks are more distinctive
in this mode. The confocal mode graph is closer to the baseline compared to the
standard mode; however the differences between the two lines are of no consequence.
Figure 3-6 : Raman micrograph of HA powder showing the effect of standard mode and
confocal mode.
Other forms of calcium phosphate were also studied to observe the effect of the
standard and confocal modes on the output spectrum. From Figure 3-7 it is evident that
the confocal mode has minimized the fluorescent effect by sacrificing the intensity. The
ACP and ATCP (Figure 3-7b and Figure 3-7c) exhibits a significant amount of
fluorescence, compared to HA and BTCP (Figure 3-7a and Figure 3-7d). A particular
phase can be identified more conclusively by comparison of spectrums that have been
obtained by employing both modes.
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 73
Figure 3-7 : Different forms of CaP studied under standard mode and confocal mode a)
HA, b) ACP, c) ATCP, d) BTCP
3.3.2 X-Ray Diffraction (XRD) analysis of feedstock
The XRD data acquisition was carried out using a Bruker (based in Billerica,
Massachusetts, USA) D8 Advance XRD machine. The software associated with the
device is named DIFFRAC plus XRD commander. The operating voltages used were
40 kV and at a current of 40 mA. Two theta ranges set for the observations were from
100 to 900. The feedstock samples observed using XRD were the same powders that
had been characterized using Raman microscopy and which had also been used for
deposition. They were Ti-6Al-4V powder, procured from TLS Technik, HA Powders
procured from Sigma Aldrich and Plasma Biotal. The other forms of CaP (ATCP and
BTCP) procured from Plasma Biotal were also tested for a comparative study.
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 74
Qualitative analysis was performed by comparing the peaks observed in the XRD
experiment with a standard data base (PDF-2) [93] and with data available from the
literature. An additional two important open-source databases have been compared in
this regard; i.e.,
1. Crystallography open database (COD) [139]
2. The American Mineralogist Crystal Structure database [90]
The XRD data indicates 8 prominent peaks for Ti-6Al-4V that comply with a study done
by Wen et al. [140]. The XRD (Figure 3-8) also imitates the XRD pattern presented by
Ahmet Hascalik and Ulas Caydas [141].
Figure 3-8 : XRD graph for Ti-6Al-4V powder.
The peaks observed in the XRD profile of Ti-6Al-4V (Figure 3-8) and SS 316L (Figure
3-9) are listed in Table 3-3. The position of the peaks for SS AISI 316L agrees with the
XRD observation of the 316L substrate, perfomred by Chakraborty et al. [142].
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 75
Table 3-3 : Prominent peak positions for Ti-6Al-4V indicated in Figure 3-8 and
substrate SS 316L indicated in Figure 3-9.
Material Peaks observed at Two theta (2ϴ0)
Ti-6Al-4V 34.95, 38.074, 39.93, 53.07, 63.18, 70.793, 76.45,
82.422
SS 316L 44.017, 51.13, 74.95
Figure 3-9 : XRD profile for SS AISI 316L substrate.
The data collected for HA samples were smoothed and indexed using Origin-Pro 9.0
software [143] for better identification of peaks. The Savitzky-Golay method was used
for smoothing, with 10 window points and a polynomial of order 2. The optimized
condition yielded better XRD profiles with the least sacrifice in terms of intensity and
peak position. The XRD profile of HA is presented in Figure 3-10.
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 76
Figure 3-10 : The XRD profile of HA (Sigma Aldrich).
The peak positions are listed in Table 3-4 for XRD data acquired for the HA powders
collected from Plasma Biotal and Sigma Aldrich. The highlighted peaks in the table
indicate the most intense and prominent peaks.
Table 3-4 : Peaks observed in the XRD profile for two types of HA feedstock.
Material Peaks observed at Two theta (2ϴ0)
HA PB (HA procured
from Plasma Biotal)
21.801, 22.88, 25.868, 28.096, 28.92, 31.78, 32.16, 32.93, 34.07, 35.41, 39.16, 39.794, 42.022, 43.865, 45.33, 46.66,
48.06, 48.57, 49.46, 50.48, 51.24, 52.07, 53.15, 54.51,
55.87, 57.15, 57.98, 59.94, 61.673, 63.107, 64.089, 65.066,
66.274, 69.81, 71.56, 73.89, 75.55, 77.05, 78.341, 81.66,
83.32, 84.3, 85.42, 87.24, 88.44
HA SA (HA procured
from Sigma Aldrich)
21.43, 22.52, 25.49, 26.04, 27.80, 28.52, 29.86, 31.43,
32.53, 33.67, 39.43, 41.501, 43.502, 45. 019, 46.35, 47.74,
49.14, 50.112, 50.84, 51.749, 52.77, 54.30, 55.572, 56.911,
57.803, 59.736, 61.298, 62.566, 63.753, 66.136, 68.662,
69.707, 71.863, 72.755, 73.795, 75.357, 76.696, 77.81
20 25 30 35 40 45 50 55 60 65 70 75 80 85 900
20
40
60
80
100
Inte
nsity
(Cou
nts)
Two theta (2)
HA SA
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 77
Figure 3-11 : XRD pattern of HA (procured from Plasma Biotal).
The XRD of both feedstock’s indicate the presence of HA; but the intensity of the peaks
for Sigma Aldrich (Figure 3-10) HA are relatively lower than those for Plasma Biotal HA
(Figure 3-11). The XRD peaks for HA SA are also broader compared to HA PB. The
broader peaks indicate two phenomena, one is reduction of crystallite size [144]; and
the other is the presence of small amounts of ATCP and BTCP. The presence of
ATCP, BTCP is not unlikely for HA SA. HA, ATCP and BTCP share some common
and close peak positions.
ATCP and BTCP procured from Plasma Biotal were studied using XRD and compared
with HA. The peak positions observed for HA, ATCP and BTCP were collected from the
PDF-2 [93] database and organized in a tabular fashion (Table 3-5). The bold peak
positions in the table indicate the intense prominent peaks. The peaks observed for HA
SA and HA PB matched with the PDF-2 database in terms of position. The associated
pattern number in the PDF-2 database is 00 - 009 - 0432 for HA, 00 – 006 – 0200 for
ATCP and 00 – 003 – 0691 for BTCP.
20 25 30 35 40 45 50 55 60 65 70 75 80 85 900
50
100
150
200
250
300
Inte
nsity
(Cou
nts)
Two theta (2)
HA PB
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 78
Table 3-5 : Peak positions for HA, ATCP and BTCP collected from PDF-2 database
[93].
Material Peaks at Two theta (2ϴ0)
HA 21.820, 22.902, 25.354, 25.879, 28.127, 28.967, 31.774,
32.197, 32.902, 34.049, 35.481, 39.205, 39.819, 40.453,
42.030, 42.319, 43.805, 44.370, 45.306, 46.713, 48.104,
48.624, 49.469, 50.494, 51.285, 52.102, 53.145, 54.442,
55.881, 57.129, 58.075, 59.940, 60.459, 61.662, 63.013,
63.444, 64.080, 65.033, 66.388, 66.388, 69.701, 71.653,
72.288, 73.997, 75.025, 75.586, 76.156, 77.177, 78.230
ATCP 22.902, 24.367, 26.914, 28.127, 30.808, 34.605, 36.650,
38.101, 40.606, 41.989, 43.894, 44.974, 47.150, 47.437,
49.440, 50.584, 52.358, 54.198, 55.441, 57.637, 60.112,
61.571, 62.917
BTCP 21.929, 26.189, 28.037, 30.917, 32.173, 33.153, 34.467,
35.598, 37.442, 39.856, 41.385, 43.917, 44.833, 47.046,
48.376, 50.079, 51.285, 53.211, 54.582, 56.403, 57.955,
59.599, 61.345, 63.687, 64.678, 66.229, 67.861, 68.999,
71.403, 73.330, 76.084, 78.306, 81.506, 84.107, 86.907,
88.898
The comparative experimental XRD profile for HA, ATCP and BTCP are also presented
in Figure 3-12. The intense region of closely associated peaks of HA, ATCP and BTCP
is shown in this figure.
Quantitative analysis was performed for HA SA and HA PB using Rietveld analysis. In
Rietveld analysis, a crystal structure model was refined by using a least square fit for
the full diffraction pattern. For the Rietveld analysis GSAS [85] and EXPGUI [86] were
used. EXPGUI is the graphical user interface of GSAS. Rietveld analysis has been
applied by different researchers in performing quantitative analysis and measuring the
lattice parameters of HA, the aim being to study changes in the crystal structure [88].
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 79
Figure 3-12 : XRD of three different types (HA, ATCP and BTCP) of calcium
phosphate.
The crystallographic information (CIF) file used for the analysis was collected from the
American Mineralogist Crystal Structure database [90] and had index number 0009353.
The graph obtained through Rietveld analysis of HA PB and HA SA is presented in
Figure 3-13 and Figure 3-14. The R and reduced Χ2 value obtained after Rietveld
analysis of HA PB powder is 0.16 and 1.245. And the R and reduced Χ2value obtained
for HA SA powder is 0.29 and 7.06. The physical meaning of these two terms (R and
reduced Χ2) has been discussed in chapter 2 at section 2.6.3. Rietveld analysis yielded
lattice parameters of the crystal. Lattice parameters are listed in Table 3-6 and
compared to the PDF-2 database [93]. The lattice parameter found after Rietveld
analysis is the same for HA PB powder but slightly changed for HA SA powder when
compared to the PDF-2 database.
Table 3-6 : Lattice parameters yielded after Rietveld analysis for HA PB and HA SA.
a (A0)
b (A0)
C (A0)
Plasma Biotal 9.422586 9.422586 6.887734
HA Sigma Aldrich 9.442797 9.442797 6.84816
26 28 30 32 34 36 38 40 42
50
100
150
200
250
300
350
400
Inte
nsity
(Cou
nts)
Two theta (2)
HA ATCP BTCP
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 80
Figure 3-13 : Rietveld analysis of HA powder procured from Plasma biotal.
Figure 3-14 : Rietveld analysis of HA powder procured from Sigma Aldrich.
Chapter 3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES
Muhammad Rakib Mansur Page 81
3.4 Conclusion Morphological, microstructural and chemical analysis of feedstock material and
substrates were carried out using SEM, EDS, Raman microscopy and XRD. The study
yielded the particle size, shape and morphology of the powders. The microstructural
information includes feret diameter, aspect ratio, roundness, circularity, and major and
minor diameters. Results in relation to different microstructural parameters obtained
after image analysis of feedstock powder are presented on Table 3-1.
Raman microscopy and XRD analysis identified the chemical phases present in the
feedstock. Both techniques confirmed the presence of crystalline HA in powders
procured from Sigma Aldrich and Plasma Biotal. As it is expected that thermal change
can transform one form of calcium phosphate to another, Raman microscopy and XRD
were used to perform a comparative study of the major types of calcium phosphate.
EDS revealed the elements present in the feedstock material and, thus, allowed the
calculation of the Ca/P ratio for HA powders.
This chapter also discusses Raman microscopic and XRD results of other major forms
of CaP, including ATCP, BTCP and ACP. Rietveld analysis of both types of HA
powders provided information regarding the lattice parameters of unit cells. The lattice
parameter ‘a’ is slightly greater for HA SA, but ‘c’ is smaller compared to the standard.
The lattice parameters (a and c) for HA PB yield the same value compared to the
standard.
Chapter 4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS STEEL
Muhammad Rakib Mansur Page 82
4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS STEEL
Chapter 4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS STEEL
Muhammad Rakib Mansur Page 83
4.1 Introduction Single-phase austenitic stainless steel (316L having 18Cr-14Ni-2.4Mo) is a material
used widely for implant applications, due to both its resistance to pitting and crevice
corrosion from the body plasma [3] and its cost effectiveness. Various passivation
processes have been adopted to improve the in-vitro and in-vivo corrosion resistance
of 316L stainless steel by creating an oxide layer on the surface [64]. Passivation can
be performed thermally, electrochemically, and by using nitric acid. However, coatings
of HA and titanium can eliminate the need for passivation. Furthermore, the formation
of HA and titanium as a surface coating will allow an apatite layer and functional groups
such as Ti-OH that will permit the prospective prosthesis to become bioactive.
Composites have been examined, as listed below, to develop a material that will be
bioactive and able to exhibit good mechanical properties.
Chang et al. developed a zirconia reinforced HA coating on titanium by using a
plasma spray method [124].
A HA-Ti/Ti/HA-Ti multilayer composite has been developed using the hot
pressing technique [145, 146].
A composite of hydroxyapatite and ethylene methacrylic acid has been created
by using a flame spray method [21].
A cold spray technique was deployed to deposit HA and titanium on aluminium
and titanium substrates by Choudhuri et al. [20].
Oktar et al. developed composites from biologically derived HA doped with
SiO2, MgO, Al2O3 and ZrO2 using a sintering method [147].
HA composite coatings have been reinforced with carbon nanotubes by using
laser surface alloying [31, 32].
A ceramic slurry mixing method was deployed by Khor et.al. for making Ti-6Al-
4V powder coated with yttria stabilized zirconia (YSZ), and HA. Then the
powders were used to produce composite coatings by a plasma spray method
to enhance the mechanical properties [96].
Various thermal spray techniques deployed to produce bioceramics have been
discussed by Berndt et al. [44]. Laser-based calcium phosphate deposition techniques
were discussed by Paital and Dahotre [3, 78]. The various routes adopted for calcium
phosphate deposition are represented in Figure 4-1.
Chapter 4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS STEEL
Muhammad Rakib Mansur Page 84
Figure 4-1: Different routes used for deposition of calcium phosphates.
The current work develops a composite from HA and titanium on austenitic stainless
steel substrates by employing two techniques; one is a CO2 laser-aided direct metal /
material deposition (DMD) technique, and the other one is Nd:YAG laser-assisted
deposition or cladding.
The DMD process is a closed loop optical feedback system where a laser is used to
create a melt pool on the substrate [35, 148]. The powders are melted and gradually
coated onto a substrate to create micro and macrostructures [36].
4.2 Experimental details Several experiments were performed to develop and characterize composite coatings
on austenitic stainless steel. They are:
Chapter 4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS STEEL
Muhammad Rakib Mansur Page 85
Deposition of CaP and Ti-6Al-4V composite coating on austenitic stainless steel
(AISI 316L) using a CO2 laser-assisted Direct Material Deposition (DMD)
technique.
Characterization of CaP and Ti-6Al-4V single layer composite coating
developed by DMD.
Deposition of CaP and Ti-Al-4V multilayer composite material using DMD.
Characterization of CaP and Ti-Al-4V multilayer composite.
Deposition of CaP and Ti-6Al-4V coating with a Nd:YAG laser on austenitic
stainless steel (SS).
Characterization of Nd:YAG laser-assisted coating on SS.
Determination of temperature and cooling rate of CO2 laser-radiated SS.
Determination of temperature and cooling rate of Nd:YAG laser-radiated SS.
Hence, the deposition process involves different process and parameter settings; the
experimental deposition procedure of CO2 (DMD), and Nd:YAG laser-assisted coating
as described in this chapter.
This chapter and chapter 7 are linked in the following manner: Experiments relating to
temperature and cooling rate determination are described at the beginning of chapter
7. A characterization process is described in generic form in this chapter, and in detail,
at the beginning of following chapters.
4.2.1 Deposition of CaP and Ti-6Al4V composite coating developed by DMD A DMD 505 laser centre (manufactured by Precision Optical Manufacturing, Michigan,
USA), which comprises a CO2 laser of maximum power 5 kW, was used for the
deposition. The CaP powders from Sigma Aldrich were of density 3.14 g/cc and
molecular weight 502.31 g/mol. Ti-6Al-4V alloy was procured from TLS Technik
International, Inc. Bitterfeld, Germany (a producer of gas-atomized metal powder). The
average particle size of Ti-6Al-4V powder was 75 micrometers. Ti-6Al-4V alloy was
selected due to its demonstrated biocompatibility with tissue or bone. The CaP and Ti-
6Al-4V alloy powders were mixed in a weight ratio of 1:3, respectively. The weight of
HA and Ti-6Al-4V was selected 1:3 to increase the bonding between the substrate and
the coating as well as to ensure better flow thorough the delivery system. Two powders
were blended together manually. Care has been taken to make sure the best possible
mixing Ball milling could increase the homogeneity of the mixer but for this case it was
not possible due to the probability of breaking of the particles into smaller size that
could significantly alter the flow ability.
Chapter 4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS STEEL
Muhammad Rakib Mansur Page 86
Substrate discs of single phase austenitic stainless steel (AISI 316L; 18Cr-14Ni-
2.4Mo), with a diameter of 80 mm and 10 mm thick, were used. Thick substrates were
used to avoid bending and deformation due to the high temperature flux and cooling
rate. The substrates were sandblasted and cleaned using compressed air before
deposition. Powder calibration was performed and set to a powder feed rate of
2.15 g/min. In DMD, two types of carrier gas and nozzle gas (Ar and He) are supplied
for deposition. The parameters kept constant throughout the experiment were: cover
gas (Ar, 10 SLPM); carrier gas (Ar, 7 SLPM and He, 2 SLPM); nozzle gas (He, 5
SLPM); and shaping gas (Ar, 16 SLPM) flow rates. Two types of nozzle gas is used in
DMD, one is He (that was kept constant) and the other is Ar (varied during the
experiment and presented in Table 4-1). The beam diameter used for the experiment
was 1.8 mm, which was kept constant throughout the experiment. The parameters
which varied during the experiment are presented in Table 4-1. The balance gas
mentioned in Table 4-1 is used to balance the effects of air knife pressure at the
nozzle.
Table 4-1 : Experimental runs and corresponding DMD parameters (variable
parameters).
Experimental
runs
Power
( W )
Traverse
speed
( mm/min )
Balance gas,
Ar
( SLPM )
Nozzle gas,
Ar
( SLPM )
1 1500 120 5 25
2 1500 300 5 25
3 1500 300 5 20
4 1500 300 20 20
5 1500 300 5 25
6 1500 300 5 25
7 1500 300 5 25
8 1000 300 5 25
9 500 300 5 25
10 200 300 5 25
The experimental run 7 is distinct from the other runs. It has been deposited using a
nickel based powder from Sulzer Metco. This metal coating was used for comparison
purposes against the composites. Experimental runs 2 and 5 are replicates to check
the reproducibility of the coating. A schematic diagram of the experimental process
along with an image of the coated sample is presented in Figure 4-2. The diagram
Chapter 4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS STEEL
Muhammad Rakib Mansur Page 87
shows the laser focusing head containing the powder delivery nozzle intricately
arranged (by the DMD manufacturer) to feed powder for the process. The schematic
also shows the feedback control (mentioned as electronics in Figure 4-2) associated
with the system.
Figure 4-2: (a) A schematic diagram of the DMD process (b) CaP and Ti-6Al-4V
composite coatings deposited onto austenitic stainless steel. The run numbers are
indicated.
Chapter 4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS STEEL
Muhammad Rakib Mansur Page 88
4.2.2 CaP and Ti-6Al-4V multilayer composite developed by DMD The same materials described in section 4.2.1 were used at the same weight ratio to
develop multilayer composite coating on stainless steel substrate. The dimension of the
substrate and preparation of the substrate is also described in section 4.2.1.
The parameters used to generate multilayer coating were powder feed rate 2.15 g/min;
cover gas (Ar) flow rate 10 SLPM; carrier gas (Ar) flow rate 7 SLPM; carrier gas (He)
flow rate 2 SLPM; nozzle gas (Ar) flow rate 25 SLPM; nozzle gas (He) flow rate 5
SLPM; shaping gas (Ar) flow rate 16 SLPM; and balance gas (Ar) 5 SLPM. Power and
traverse speeds used for the experiment were 1500 W and 300 mm/min respectively.
The beam diameter used for the experiment was 1.8 mm, which was kept constant
throughout the experiment. The dimension of the multilayer coating pad developed on
the substrate was 20 X 10 X 6 mm3. To generate the multilayer coating pad the
distance maintained for overlapped tracks was 0.65 mm. The distance between the
nozzle and the substrate was kept at 20 mm.
4.2.3 Deposition of CaP and Ti-6Al-4V composite coating developed by Nd:YAG Two powders (CaP and Ti-6Al-V) were used for the experiment. The CaP (HA) and Ti-
6Al-4V alloy powders were mixed in a weight ratio of 1:3, respectively. The CaP
powders from Plasma Biotal and Ti-6Al-4V alloy powder procured from TLS Technik
International, Inc.
A Nd:YAG laser centre, Rofin CW 025 (manufactured by Rofin-Sinar, a German based
company headquartered in Michigan, USA) comprising a Nd:YAG laser of maximum
power 2.3 kW, was used for the process. The powder feeder used was a
SINGLE 10 – C manufactured by Sulzer Metco. Substrate coupons of single-phase
austenitic stainless steel (AISI 304L; 18 Cr - 14 Ni - 2.4 Mo), with dimension
30 mm X 30 mm X 2 mm, were used. The substrates were sand-blasted using 80
mesh-size garnet sand and cleaned using compressed air before processing. The
substrate coupons of 2 mm thickness were tightly clamped to the substrate holder to
avoid bending or buckling during the coating process.
The parameters used to generate multilayer coating were powder feed rate 2.03 g/min
and track length 24 mm. The distance between the lens and the substrate was 211
mm, the laser beam radius 3 mm and the distance between the substrate and the
Chapter 4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS STEEL
Muhammad Rakib Mansur Page 89
nozzle was 11 mm. The parameters varied throughout the experiment are presented in
Table 4-2.
Table 4-2 : Variable parameters for each experimental run.
Experimental
runs
Power
( W )
Traverse speed
( mm / min )
1 150 150
2 250 120
3 350 120
4 300 120
5 400 120
6 400 240
7 400 180
4.2.4 After-deposition characterization The samples were cut from the substrate by using either a wire cut EDM or by using a
cutting saw made of carbide or diamond. The sectioned samples were cold mounted to
observe the top, bottom and the cross section. The samples were ground using grade
120, 240, 320, 400, and 600 grit SiC abrasive papers, and then polished further using
6 µm and 1 µm diamond pastes. An inverted optical microscope (Leica MEF4M) was
used for metallographic examination. Leica MEF4M is a light optical microscope
manufactured by Leica Microsystems Gmbh, Wetzlar, Germany. A Durascan 80,
manufactured by Struers, Denmark, was used to measure the Vickers microhardness
of the substrate and the cross section of the polished coating. A field emission
scanning electron microscope (FESEM, ZEISS SUPRA 40 VP) and an energy
dispersive X-ray spectrometer (EDS, Oxford instruments INCA suite v.4.13) were used
to observe the microstructure and surface morphology, and to analyse the elemental
composition of the coating. The XRD data acquisition was carried out using a Bruker
D8 Advance XRD machine. The specification of the device is mentioned in chapter 3 at
section 3.3.2.
Chapter 4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS STEEL
Muhammad Rakib Mansur Page 90
4.3 Conclusion Composite coatings were developed from CaP and Ti-6Al-4V using CO2 and Nd:YAG
lasers. For CO2 laser, direct material deposition technique was adopted. DMD has a
unique closed-loop optical feed-back system that helps to create micro and
macrostructures. The DMD system possesses a coaxial powder delivery whether
Nd:YAG system possesses angular powder delivery. The DMD system comprises a
unique gas delivery system, that creates a protective atmosphere to reduce oxidation.
This chapter mainly describes the procedure and process parameters which includes
fixed and variable process parameters, adopted during the experiment.
The first section of the experimental details describe the process parameters and
experimental arrangements made for producing CaP (HA) and Ti-6Al-4V composite
coating (single layer) developed on austenitic stainless steel. The second section
describes the experimental parameters used for producing multilayer composite made
from CaP (HA) and Ti-6Al-4V using DMD (using CO2 laser). The third section describes
the CaP (HA) and Ti-6Al-4V single layer composite coatings produced using Nd:YAG
laser. The instruments and generic characterization procedure adopted for
microstructural, chemical and mechanical characterization are narrated in the fourth
section. The techniques and equipment used for characterization are similar for all of
the produced coatings and composite materials.
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 91
5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 92
5.1 Introduction Composite coatings made of hydroxyapatite (HA) and titanium alloy (Ti-6Al-4V) have
potential biomedical applications because of their biocompatibility and bioactive
characteristics. In this chapter, the developed composite coatings (deposition
technique and parameters used are described in chapter 4, section 4.2.1) of HA and Ti-
6Al-4V are characterized. Roughness averages were assessed by contact profilometry.
The microstructures and surface topography were characterized using optical
microscopy and scanning electron microscopy (SEM). The microstructure of the
deposited coating was studied to collect qualitative and quantitative information
concerning the physical and structural properties of the coating. The microstructure of
the heat affected zone (HAZ) where diffusion was dominant was inspected.
The results indicate that average roughness increases with traverse speed and
depends significantly on the power level. The crack orientation was found to be
sensitive to traverse speed, while the number of cracks was related to the power level.
Porosity decreased as the power level increased. Correlation was established between
microstructural parameters such as (i) crust height, (ii) aspect ratio (i.e., the ratio
between the width and height of deposited tracks), and (iii) the angle of the coating in
relation to the substrate surface; all with respect to the power and specific energy. The
characterization also included the study of surface morphology, roughness average,
mechanical property (micro-hardness) and the chemical composition of the coating.
The chemical composition and hardness of the composite coatings were investigated
using energy dispersive X-ray spectroscopy (EDS) and Vickers microhardness tests. In
the experiment, the ratio of calcium and phosphorus (Ca/P) for different experimental
runs was determined and correlated with the energy and power levels. The diffusion of
titanium was analysed and the diffusion coefficient of titanium in iron was estimated.
The diffusion coefficient of titanium provided an indication of the solubility of titanium in
stainless steel (316L), which influenced the bond and adhesion of the coating with the
substrate. The ratio of calcium to phosphorous in the coating and the diffusion rate of
titanium into the substrate vary with power and traverse speed. Traverse speed has
more influence on surface morphology and Ca/P ratio than power. The variation in
microhardness along the cross section of the heat-affected zone was investigated and
the diffusion coefficient of titanium into stainless steel estimated.
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 93
5.2 Experimental details The details of the performed experiment are described in chapter 4 at section 4.2.1.
Ten experimental runs along with their associated process parameters are mentioned
in the above mentioned section.
The roughness was measured using a Surtronic 25 profilometer manufactured by
Taylor Hobson. The sample preparation procedure is described in chapter 4 at section
4.2.4. A Durascan 80, manufactured by Struers, Denmark was used to measure the
Vickers microhardness of the substrate and the cross section of the polished coating. A
field emission scanning electron microscope (FESEM, ZEISS SUPRA 40 VP) and an
energy dispersive X-ray spectrometer (EDS, Oxford instruments INCA suite v.4.13)
were used to observe microstructure and surface morphology, and to analyse the
elemental composition of the coating.
5.3 Results and discussion
5.3.1 Surface roughness and morphology The roughness of the coated samples, Figure 5-1, was measured using a Surtronic 25
profilometer manufactured by Taylor Hobson. The evaluation length for the roughness
measurement was 16 mm and ten readings (n=10) were taken to calculate average
roughness. The determined roughness values are presented in appendix, at Table
11-1. The average roughness of the sand blasted stainless steel substrate was 2.9
µm, while the maximum average roughness of the coated sample was 11.22 µm and
the minimum was 6.20 µm for the composite. An average roughness for the nickel-
based metal was 4.02 µm; a value that is distinct from the composite coatings. Average
roughness increased with an increase in traverse speed. A variation of traverse speed
from 120 mm/min to 300 mm/min does not unduly influence the average roughness.
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 94
Figure 5-1: (a) Average roughness (Ra) of experimental runs showing the surface
profile (b) Average roughness of the 10 coating trials, n = number of roughness
measurements carried on each trial/run.
The average root mean squared values of the roughness (expressed by Rq or RMS)
were also determined. Ra values are complemented by Rq values. Rq value is typically
10 to 25% larger than Ra values depending on the nature of the surface [149]. RMS or
Rq values are more weighted by the large values of peak height and valley depth. The
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 95
determined Rq values are presented in the appendix, in Table 11-2. For the performed
experiment, the Rq values were larger than Ra and within 25% of the Ra value.
Besides traverse speed, average roughness is also affected by power. Roughness
variations are related to changes in power. Figure 5-2 indicates that average
roughness attains a minimum at 1,000 W.
Figure 5-2: Variation of average roughness with power, n = number of roughness
measurements carried.
Both power and traverse speed influenced the surface morphology, but traverse speed
had the greater influence on morphology. With increasing traverse speeds from
120 mm/min to 300 mm/min more powdery, flake-like particles were visible on the
surface (Figure 5-3a and Figure 5-3b). At a specific power, a lower traverse speed led
to more uniform melting due to higher specific energy. At the same traverse speed, low
power yields less melting and larger sized flaky ceramic particles are prominent on the
surface (Figure 5-3g).
Surface morphology was also influenced by the nozzle gas (Ar) flow rate. Decreasing
nozzle gas (Ar) from 25 to 20 SLPM (Run 03, Table 4-1) resulted in a patterned surface
that exhibited a distinctive texture (Figure 5-3c). An increase in the argon balance gas
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 96
flow rate from 5 to 20 SLPM (Run 04, Table 4-1) further enhanced the effect (Figure
5-3d), which was visible only on the top layer of the coating.
Figure 5-3: Surface morphology of the coatings deposited using different parameters
(a) Run 01, (b) Run 02, (c) Run 03, (d) Run 04, (e) Run 08, (f) Run 09, (g) Run 10 and
(h) cross sectional view of run 04 showing the top ceramic layer .
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 97
5.3.2 Crack orientation More extensive cracking behaviour was observed for Run 1, where a predominantly
longitudinal morphology (Figure 5-4a) that followed the coating boundary was
observed. Some areas exhibited curved orientations that probably reflect the residual
stress conditions. The other runs revealed cracks in the transverse direction (Figure
5-4b). The difference can mostly be attributed to the large heat flux gradient because of
different traverse speeds. Crack direction changes from longitudinal to transverse as
the traverse speed increases. The number of cracks reduced with a reduction in power.
Figure 5-4: Cracks formed in longitudinal and transverse orientation with respect to
traverse direction (a) Run 1, and (b) Run 2.
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 98
5.3.3 Microstructural observation Two distinct layers were observed on the coating (Figure 5-5a). Figure 5-5 (a) and (b)
show the cross-sectional picture and the schematic description prepared after
observing the cross-section. The top layer of the coating was identified as ‘the crust’
and is shown in the schematic diagram in Figure 5-5b. The heat-affected zone, ‘HAZ’,
is the second layer that lies beneath the crust and diffusion processes occurred within
this zone.
Figure 5-5 : Cross-sectional view of experimental run 01 (a) and the schematic model
(b) of the cross section.
The surface topography (Figure 5-3d) and the cross-sectional microstructures of
experimental run 04 (Figure 5-3h) exhibited similar features. The top and the cross
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 99
section of the crust revealed an oriented and crystalline microstructure compared to the
other experimental runs. This is attributed to the nozzle gas flow rate.
The variation of power and traverse speed ultimately affects the specific energy. The
relationship between specific energy, power, beam diameter and traverse speed is
given below [150]:
(
)
(
)
(
) ( )
5-1
Experimental values regarding power, traverse speed, beam diameter, along with the
calculated value of specific energy are presented in Table 5-1.
Table 5-1 : Different microstructural parameters (height, width and aspect ratio) for run
01, 02, 08, 09 and 10 varying with power, traverse speed and specific energy.
Power (W)
Traverse speed
(mm/min)
Beam diameter
(mm)
Specific energy (J/mm2)
Height of the coating
(µm)
Width of the
coating (µm)
Aspect ratio
(Width/ height)
1500 120 1.8 416.67 2250 3362 1.49
1500 300 1.8 166.67 1273 3020 2.37
1000 300 1.8 111.11 629 2795 4.44
500 300 1.8 55.56 525 2043 3.89
200 300 1.8 22.22 145 858 5.92
The aspect ratio, that is the ratio of the coating width to the coating height, decreased
linearly with an increase in specific energy (Table 5-1).
Variations in the depth of the HAZ, the average crust height and the angle of the
coating with respect to power and traverse speed were investigated, as shown in Table
5-2. The curvature angle of the coatings also depended on power and the relation
followed a linear increment with power (Table 5-2).
The depth of the HAZ and crust height increased linearly with the increase in power
(Figure 5-6), for a constant traverse speed of 300 mm/min. But at the same power, the
crust height decreased with an increase in traverse speed (for run 01 and run 02, Table
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 100
5-2), due mainly to the decrease in specific energy. Areas of the HAZ’s also
demonstrated linear relationship with respect to specific energy. The linear relationship
between HAZ area and specific energy is shown in Figure 11-8 within the Appendix.
Table 5-2 : Different microstructural parameters (crust height, angle of coating, height
of HAZ and HAZ area) for run 01, 02, 08, 09 and 10 varying with power, traverse speed
and specific energy.
Power (W)
Traverse speed
(mm/min)
Beam diameter
(mm)
Specific energy (J/mm2)
Crust height (µm)
Angle of the
coating (o)
Height of HAZ
(µm)
HAZ Area
(mm2)
1500 120 1.8 416.67 157.02 19.39 2093 4.7
1500 300 1.8 166.67 91.59 10.22 1182 2.6
1000 300 1.8 111.11 81.97 7.50 547 1.3
500 300 1.8 55.56 70.69 5.43 455 0.8
200 300 1.8 22.22 68.22 3.77 77 0.1
Figure 5-6: Variation of depth of HAZ and crust height with change in power at a
constant traverse speed 300 mm/min.
The austenitic SS substrate (AISI 316L) was etched using a tint etchant (20 mL HCl,
100 mL water and 1g potassium meta-bisulfite) and observed under optical microscopy
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 101
(Figure 5-7a ). Tint etchant is an immersion etchant that creates colour contrast on the
specific microstructure because of the thin sulphide or oxide film formed. It thus reveals
the structure due to variations in light interference effects as a function of the film
thickness [151]. The microstructure reveals the typical structure of an austenitic
stainless steel.
Another etchant was used for etching the HAZ to reveal the Ti-diffused HAZ. This
etchant contained 50% HCl and 50% water and the etched microstructure was
observed (Figure 5-7b) using an optical microscope. The etchant had etched the
titanium diffusion zone.
Figure 5-7: Etched microstructure of the (a) stainless steel substrate and (b) diffused
heat-affected zone (HAZ).
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 102
The microstructure of the HAZ is different from that of the substrate in that the grain
boundaries in the HAZ are globular and equiaxed, which is similar with the alpha phase
of the Ti-6Al-4V. Beta phases are not prominent at the grain boundaries. For pure Ti-
6Al-4V, the alpha phase dominates (90 vol%) at room temperature and determines the
physical and mechanical properties of the alloy [152]. The grains contain small sphere-
like particles, that are similar to spheroidites observed in heat-treated steels [153, 154].
Inter-diffusion occurred between Ti-6Al-4V and austenite and since laser-assisted
deposition is a non-isothermal process, the diffusion-affected zone does not increase
proportionately to the square root of time [155].
Microstructures of the crusts were observed at 200X and 500X magnifications using a
Leica MEF4M optical microscope and SEM, respectively. White spots inside the
images (Figure 5-8) represent an accumulation of CaP, while black cavities are pores.
Porosity was analysed using imageJ [137, 138] software. Figure 5-8 shows the SEMs
of four microstructures of different runs.
Figure 5-8: Microstructure of the crust of CaP and Ti-6Al-4V composite coatings (a)
experimental run 01, (b) experimental run 08, (c) experimental run 09, and (d)
experimental run 10.
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 103
The porosity of the crust for runs 2, 8, 9 and 10 was 7.1%, 8.63%, 16.05% and 22.73%,
respectively. Porosity decreases with an increase in power (Figure 5-9).
Figure 5-9: Variation of porosity with power (a) and specific energy (b).
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 104
At the same power, porosity becomes higher (7.1%, Run 2) for a 300 mm/min traverse
speed compared to 120 mm/min traverse speed (5.3%, Run 1). This phenomenon is
attributed to the change of specific energy and heat transfer rate. At the same power,
slower traverse speed produces more specific energy, which increases melt pool
temperature and helps to melt more powder particles uniformly.
5.3.4 Diffusion between iron and titanium
The main elements that play a major role in the diffusion inside the HAZ and which
change the microstructure beneath the crust are Ti, Fe, Ni and Cr. Titanium diffused
into iron and its concentration gradually decreased from the surface of the crust to the
depth of the HAZ. The weight percentage of iron was increased with an increase in
depth. On the other hand, the titanium percentage decreased along the depth, Figure
5-10. The concentration gradient of titanium follows a linear tendency with specific
energy is presented in Figure 5-11.
Figure 5-10: Change of Ti concentration with the depth, for different runs.
Diffusion of titanium into iron is non-steady state in nature. The diffusion coefficient of
titanium into iron was calculated using the following equation (5-2) [104] and the results
are presented in Table 5-3.
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 105
(
√ )
5-2
In the above equation Cx represents the concentration at depth x after time t, Co is the
concentration before diffusion, Cs is the concentration of solute after time t and D is the
diffusion coefficient. The concentration values used in this equation were in wt.%.
The theory of diffusion is discussed in chapter 2 at section2.6.4.
The determined diffusion coefficients (Table 5-3) exhibited a linear relationship with
power and specific energy (Figure 5-10). At very low specific energy (for run 10), there
was almost no diffusion of titanium into iron and the concentration gradient of titanium
became insignificant.
Table 5-3: Concentration gradient and diffusion coefficient of titanium in austenitic
stainless steel.
Power (W)
Traverse speed
(mm/min)
Beam diameter
(mm)
Specific energy (J/mm2)
Concentration gradient (kg/m3)
Diffusion coefficient
(m2/s)
1500 120 1.8 416.67 1321 2.74371E-07
1500 300 1.8 166.67 843 2.18001E-07
1000 300 1.8 111.11 647 6.65394E-08
500 300 1.8 55.56 526 3.01358E-08
200 300 1.8 22.22 - -
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 106
Determined diffusion coefficients exhibited a linear relationship with power and specific
energy (Figure 5-11). At very low specific energy (for run 10) there was almost no
diffusion of titanium into iron and the concentration gradient of titanium became
insignificant.
Figure 5-11: Dependence of diffusion coefficient and concentration gradient of titanium
on specific energy.
5.3.5 Chemical Analysis A field emission scanning electron microscope (FESEM, ZEISS SUPRA 40 VP) and an
energy dispersive X-ray spectrometer (EDS, Oxford instruments INCA suite v.4.13)
were used to analyse elemental composition of the top and cross section of the
coating. Energy dispersive X-ray spectroscopy (EDS) results of the top surface reveal
the chemical constituents present in the coating, Figure 5-12. The white spots on the
Figure 5-12a (SEM picture) are the phosphorous and calcium rich zone. The peaks on
the picture are titanium rich and lack the presence of calcium and phosphorous.
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 107
Figure 5-12: Elemental mapping of the coated composite sample, (a) SEM image, and
elemental map of (b) Calcium, (c) Phosphorous, (d) Titanium, (e) Oxygen, (f)
Aluminium (Run 08).
The spectrum of the elements is shown in the appendix, Figure 11-7. The probable
source of the small amount of silicon may arise from sand-blasting, and manganese,
iron, copper and zinc is likely to be a diffusion product from the substrate.
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 108
The EDS of the coating cross-section revealed that the top crust layer contained a
higher concentration of calcium and phosphorous, as well as titanium, aluminium,
oxygen, and carbon. The HAZ layer beneath the crust consisted mostly of elements
such as titanium, iron, nickel, chromium, magnesium, and molybdenum. The titanium
diffused from the crust layer to the bottom layer (HAZ) and iron, nickel, chromium,
manganese, molybdenum belongs to the substrate. In this case, the iron is the solvent
and the titanium is the solute. There is little diffusion of aluminium from the top layer.
Trace amounts of vanadium were found in the bottom layer of Run 9 and 10. A trace
amount of iron diffused from the substrate to top layer but only for Runs 2, 9 and 10.
The amount of calcium, phosphorous, oxygen and aluminium on the crust changes with
specific energy (specific energy is calculated using Eq 5-1) and this is presented in
Figure 5-13.
Figure 5-13: Change of concentration (wt.%) calcium, phosphorous, oxygen and
aluminium along with the change of specific energy of laser.
The Ca/P ratio of the powder used for deposition was 1.47 and determined using EDS
analysis. This indicates the powder may contain trace amounts of other forms of
calcium phosphate along with HA. The Ca/P ratio is significantly affected by traverse
speed. With the increase in traverse speed from 120 to 300 mm/min, specific energy
sharply drops from 417 J/mm2 to 167 J/mm2, and Ca/P ratio declines from 43 to 1.66.
At a constant traverse speed of 300 mm/s, the variation of Ca/P ratio is less drastic
(1.66 for 1500 W, 1.45 for 1000 W, 0.26 for 500 W, and 2.4 for 200 W) with the change
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 109
of power. The data indicates that, up to a certain specific energy, little change in Ca/P
ratio is observed, but beyond that energy level, a drastic increment in the Ca/P ratio
occurs. When high energy is used, phosphate could break into phosphorous and
oxygen and then the phosphorous becomes vaporized, which may result in a dramatic
increase in the calcium phosphorous ratio. The oxygen evolved in this process could
contribute to the formation of metal oxides by oxidation process. For oxidation of a
metal, temperature is a prime factor [156]. The increase of oxygen content with higher
specific energy at the top layer of the coating justifies the hypothesis. A small variation
in Ca/P ratio might arise due to the inhomogeneous distribution of calcium
phosphorous and titanium alloy powders.
5.3.6 Microhardness Microhardness is an indicator of the coating’s resistance to localized plastic
deformation. The variations of microhardness along the depth of HAZ for different
experimental runs were plotted along with the standard deviation, Figure 5-14. The
microhardness value of the stainless steel substrate is 157 HV, having a standard
deviation of 2.6. The specific energy affects the hardness of the HAZ and the
microhardness value in the HAZ is much higher than that in the substrate.
Figure 5-14: Variation of hardness in the heat-affected diffusion zone of the coatings.
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 110
The HAZ possesses complex chemistry because of the presence of different elements.
Iron and titanium are the two major elements and contribute most in terms of
mechanical, chemical and microstructural properties. As the HAZ is small in size, the
load used for the microhardness measurement is 500 gf. Microhardness increased
from the bottom to the top of the HAZ, and the trend is similar for all the runs. At high
traverse speed (300 mm/min), the microhardness value is relatively low in the middle,
compared to top and the bottom of the HAZ, showed in Figure 5-14. For less traverse
speed (120 mm/min, Run 01), microhardness at the middle of the HAZ shows a slightly
different trend. This difference can be attributed to the difference in the cooling rate that
was caused by the change in traverse speed. As run 10 has the smallest HAZ, it was
not possible to check the microhardness along with the increase in height or depth. The
microhardness value obtained for run 10 is 192 + 10 HV due to the low specific energy
and cooling rate.
5.4 Conclusion Experimental results show that the surface profile depends on power. In the
experimental range investigated the optimum power level to produce minimum
roughness is 1,000 W. The crack orientation was sensitive to traverse speed, while the
number of cracks was related to the power level. Elemental distribution of the coating
top revealed that the partial unmelts of titanium contained no calcium and
phosphorous.
The produced coatings exhibited two distinct layers; the top layer is ceramic in nature
and the bottom is HAZ. Diffusion of titanium into iron occurred in the HAZ. The change
of traverse speed and power ultimately changes specific energy, which is responsible
for the change in surface morphology and microstructure. The higher the specific
energy, the more changes can be observed on the coating microstructure in terms of
crust height, aspect ratio, angle of the coating and depth of HAZ. Nozzle gas flow rate
contributes towards a structured and crystalline-looking surface at the top.
The study reveals that porosity decreases with the increase in power and the minimum
amount of porosity can be achieved at 1,500 W power. Inside the coating crust weight
percentage of calcium, phosphorus, oxygen, and aluminium varies with specific energy.
The Ca/P ratio also depends on specific energy. A significant change in the Ca/P ratio
occurs with the change in traverse speed. The Ca/P ratio (1.66) nearest to human bone
was obtained for experimental run 02 with a traverse speed 300 mm/min and power
Chapter 5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING
Muhammad Rakib Mansur Page 111
1500 W. From the experiment, it is observed that the specific energy at or below
167 J / mm2 is suitable to achieve a Ca/P ratio less than 1.67; i.e., the ratio desired for
cell apoptosis and clinical practice. The concentration profile of titanium inside iron
pointed towards non-steady state diffusion. The diffusion coefficient of titanium inside
iron was determined. The diffusion trend exhibited a linear variation with specific
energy. The microhardness of the HAZ was determined and the top section of the
coating yielded a greater hardness than the bottom section.
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 112
6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 113
6.1 Introduction Multilayer composites could be the building blocks to produce a bone graft material for
morphological fixation. For grafting, an autogenous bone graft - where bone harvested
from the same patient is used as a graft to fill the bone defect - is the first option.
Although this is the best option to repair the defect, it has some drawbacks considering
the possibilities of graft rejection. The drawbacks include additional pain in the harvest
site, the increment in rehabilitation time and the risk of post-operative complications
[28, 157, 158].
Different forms of CaP (HA, ATCP, BTCP, TTCP) have been tested as bone
substitutes and are used as bone graft materials for orthopaedic and maxillofacial
surgery. HA is non-toxic, bioactive and forms an interfacial bond between the material
and tissue [13]. HA can promote bone ingrowth and provide enhanced fixation. Ti-6Al-
4V is an alloy comparable to commercially pure titanium: it demonstrates superior
mechanical properties such as higher fatigue limit, yield strength and ultimate tensile
strength [6]. Ti-6Al-4V alloy also exhibits excellent corrosion resistance properties. The
corrosion potential of Ti-6Al-4V is similar to commercially pure titanium, but the passive
current density is lower, which indicates better corrosion resistance [7].
A composite material made from Ti-6Al-4V and HA could be a potential graft material.
The current work employs HA with Ti-6Al-4V to promote bone ingrowth by creating a
protective ceramic layer on the top of Ti-6Al-4V using the DMD (rapid prototyping)
technique. Because of the inherent property of the used materials, it is expected to
promote bone ingrowth by creating a protective ceramic layer on the top of Ti-6Al-4V.
In this fashion, the composite will be more bioactive whilst retaining biocompatibility.
This chapter contains the morphological, microstructural and chemical characterization
of a multilayer composite coating prepared from Ti-6Al-4V and HA. Besides this, effort
has been given to making a comparative study between single layer and multilayer
composite coatings made from Ti-6Al-V and HA. A comparison between composite and
pure HA coating on stainless steel is also presented in this chapter. The
microstructures were characterized using optical microscopy and scanning electron
microscopy (SEM). The microstructure of the deposited material was studied to collect
qualitative and quantitative information concerning the physical and structural
properties of the coating. Chemical composition and hardness of the composite were
investigated, using energy dispersive X-ray spectroscopy (EDS) and Vickers
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 114
microhardness tests. XRD was used to identify the different phases evolved in the
material.
6.2 Experimental details The details of the performed experiment are described in chapter 4 at section 4.2.2.
Process parameters used for the experiment are mentioned in the above mentioned
section.
The produced sample was cut from the substrate using a diamond cutting saw. The
sample was then sectioned into three pieces and mounted using a cold mounting
technique to observe the top, bottom and the cross-section. The samples were ground
using grade 120, 240, 320, 400, and 600 grit SiC abrasive papers, and then polished
further using 6 µm and 1 µm diamond pastes. An inverted optical microscope (Leica
MEF4M) was used for metallographic examination. The hardness testing machine
(micro-indenter) used for the experiment was manufactured by Brukers (a German-
based scientific instrument manufacturer, headquartered in Billerica, Massachusetts,
USA). This machine was used to measure Vickers microhardness on the substrate and
the cross section of the polished coating. The machine has the capability to use both
the Vickers and Knoop micro-indenter. A Knoop indenter was also used to measure the
hardness in order to calculate elastic modulus. A field emission scanning electron
microscope (FESEM, ZEISS SUPRA 40 VP) and an energy dispersive X-ray
spectrometer (EDS, Oxford instruments INCA suite v.4.13) were used to observe the
microstructure and surface morphology, and to analyse the elemental composition of
the coating. The XRD data acquisition was carried out using a Bruker D8 Advance
XRD machine. The software associated with the device was named DIFFRAC plus
XRD commander, and the used operating voltage and current were 40 kV and 40 mA.
The two theta ranges set for the observations were from 100 to 900.
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 115
6.3 Results and discussion
6.3.1 Microstructural characterization Polished samples were observed under inverted optical microscope (Leica MEF4M)
and SEM. Optical micrographs were analysed using imageJ. The sample optical
micrographs used for image analysis are presented in appendix, Figure 11-6. Two
layers were observed in the composite cross-section; the top layer is termed as ‘crust’.
The average thickness/height of the crust was 222 µm with standard deviation 48.8
µm.
Analysis revealed that the top section is more porous than the bottom section. The
maximum and minimum feret diameter of the voids on the top is larger compared to
bottom. The circularity and solidity of the pores on the top side of the coating are
slightly less than the bottom side. The microstructural factors expressed here have
been discussed in sections 2.6.1and 2.6.6.
Table 6-1 : The microstructural parameters of pores.
% Area
Maximum feret
diameter
Min feret
Diameter Circularity Solidity
Top 8.197 10.777 6.802 0.902 0.923
Bottom 6.577 9.101 5.707 0.932 0.932
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 116
Figure 6-1 : SEM micrograph of HA and Ti-6Al-4V composite (a) Top (b) bottom.
Histograms were plotted using data procured from analysed images. The histograms
are incorporated in Figure 6-2 and Figure 6-3. The histograms of feret diameter, aspect
ratio, roundness, circularity and solidity of the top and the bottom section are compared
side by side. From the histogram (Figure 6-3), it is palpable that most of the pores on
the top section have medium to high circularity, but for the bottom section, a large
amount of pores have low and medium circularity. But the frequency distribution of the
pores is such that the average circularity of pores on the top side is less than the
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 117
average circularity of pores on the bottom. The shape factor that indicates the
eccentricity of a spheroid is also analysed. A shape factor close to zero indicates a
spheroidal or circular shape and a value close to 1 indicates the shape of an ellipse or
ellipsoid.
Figure 6-2 : Maximum feret diameter, minimum ferret diameter and aspect ratio of
voids for HA and Ti-6Al-4V composite coating top (a, c and e) and bottom (b, d and f).
No of particles in y direction actually indicates the no of pores.
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 118
Figure 6-3 : Roundness, circularity and solidity of voids for HA and Ti-6Al-4V composite
coating top (a, c and e) and bottom (b, d and f). No of particles in y direction actually
indicates the no of pores.
6.3.2 Chemical characterization EDS has been performed to check elemental weight percentage along the cross-
section of the coating. Elemental percentages along a straight line containing 10 points
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 119
have been examined. The data is presented in appendix (Table 11-3) and plotted in
Figure 6-4. “0” represents the bottom of the composite and “3.5” indicates the top of the
composite. From the graph, it is evident that iron, chromium, nickel is diffused from the
substrate. The percentage of iron is greater at the bottom of the composite. Chromium
and nickel show the same tendency. On the other hand, the percentage of titanium is
greater at the top and decreases gradually with the depth. The change of concentration
for aluminium and vanadium is not significant due to their small weight percentage, but
the trend is similar to titanium.
Figure 6-4 : Change of concentration along the cross-section of the composite.
The presence of calcium was detected in the top section but not in the bottom. The
atomic percentage of calcium and phosphorous on the top side of the composite is
9.01 and 1.65, which yields a Ca/P ratio 5.461. The Ca/P ratio of the composite (5.461)
is high compared to the feedstock HA (1.47). The reason is so because, when high
energy is used, phosphate can break into phosphorous and oxygen and then the
phosphorous become vaporized, which may result in a dramatic increase in calcium
phosphorous ratio. The same phenomenon was observed for a single layer coating on
austenitic stainless steel (section 5.3.5). The Ca/P ratio can be decreased by using
less specific energy for deposition process.
XRD was performed on the top section of the composite and revealed the presence of
HA, ATCP, BTCP phase of CaP (Figure 6-5). The XRD pattern shows a significant
amount of amorphous phase. Due to the high temperature evolved during laser
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 120
processing, ATCP and BTCP were formed from HA. Titanium can also be observed in
the XRD graph. The titanium present in the composite is mainly alpha titanium; the
presence of beta titanium is not prominent in the diffraction graph.
Figure 6-5 : XRD of the top area of the composite.
6.3.3 Mechanical characterization Four important mechanical properties of the composite were determined using a micro-
indentation technique. They are: Vickers microhardness, fracture toughness, Knoop
microhardness and modulus of elasticity. The microhardness measured in the
experiment followed the standard test method, ASTM E 384-99 [159] for testing micro
indentation hardness of materials. Modulus of elasticity was determined using Knoop
indention, but the other two properties were determined using a Vickers micro-
indentation technique. The formula used for the determination of fracture toughness
(Kc) was devised by Evans and Charles is presented [160] below.
⁄ 6-1
30 35 40 45 50 55 60 65 70 75 80 85 900
20
40
60
80
100
120
140
HA
HA
HA
TiHA
HA
HAHA
HATi
HAHA
Inte
nsity
(Cou
nts)
Two theta (2)
HA
Ti
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 121
Where P is load and c is the diagonally measured crack length that originates at the
corners of the Vickers indent length. The critical load determined for crack initiation is
500 gf for the composite. The cracks generated are considered as radial/median crack.
Modulus of Elasticity (E) was measured using Knoop micro-indentation, following the
research undertaken by Leigh and Berndt [109]. The formula used for the
determination of modulus of Elasticity is described below.
6-2
Where a and b are the major and minor diagonals of the Knoop indenter, a’ and b’ is
the reduced major and minor diagonal length after elastic recovery. Kweh et al. [110]
considered α as 0.45 for plasma-sprayed HA coatings. For our calculation, the value of
α is also 0.45. The theoretical detail associated with this determination is discussed in
section 2.6.5.
A graph (Figure 6-6) is plotted to show Vickers micro-hardness and fracture toughness
of the composite.
Figure 6-6 : Vickers micro-hardness and fracture toughness of composite made of HA
and Ti-6Al-4V with respect to depth.
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 122
It is observed that the hardness and fracture toughness decrease from the top to the
bottom. The reason behind the relatively high hardness on the top is the faster cooling
rate.
The average value of Modulus of Elasticity (E) of the composite is found to be 118.37
GPa, with standard deviation + 33.71 GPa. The average Knoop micro-hardness value
obtained for the composite is 840.35, with standard deviation + 17.99. The
experimental data is presented in Table 6-2.
Table 6-2 : Table contains Knoop micro-hardness and Modulus of Elasiticity data for
composite made of HA and Ti-6Al-4V.
Ser Load (gf)
Long Diagonal 2a (µm)
a short Diagonal 2b (µm)
b Hardenss Hk
b' Elastic Modulus E (Gpa)
1 500 92.8 46.4 15.7 7.85 826.1 6.70 88.62
2 500 93.2 46.6 14.9 7.45 819 6.72 141.61
3 500 91.1 45.55 14.4 7.2 857.2 6.57 160.15
4 500 91 45.5 15.6 7.8 859.1 6.57 81.28
5 500 91.8 45.9 14.6 7.3 843 6.62 149.58
6 500 92.5 46.25 15.5 7.75 830 6.67 94.77
7 500 91.7 45.85 14.9 7.45 845 6.62 121.24
8 500 92.3 46.15 15.4 7.7 836 6.66 97.79
9 500 91.5 45.75 14.9 7.45 849 6.60 118.92
10 500 93 46.5 15 7.5 825 6.71 129.78
Mean 92.09 46.05 15.09 7.55 838.94 6.64 118.37
Standard deviation 0.99 0.49 0.53 0.27 17.99 0.07 33.71
6.3.4 Comparison between single-layer coating and multilayer composite Only the crust layer was visible for the single layer coating and multilayer composite.
The crust observed at the composite coating (at power 1500W and traverse speed 300
mm/min) is similar to single-layer, but the thickness/height of the crust for the
composite is more than the single-layer coating. The average thickness/height of the
crust is 222 µm for the composite; but for a single-layer, the thickness/height is 91.59
µm. In both cases, the crusts are ceramic and porous in nature. The porosity of the
crust of the composite is very high of 32%, compared to the single-layer coating crust
of 7.1%.
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 123
The Ca/P ratio of the multilayer composite is higher (5.46) than the single-layer coating
(1.66). The reason behind the high Ca/P ratio for the composite is that the temperature
generated for the multilayer composite is higher because of the successive back and
forth motion of the laser beam across the coating compared to the manufacturing
process of the single-layer coating. High temperature caused more phosphate to
dissociate into phosphorous and oxygen and then more phosphorous became
vaporized, and shifted the calcium to phosphorous. The elements present in the top
section of the single-layer coating and multilayer composite are presented in tabular
form in the appendix (Table 11-6). The crust of the single layer composite contains less
iron, titanium, aluminium and vanadium but more carbon and oxygen compared to the
multilayer composite crust.
The Vickers microhardness value observed for the top section of the single-layer
coating is 534 kgf/mm2. On the other hand, the Vickers micro-hardness value for the
multilayer composite is 846 kgf/mm2 (at the top section), which is much higher than that
in the single-layer composite. This phenomenon is due to the higher cooling rate for
single-layer deposition. Fracture toughness could not be determined for the single-layer
because, compared to the multilayer composite, it was more metallic in nature and
cracks were not observed, even with higher loads. The multilayer composite is more
ceramic in nature and micro-indentations were able to produce cracks at, or over,
critical loads.
6.3.5 Comparison between pure HA and composite coating on SS Single-layer and multilayer pure HA coating was produced to identify the change in the
HA phase due to laser processing on austenitic stainless steel. X-ray diffraction was
used for the analysis. For pure HA coating, some HA was converted to ATCP and
BTCP during laser processing. The same result was found for the multilayer composite
coating, but the multilayer composite contains a more amorphous phase of HA and
titanium because of the higher temperature. A pure HA coating is shown in Figure 6-7
to contain HA, ATCP and BTCP. As the whole area of the sample was not uniformly
coated with HA, three peaks for stainless steel were detected.
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 124
Figure 6-7 : X-ray diffraction pattern of single-layer pure HA coating on stainless steel
substrate.
An X-Ray diffraction graph of a single-track and multi-track HA coating is presented in
Figure 6-8. The XRD graph shows the multi-track has prominent HA, ATCP and BTCP
peaks compared to the single track. In the graph SS stands for stainless steel and HA
for hydroxyapatite. The intensity of stainless steel (SS) peaks has been reduced for
multi-tracks compared to single HA track on the substrate.
20 30 40 50 60 70 80 900
20
40
60
80
100
120
140
160
HA
HAHA
HAHAHA
HAHA
HA
SS
SS
Inte
nsity
(Cou
nts)
Two theta (2)
SS
HA
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 125
Figure 6-8 : XRD graph of single-layer and multi-layer pure HA coating on stainless
steel substrate.
Micro-hardness and fracture toughness of laser assisted HA coating was determined
using Vickers micro-hardness test and by measuring the crack length evolved after
micro-indentation. The Vickers micro-hardness and fracture toughness of pure HA
coating is 275 + 13.66 and 1.44 + 0.41 respectively. The experimental data is
presented in appendix, at Table 11-5.
6.4 Conclusion This chapter discussed the deposition and characterization of a composite material
made from HA and Ti-6Al-4V. The characterization of the material covered
microstructural (porosity, pore size and shape), chemical (elemental organization,
phases and Ca/P ratio) and mechanical (microhardness, fracture toughness and elastic
modulus) perspectives. The composite is compared with single-layer composite coating
(made from HA and Ti-6Al-4V) and pure HA coating. The bottom of the composite is
less porous than the top. The crust (top section) of the composite is ceramic in nature
Chapter 6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE
Muhammad Rakib Mansur Page 126
and is more porous compared to the crust of the single-layer coating deposited using
the same experimental parameters. The Ca/P ratio of the composite material is higher
compared to its single-layer counterpart. The composite contains more amorphous
content compared to pure HA coating. Vickers micro-hardness and fracture toughness
of the composite reduce with the increase of depth (more on the top than the bottom).
Vickers hardness of the multilayer composite is greater compared to the single-layer
composite and the elastic modulus is slightly higher than pure Ti-6Al-4V alloy.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 127
7. TEMPERATURE AND COOLING RATE
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 128
7.1 Introduction Laser processing techniques involve complex thermal, chemical, mechanical and
metallurgical processes and the interaction between them. Laser surface processing
involves rapid heating and cooling that provides an opportunity to produce novel
materials without them being constrained by an equilibrium phase diagram [37]. The
surface chemistry of a material can be significantly improved by rapid solidification
during the laser cladding process. The increased solubility of the solute atoms as a
result of the high cooling rate can produce metastable materials [37]. So, for laser
processing, temperature and cooling rate are important pieces of information to study
the correlation among thermal, chemical, mechanical and metallurgical parameters.
Additionally, correlation of temperature and cooling rate and the after-effects of process
parameters can pave the way towards a better understanding of the interaction.
Various techniques have been adopted to determine the temperature and cooling rates
of laser materials processing. A thermal imaging technique was used by Hofmeister et
al. to measure the temperatures and cooling rates around the melt pool of AISI 316L
stainless steel and H13 tool steel processed by laser-engineered net shaping (LENS)
technique [38]. They used Rosenthal’s solution [39, 40] scaled with traverse velocity.
Ueda et al. used fused fiber-coupled infrared radiation pyrometer to measure the
temperature of a work piece made of partially stabilized zirconia (PSZ) and Al2O3
irradiated by CO2 laser [41]. They compared the experimental results with the
numerically determined results using a finite element method (FEM). Smurov et al.
used two pyrometers (one 2D and another monochromatic pyrometer) placed on the
same plane at an angle of 450, in conjunction with an infrared camera set on a plane
perpendicular to the pyrometer plane at 600 angle, to monitor brightness temperature
[42].
The development of thermal models to predict temperature or cooling rate was initially
devised for welding. Rosenthal developed a solution for temperature distribution of a
moving point source considering a semi-infinite work piece and pertinent for welding
[39]. T. W. Eagar and N. S. Tsai evaluated the effect of welding process variables on
the shape of the weld [161]. They made theoretical predictions along with experimental
verification of weld width, comparing it with different process variables of welding for
carbon steel, stainless steel, titanium and aluminium. Most of the mathematical models
used to determine temperature for laser processing were developed following the
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 129
approach adopted to solve the welding problem because of the close resemblance of
the physical phenomenon.
For laser processing, various mathematical models have been devised adopting
different solution techniques and considering particular boundary conditions suitable for
solution. M. F. Ashby and K. E. Easterling devised an analytical solution from a
governing differential equation to determine the temperature field created by a high
power laser [162]. They combined the approximate heat flow equation with kinetic
models to predict the near surface structure of plain carbon steels after laser
processing.
A thermal analysis was performed by Cline and Anthony, which considered a moving
Gaussian source at constant velocity [163]. They calculated temperature distribution,
cooling rate distribution and depth of melting, and related those parameters to the
velocity, power and laser spot size.
Manca et al. correlated the temperature distribution where the maximum mid-plane
temperature was presented as a function of Peclet number, solid thickness and width
[164]. A semi-implicit finite difference method was used by Han et al. to solve mass
momentum and the energy equation of a laser cladding process with powder injection,
considering laser-substrate, laser-powder, and powder-substrate interactions [165].
They compared the influences of powder injection on the melt pool shape, penetration
and flow pattern.
R. Jendrzejewski et al. used a computer code named nonlinear heat transfer variable
step analysis procedure (NHTVSAP) to analyse a 2-D heat transfer problem for
multilayer structures prepared by direct laser remelting of metal powders [166]. They
considered phase change and heat exchange with the environment for a given set of
boundary conditions. A comprehensive literature review was carried out by Mackwood
and Crafer, emphasizing thermal modelling and the prediction of laser welding in
metals [167]. William M. Steen and Jyotirmoy Mazumder dedicated a complete chapter
in their famous book, Laser Material Processing, to mathematical modelling and
simulation, where they have discussed different significant analytical models invented
for laser material processing [70].
For laser processing, temperature and cooling rate during the processing has a deep
impact on the microstructural, chemical and mechanical properties of a deposited
material. Two configurations have been used to perform temperature measurement. In
one configuration, temperature and cooling rate was determined using a two colour
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 130
infrared pyrometer for Nd:YAG Laser treated AISI 304L austenitic stainless steel. In
another configuration, temperature and cooling rate were determined using contact
pyrometers fixed at the bottom of the AISI 316L austenitic stainless steel substrate.
The temperature profile was recorded and analysed to find out peak temperature and
cooling rate. Two analytical models were used to determine temperature and cooling
rate numerically for prediction and evaluation against the obtained experimental values.
7.2 Experimental plan for configuration one A Nd:YAG laser centre (manufactured by Precision Optical Manufacturing, Michigan,
USA) which comprises a Nd:YAG laser of maximum power 2.3 kW, was used for the
process. Substrate coupons of single-phase austenitic stainless steel (AISI 304L; 18 Cr
- 14 Ni - 2.4 Mo) with dimension 30 mm X 30 mm X 2 mm were used. The substrates
were sand blasted using 80 mesh size garnet sand and cleaned using compressed air
before processing. Substrate coupons were tightly clamped with the substrate holder to
avoid bending or buckling during the process.
The two-colour pyrometer was coupled to the LASCON controller using a fibre optic
cable. The set-up used for the measurement was procured from Dr Margenthaler
GmBH & Co.KG in Neu-Ulm, Germany (a specialized company in the field of
pyrometry). It measured the highest temperature in the field of-view of the pyrometer.
The pyrometer and the powder nozzle were placed 200 mm and 11 mm apart,
respectively, from the substrate. The schematic and the experimental set-up are
presented in Figure 7-1.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 131
Figure 7-1 : (a) Schematic of the experimental set-up and (b) Experimental
arrangement for measuring temperature using pyrometer.
The pyrometer had a built-in laser to focus it on the substrate. The pyrometer was
focused on the middle of the substrate in such a way that the laser passed through the
pyrometer focus during the process. Five power settings (100W, 150W, 200W, 250W,
300W) along with three traverse speed or scan rate (120 mm/min, 240 mm/min, 360
mm/min) were used for the experiment. In total, 15 runs were performed. The diameter
of the laser beam was fixed to 3 mm during the experiment.
7.3 Results and discussion for configuration one Two analytical models were adopted to evaluate temperature and cooling rate. The first
model followed that of Ashby and Easterling [162] where the second was the Cline and
Anthony model [163]. The equation derived by Ashby and Easterling to determine peak
temperature is:
[ ( )] (( ))
)⁄ 7-1
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 132
Where T0 = initial temperature, A = absorptivity of the surface, t = time, t0 = the time for
heat to diffuse over a distance equal to the radius of the beam (rb), so t0 = rb2/4α, λ =
thermal conductivity (J s-1m-1K-1), α = the thermal diffusivity (m2s-1), q = laser power
(W), v = tracking velocity or traverser speed (m/s) for continuous laser, Z = depth below
surface (m), J1 = correction factor, and Z0 = distance over which heat can diffuse during
the beam interaction time (rb / v).
Since the temperature on the surface is calculated, Z is considered as zero. The time t
is considered as half of the radius of the laser beam divided by velocity. The correction
factor J1 is considered as 1 [162]. Absorptivity of the surface was estimated as 0.8
using a trial and error method and by equating the relationship with the experimental
value of the experimentally obtained temperature.
Another mathematical model developed by H. E. Cline and T. R. Anthony [163] to
determine temperature is:
( )
( ) 7-2
Where P = power (W), Cp = specific heat per unit volume (W / cm2 0C), D = thermal
diffusivity (cm2/s), V = velocity of scanning laser beam in x direction (cm/s), r = distance
from source, R = radius of laser beam (cm), f = distribution function and the value of f is
determined from the literature [163], depending upon the RV/D value. For Cline and
Anthony model the units used for distance and length are in cm.
In the experiment, all the parameters were constant except power and traverse speed.
The two parameters affecting temperature were power and traverse speed. The
distribution function interpreted from the literature [163] is f = 0.15, 0.13 and 0.115 for V
= 120 mm/min, 240 mm/min and 360 mm/min.
The required data that are related to thermal properties of materials were collected
from the MatWeb [65] website and are presented in Table 7-1.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 133
Table 7-1: Material data used for calculation (for AISI 304L austenitic stainless steel)
[65]
Density, ρ 8000.00 kg/m3
Specific heat capacity, C 500.00 J / kg.0C
Thermal conductivity, λ 16.20 W/m.0C
Thermal diffusivity, α 0.00000405 m2/s
The variation of power and traverse speed affects the specific energy. The relationship
between specific energy, power, beam diameter and traverse speed is given below
[150]:
(
)
(
)
(
) ( )
7-3
This relationship is used to calculate specific energy and to show the relationship
between temperature, cooling rate and specific energy.
The measured temperature and the calculated temperature using both the models are
presented in Table 7-2.
The data plotted in Figure 7-2 shows the variation of temperature with power
determined experimentally and by using the Ashby-Easterling model for different
traverse speeds. The graph shows that calculated temperature and the measured
values are in good agreement.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 134
Figure 7-2 : Variation of temperature with power at different traverse speed.
Table 7-2 : Temperature presented in tabular form.
Laser
Power
(W)
Scan
Rate
(mm
/min)
Specific
Energy
(J/mm2)
Temperature
(0C)
(Experim-
-ental)
Temperature
(0C)
(Numerical,
Ashby)
Difference
(Exp –
Num,Ash)
Temperature
(0C)
(Numerical,
Cline)
Difference
(Exp –
Num,Cline)
100 120 17 945 917 28 617 328 150 120 25 1326 1365 -39 926 400 200 120 33 1810 1790 20 1235 575 250 120 42 2420 2238 182 1543 877 300 120 50 2575 2686 -111 1852 723 100 240 8 1080 758 322 535 545 150 240 13 1187 1127 60 802 385 200 240 17 1635 1495 140 1070 565 250 240 21 2025 1863 162 1337 688 300 240 25 2175 2231 -56 1605 570 100 360 6 - 651 - 473 - 150 360 8 798 966 -168 710 88 200 360 11 1316 1281 35 947 369 250 360 14 1765 1595 170 1183 582 300 360 17 1950 1910 40 1420 40
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 135
Experimentally observed temperature along with analytically (numerically) calculated
temperature values are presented in Table 7-2. The data plotted in Figure 7-2 shows
the variation of temperature with power determined experimentally and by using the
Ashby-Easterling model for different traverse speeds. The graph shows calculated
temperature and the measured values are in sensible agreement with each other.
Calculated temperatures using both the models were plotted as a power versus
temperature graph in Figure 7-3. The graph shows the temperature is increasing
linearly with an increase in power for different traverse speed.
Analytical values obtained from the Ashby and Easterling model are in sensible
agreement with the experimental values and are considered as a better predictor,
compared to the Cline and Anthony model. Figure 7-3 shows that, for a small amount
of power, the Cline and Anthony model yields temperature values close to the Ashby
and Easterling model, but the line deviates more as the power increases.
Figure 7-3 : Variation of temperature with power for different traverse speed.
The heating and cooling curve has been recorded by the two-colour pyrometer. The
measurement range of the pyrometer is from 422 0C to 1475 0C. The temperatures
exceeding the range have been calculated by extrapolating the heating and the cooling
curve obtained from the pyrometer reading.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 136
The measured temperature data is plotted against time for power 150 W and scan rate
360 mm/min in Figure 7-4. In the figure, the rising curve depicts the heating curve; after
reaching the top, the curve climbs down, which resembles the cooling curve. The peak
temperature is achieved when the laser coincides with the pyrometer focus.
Figure 7-4 : Temperature versus time to obtain the heating and cooling curve.
The cooling rate is the thermal gradient in the direction of motion [163], and is
described by the relationship:
7-4
The experimental cooling rate is calculated from the gradient of the cooling curve
obtained from the pyrometer reading.
Like temperature, the analytical cooling rate is also determined by using two models.
The formula for cooling rate determined by the Ashby and Easterling model is:
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 137
( )
7-5
Where T0 = initial temperature, T = peak temperature, A = absorptivity of the surface, t
= time, λ = thermal conductivity (J s-1 m-10C-1), α = the thermal diffusivity (m2s-1), q =
laser power (W), and v = tracking velocity (m/s) for continuous laser.
According to Cline and Anthony the complete equation for the determination of cooling
rate is [163]:
[
(
)]
7-6
Where T = temperature (oC), t = time (sec), V = velocity of scanning laser beam in x
direction (cm/s), x = direction of motion of laser, r = distance from source, and D =
thermal diffusivity (cm2/s).
The cooling rate, calculated using the Ashby-Easterling and Cline-Anthony equations is
presented in Figure 7-5 and compared with the experimental cooling rate.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 138
Table 7-3 : Cooling rate in tabular form.
Laser
Power
(W)
Scan
Rate
(mm/min)
Specific
Energy
(J/mm2)
Cooling
Rate
(0C/s)
(Experim
ental)
Cooling
Rate
(0C/s)
(Numerical
, Ashby)
Difference
(Exp –
Num,Ashby)
Cooling
Rate (0C/s)
(Numerical,
Cline)
Difference
(Exp –
Num,Cline)
100 120 17 -934 -680 -254 -305 -629 150 120 25 -962 -1021 59 -457 -505 200 120 33 -1274 -1361 87 -610 -664 250 120 42 -1838 -1701 -137 -762 -1076 300 120 50 -1985 -2041 56 -914 -1071 100 240 8 -1367 -951 -416 -264 -1103 150 240 13 -1568 -1426 -142 -396 -1172 200 240 17 -1883 -1901 18 -528 -1355 250 240 21 -2808 -2377 -431 -660 -2148 300 240 25 -3171 -2852 -319 -793 -2378 100 360 6
-1068 1068 -234 234
150 360 8 -1832 -1601 -231 -351 -1481 200 360 11 -2741 -2135 -606 -467 -2274 250 360 14 -2827 -2669 -158 -584 -2243 300 360 17 -3430 -3203 -227 -701 -2729
The change of cooling rate with the change of power for different traverse speed is
presented in Figure 7-5. The numerical cooling rate represented in the graph is
calculated using the Ashby-Easterling equation. Both numerical and experimental
values are in good agreement for traverse speed 120 mm/min, but only in reasonable
agreement for traverse speeds of 240 mm/min and 360 mm/min.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 139
Figure 7-5 : Cooling rate V power for experimental and analytical results.
The cooling rate calculated by the Ashby-Easterling and Cline-Anthony models is
plotted in Figure 7-6 for different traverse speeds. The graph reveals a large difference
between the two lines. It is also noticeable that, in the Ashby-Easterling model, the
gradient of the line increases with an increase in traverse speed. But the gradient of
the line decreases with the increase in traverse speed for the cooling rates calculated
by The Cline and Anthony model. Thus, the experimental trend supports the Ashby and
Easterling model (Figure 7-5).
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 140
Figure 7-6 : Variation of cooling rate with power calculated by different models for
different traverse speeds.
The beam diameter was constant (3 mm). The two main parameters affecting specific
energy are power and traverse speed and therefore cooling rate and temperature
varies with the specific energy. A contour map (Figure 7-7) was drawn to show the
variation of temperature and cooling rate with traverse speed and power.
The Ashby and Easterling model is a better predictor (than Cline and Anthony model)
of the temperature and cooling rate when compared with experimental values. The
contour map has been created using the data obtained from the Ashby and Easterling
model. According to the graph, the maximum temperature zone is between power 272
W to 175 mm/min traverse speed (Figure 7-7 a) and the maximum cooling rate zone
(Figure 7-7b) is between power 270 W to 260 mm/min traverse speed. The
temperature is higher for high power and low traverse speed, and the cooling rate is
higher for relatively low power and traverse speed. The graph provides an idea of the
temperature and cooling rate zone based upon power and traverse speed.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 141
Figure 7-7 : (a) Variation of temperature with power and traverse speed (b) variation of
cooling rate with power and traverse speed.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 142
7.4 Experimental set-up for configuration two Substrate discs of single-phase austenitic stainless steel (AISI 316L; 18Cr-14Ni-2.4Mo)
with a diameter of 80 mm and 10 mm thick were used for this experiment. Five
4+0.1 mm diameter holes were drilled (each 10 mm apart) at the bottom of the
substrate to fit five thermocouples.
The thermocouples used for the experiment were procured from Omega Engineering
Inc., USA. The thermocouples used for the experiment were K-type thermocouples,
having part number CHAL – 032, diameter 0.81 mm and maximum service temperature
of 982 oC. As the thermocouples were used beneath the substrate, and the distance
between the surface and thermocouple was 8 mm, there was no chance of exceeding
the temperature range.
A ceramic tube made of zirconium was used to protect the thermocouple wires from
short circuit. The tube measured 4 mm in diameter and had two cylindrical holes of 1
mm diameter in which to place the thermocouple wires. The two wires of the
thermocouple were fitted inside the two holes of the tube.
Figure 7-8 : Picture of the (a) experimental setup (b) schematic diagram of substrate
disk fitted with thermocouples (c) substrate disk inserted with thermocouples and k-
type connectors.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 143
High temperature cement was used for heat conductivity and thermal shock resistance.
The cement also acts as an electrical insulator. The cement was also procured from
Omega. The cement contained two parts, one was powder filler (crystalline silica) and
the other was liquid binder (sodium silicate solution). Powder filler was mixed with the
binder following the ratio mentioned in the user manual and then applied.
The cement was applied inside the drilled hole on the bottom of the substrate. Then the
ceramic tube fitted with the thermocouple was inserted into the drilled hole in such a
way that the thermocouple placed inside the hole at the bottom of the substrate
maintained the appropriate distance. The distance maintained between the top surface
of the substrate and the thermocouple was 8 mm. After fixing the thermocouples with
the substrate, sufficient time (24 hrs.) was given for complete curing of the cement.
Two thermocouples were connected with the data acquisition module (data logger),
using K type connectors and thermocouple wire, and both were connected with the
data acquisition module to collect data simultaneously. Substrates were sand-blasted
and cleaned using compressed air before deposition.
The experimental set-up along with the schematic diagram is presented in Figure 7-8,
showing laser head, substrate and the connectors of the thermocouples. The process
parameters used for the experiment is presented in Table 7-4. The balance and nozzle
gases (both are Ar) flow rate maintained throughout the experiment are 5 SLPM and
25 SLPM respectively.
Table 7-4 : Parameters used for temperature determination.
Experimental runs
Power (W) Traverse speed (mm/min)
1 1500 120
2 1500 300
3 1000 300
4 500 300
5 200 300
7.5 Results and discussion for configuration two A different approach was adopted to determine temperature for the second
configuration. Thermocouples were used instead of the two-colour pyrometer. There
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 144
are two main reasons behind this approach: the first reason is the lack of suitable
optics for the two-colour pyrometer so that it can be used for CO2 laser, and the second
reason is to validate the temperature using a direct contact method. This provides
some comparative idea about the accuracy and applicability of the non-contact and
contact measurements.
For estimating temperature and cooling rate, two analytical models were adopted to
evaluate temperature and cooling rate. The first one was the Ashby and Easterling
[162] model and the second one was the transient solution described by J.P. Holman
[168]. The same calculation procedure was adopted for the Ashby and Easterling
model, which has been described in the first paragraph of section 7.3. The major
material data used for the calculation is provided below in Table 7-5.
Table 7-5 : Material data used for calculation (for AISI 316L austenitic stainless
steel)[66].
Density, ρ 8000.00 kg/m3
Specific heat capacity, C 500.00 J/kg.0C
Thermal conductivity, λ 15.9 W/m.0C
Themal diffusivity, α 0.00000398 m2/s
Figure 7-9 represent the heating and cooling curves determined by two thermocouples
placed underneath the substrate for 120 mm/min traverse speed and 1500 W power.
The linear distance from the laser heat source interacting on the surface of the
substrate to the thermocouple 1 is 8 mm and to the thermocouple 2 is 12.81 mm. Data
has been smoothed by using the Savitzky - Golay method to reduce noise, using
Origin 9.
From Figure 7-9 it is evident that the heat flux reached thermocouple 1 prior to
thermocouple 2, which was placed farther away, and the intensity of the heat flux
recorded is also less for thermocouple 2 because of the reduced heat flux due to
diffusion of heat.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 145
Figure 7-9 : Temperature plotted against time measured by thermocouple 1 and 2 for
run 1 placed underneath the substrate.
The temperature record for 120 mm/min and 300 mm/min traverse speeds at 1500 W
power is presented in Figure 7-10. As the traverse speed is different when the
temperature recording started, some heat flux already diffused to the thermocouple for
slower traverse speed (120 mm/min). Thus an increased initial temperature was
recorded by the thermocouple and showed a difference between the starting values of
the measured temperatures. The peak for 120 mm/min traverse speed shifted towards
the right compared to the peak observed for 300 mm/min. This is mainly because, the
heat flux reaches its maximum more quickly compared than when there is a slower
traverse speed.
0 5 10 15 20 25 30 35 40 45 50 55 60 650
20
40
60
80
100
120
140
160
180
Tem
pera
ture
(0 C)
Time (sec)
Thermocouple 1 Thermocouple 2
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 146
Figure 7-10 : Temperature measured by thermocouples plotted against time for two
different traverse speeds at 1500 W power.
Temperature determined experimentally using thermocouples for traverse speed 120
mm/min is presented in Table 7-6. Estimated temperatures calculated by the Ashby
model and the accompanying transient relationships are also presented in the same
table. Experimental and estimated temperatures for 300 mm/min are presented in
Table 7-7. The transient relationship presented in the heat transfer book by J. P.
Holman is [168]:
[
√( )]] (
)
7-7
Where Ti = initial temperature, A= area, ρ = density, τ = time, c = specific heat, Q0 =
heat, and α = thermal diffusivity.
0 5 10 15 20 25 30 35 40 45 50 55 60 650
20
40
60
80
100
120
140
160
180
Tem
pera
ture
(0 C)
Time (sec)
300 mm/min 120 mm/min
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 147
Table 7-6 : Experimental and estimated temperatures at 8 mm depth and on the
surface for 120 mm/min.
Power Temperature at 8 mm depth Estimated temperature on the surface
Transient Ashby model
Experimental Transient Ashby model
200 38 33 37 1478 1489
500 66 49 92 3694 3689
1000 112 76 128 7388 7357
1500 157 102 153 11082 11024
Table 7-7 : Experimental and estimated temperatures at 8 mm depth and on the
surface for 300 mm/min.
Power Temperature at 8 mm depth Estimated temperature on the surface
Transient Ashby model
Experimental Transient Ashby model
200 27 26 25 591 593
500 38 33 45 1478 1450
1000 57 43 63 2955 2878
1500 75 54 68 4433 4306
Temperatures evaluated by the Ashby and Easterling model and transient formulae for
traverse speeds 300 mm/min and 120 mm/min were determined at 8 mm beneath the
surface. The experimentally measured values are in good agreement with the transient
solution compared to the solution specified in the Ashby model. The relationship is
evident in Figure 7-11. For both traverse speeds, the same trends have been
observed.
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 148
Figure 7-11 : Relationship between temperature and power evaluated by transient
Ashby and relationship along with experimental results for traverse speed 120 mm/min
(top three lines) and 300 mm/min (bottom three lines) determined at 8 mm beneath the
surface.
However, temperature estimated on the top surface of the substrate by a transient
solution and Ashby model are in good agreement with one another (Table 7-6 and
Table 7-7). In particular, the estimated temperature values for 120 mm/min (Table 7-7)
have very little difference between them.
Cooling rate depends on power and also on traverse speed. Cooling rate increases
with power in a linear fashion. The trend is shown in Figure 7-12 and the data is
represented in Table 7-8. The gradient of the slope (cooling rate slope) is greater (in
respect to x axis) for less traverse speed (Figure 7-12).
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 149
Table 7-8 : Table representing surface temperature and cooling rate calculated using
Ashby model.
Power Traverse speed (120 mm/min) Traverse speed (300 mm/min)
Temperature (0C)
Cooling rate (0C/s)
Temperature (0C)
Cooling rate (0C/s)
200 1489 -6142 593 -2328 500 3689 -15355 1450 -5820
1000 7357 -30711 2878 -11641 1500 11024 -46066 4306 -17461
Figure 7-12 : Change of cooling rate with power at different traverse speed.
A contour map (Figure 7-13) shows the variation of temperature and cooling rate with
traverse speed and power. The contour map has been created using data obtained
from the Ashby and Easterling model. According to the graph, the maximum
temperature zone is between 1320 W power, 120 mm/min traverse speed to (Figure
7-13a) 1500 W power and 155 mm/min traverse speed. Additionally, the maximum
cooling rate is estimated between 1325 W power, 120 mm/min traverse speed to
(Figure 7-13b) 1500 W power and 154 mm/min traverse speed. The temperature and
0 250 500 750 1000 1250 1500 1750 2000-50000
-45000
-40000
-35000
-30000
-25000
-20000
-15000
-10000
-5000
0
Coo
ling
rate
(0 C/s
ec)
Power (W)
120 mm/min 300 mm/min
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 150
cooling rate zones are almost superimposed with slight variation for the 1.8 mm beam
diameter
Figure 7-13 : (a) Variation of temperature with power and traverse speed (b) Change of
cooling rate with power and traverse speed
Chapter 7. TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 151
7.6 Conclusion Two techniques have been used to determine the temperature evolved during laser
processing of the substrate. One is a non-contact type, where a two-colour pyrometer
is used to measure temperature. The technique was adopted for Nd:YAG laser
processing. The other technique is contact type, where thermocouples have been used
to measure temperature underneath the substrate. This technique was developed for
CO2 laser assisted DMD processing.
Two mathematical models were used to evaluate surface temperature for Nd:YAG
laser processing. These models are the Ashby–Easterling and Cline–Anthony models.
Experimental results showed that the Ashby–Easterling model predicted surface
temperature better in comparison with the Cline–Anthony model.
For CO2 laser-assisted deposition, an indirect approach was taken because it was not
possible to measure temperature by placing thermocouples directly underneath the
laser, since they may melt. The thermocouples were placed underneath the substrate
at a suitable distance so that temperatures could be measured without destroying the
thermocouples. Then temperature was estimated using a transient heat transfer
equation and the Ashby–Easterling model.
For estimating surface temperature, the Ashby-Easterling model provided a good result
for both cases. It was possible to calculate absorptivity of the substrate by trial and
error using the Ashby-Easterling model and experimentally obtained temperature
values. The estimation yielded the following: absorptivity value 0.8 for Nd:YAG and
0.35 for CO2 laser. The reason behind this difference is the wavelength associated with
laser types. Nd:YAG and CO2 continuous wave lasers have wavelengths 1.06 µm and
10.6 µm, respectively. For steel, reflectivity increases with increase in wavelength; this
means absorptivity decreases with the increase in wavelength.
Relationships have been established for both types of laser for temperature and
cooling rate with power and traverse speed. This will help to estimate the temperature
and cooling rate within the processing window. From the experiment, it is evident that
Ashby-Easterling model can be employed for predicting temperature and cooling rate
beyond this experimental window, using the absorptivity values for this particular
material and surface condition.
Chapter 8. EFFECT OF TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 152
8. EFFECT OF TEMPERATURE AND COOLING RATE
Chapter 8. EFFECT OF TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 153
8.1 Introduction The temperature and cooling rate plays an important role in solute distribution that
ultimately influence the microstructural, chemical and mechanical properties of the
material and coating. The maximum temperature attains at the centre underneath the
laser beam and decreases radially outwards. The variation of surface temperature
propel the molten material radially outwards. The radially outward flow of the molten
material approaches the edge of the melt pool then goes down and turns around. The
flow then moves back to the centre and complete the recirculation. Because of the
recirculating pattern of the flow, molten particles travel a long path before they freeze
and this increases the chance of forming uniform composition.
The fineness of microstructure depends upon cooling rate. Fine microstructure is
expected with a higher cooling rate. In the melt pool, the cooling rate decreases from
maximum to minimum from centreline towards the edge, similarly from surface to the
bottom [70].
This chapter discusses the relationship between different properties and temperature
and cooling rate. Coatings developed by CO2 laser were characterized in chapter 5.
Different properties, like surface average roughness and diffusion coefficient, which
were determined in that section, were correlated with temperature. A relationship was
found between micro-hardness and cooling rate.
8.2 Relationship for CO2 laser-assisted coating
The experimental procedure and parameters used for the deposition of HA and
Ti-6Al-4V composite coating on austenitic stainless steel using CO2 laser are described
in chapter 4 in section 4.2.1. Temperature and cooling rate were determined
experimentally by using thermocouples for CO2 laser-assisted deposition, which is
described in chapter 7 in sections 7.4 and 7.5. Ashby-Easterling model was found
suitable for better approximation of temperature and cooling rate. Temperature and
cooling rate for the performed experiment, along with microstructural parameters, are
presented in Table 8-1 and Table 8-2. Microstructural parameters presented in the
tables were collected and compiled from chapter 5. It was observed that microstructural
parameters varied linearly with temperature and cooling rate. Temperature has more
Chapter 8. EFFECT OF TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 154
effect on the microstructural change compared to cooling rate for the parameters
mentioned in the Table 8-1 and Table 8-2.
Table 8-1 : Height, width and aspect ratio of the coatings.
Power (W)
Traverse speed
(mm/min)
Temperature (0C)
Cooling rate
(0C/s)
Height of the
coating (µm)
Width of the
coating (µm)
Aspect ratio
(Width/ height)
1500 120 11024 -46066 2250 3362 1.49
1500 300 4306 -17461 1273 3020 2.37
1000 300 2878 -11641 629 2795 4.44
500 300 1450 -5820 525 2043 3.89
200 300 593 -2328 145 858 5.92
Table 8-2 : Crust height, angle of the coating and height of the HAZ.
Power (W)
Traverse speed
(mm/min)
Temperature (0C)
Cooling rate
(0C/s)
Crust Height (µm)
Angle of the
coating (0)
Height of HAZ
1500 120 11024 -46066 157.02 19.39 2093
1500 300 4306 -17461 91.59 10.22 1182
1000 300 2878 -11641 81.97 7.5 547
500 300 1450 -5820 70.69 5.43 455
200 300 593 -2328 68.22 3.77 77
It has been found that the height and the width of the coating increased linearly with the
temperature, but the aspect ratio (width/height) decreased. The observation is;
increased temperature causes more feedstock materials to melt and thus increases the
heights rapidly than the increase in width. The variation of aspect ratio and the angle of
the coating with temperature are presented in Figure 8-1.
Chapter 8. EFFECT OF TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 155
Figure 8-1 : The change of aspect ratio and angle of the coating with temperature.
Ahsan et al. have shown that surface roughness of laser deposited Ti-6Al-4V coating
changes with mass flow rate of the supplied powder [169]. Temperatures also have a
strong effect on the surface property of deposited coating. Surface roughness has been
measured for the developed coating and presented in chapter 5. Roughness average is
plotted against temperature in Figure 8-2, where it is demonstrated that with the
increase in temperature, the average roughness of the surface decreases up to a
certain limit then increases again. The minimum roughness was attained at 2878 oC.
The probable reason for the increase of roughness after a certain threshold could be
the increased turbulence and bubble formation due to excessive temperature.
Chapter 8. EFFECT OF TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 156
Figure 8-2 : Roughness average V temperature. Here n = number of roughness
measurements carried on each trial/run.
Cooling rate has an impact on the hardness of the material. Slower cooling rates yield
less Vickers micro-hardness compared to higher cooling rates (Figure 8-3); however
after a certain limit of cooling rate, the hardness does not change significantly.
Figure 8-3 : Variation of micro-hardness with cooling rate.
Chapter 8. EFFECT OF TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 157
8.3 Relationship for Nd:YAG laser The experimental procedure and parameters used for the deposition of HA and Ti-6Al-
4V composite coating on austenitic stainless steel is described in chapter 4 in section
4.2.3. A Two-colour pyrometer was used to determine temperature and cooling rate
experimentally for Nd:YAG laser-assisted deposition, which is described in chapter 7 in
section 7.2 and 7.3. For both CO2 and Nd:YAG laser-assisted deposition, the Ashby
and Easterling model was found suitable for better approximation of temperature and
cooling rate. As temperature and cooling rate for all the experimental runs used for
developed coatings was not determined experimentally, the Ashby and Easterling
model was used to calculate all the values. Temperature and cooling rate for
corresponding power and traverse speed is presented in Table 8-3.
Table 8-3 : Temperature and cooling rate for the experimental runs.
Exp Run
Power (W)
Traverse speed
(mm / min)
Traverse speed
(mm / sec)
Beam Ø (mm)
Specific energy (J/mm2)
Temperature Cooling Rate
1 150 150 2.5 3 20.00 1322 -685
2 250 120 2 3 41.67 2260 -807
3 350 120 2 3 58.33 3155.00 -1129.00
4 300 120 2 3 50.00 2708.00 -968.00
5 400 120 2 3 66.67 3603.00 -1290.00
6 400 240 4 3 33.33 3199.00 -3605.00
7 400 180 3 3 44.44 3383.00 -2401.00
The composite coating produced by Nd:YAG laser-assisted deposition was observed
under optical microscope (LEICA MEF4M) for the determination of microstructural
parameters. The parameters determined are presented on Table 8-4. Experimental run
01 produced no coatings because of the low temperature. Experimental run 02
produced temperature more than the melting temperature of the substrate and the
alloy, but did not develop significant coating thickness. Temperatures more than 2700 0C were able to produce significant coating thickness and width.
Chapter 8. EFFECT OF TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 158
Table 8-4 : Microstructural parameters for Nd:YAG laser-deposited composite coating.
Exp Run
Temperature Cooling Rate Height of the coating
(µm)
Width of the coating
(µm)
Aspect ratio
(Width/ height)
Angle of the coating
(0)
1 1322 -685 - - - -
2 2260 -807 - - - -
3 3155.00 -1129.00 867.3 1510 1.74 74.72
4 2708.00 -968.00 683.3 1355.6 1.98 66.83
5 3603.00 -1290.00 1925.8 2448 1.27 60.57
6 3199.00 -3605.00 524.8 1515.6 2.89 43.59
7 3383.00 -2401.00 792.4 1928.2 2.43 48.86
The width and height of the developed coating was found at linear increment with the
temperatures produced. The relationship is shown in Figure 8-4, but no significant
relationship was achieved in between Vickers micro-hardness and the cooling rate
(Figure 8-5). This is mainly because of less variation in terms of cooling rate.
Figure 8-4 : Variation of coating height and width with temperature.
Chapter 8. EFFECT OF TEMPERATURE AND COOLING RATE
Muhammad Rakib Mansur Page 159
Figure 8-5 : Vickers micro-hardness and cooling rate.
8.4 Conclusion After analysis, it was found that the microstructural parameters, such as height and
width of the coating, is dependent on temperature for CO2 and Nd:YAG laser-assisted
coating. Microhardness is dependent on cooling rate. Greater hardness is achieved
above a certain threshold of cooling rate. Above the threshold, the change in
microhardness is less drastic.
The roughness average of the surface decreases with the increase in temperature, but
only up to a certain limit, then it increases again. The minimum roughness was attained
at 2878 oC. The probable reason for the increase of roughness over a certain threshold
could be the increased turbulence and bubble formation due to excessive temperature.
Chapter 9. CONCLUSION AND FUTURE SCOPE
Muhammad Rakib Mansur Page 160
9. CONCLUSION AND FUTURE SCOPE
Chapter 9. CONCLUSION AND FUTURE SCOPE
Muhammad Rakib Mansur Page 161
9.1 Conclusion Composite coatings were developed from CaP and Ti-6Al-4V using CO2 and Nd:YAG
laser. For CO2 laser, direct material deposition technique was adopted. A
comprehensive morphological, microstructural and chemical analysis of feedstock
material and substrate were carried out using SEM, EDS, Raman microscopy and
XRD. The study yielded the particle size, shape and morphology of the powders. The
microstructural information included feret diameter, aspect ratio, roundness, circularity,
and major and minor diameter. Raman microscopy and XRD analysis identified the
chemical phases present in the feedstock. Both the techniques confirmed the presence
of crystalline HA in powders procured from Sigma Aldrich and Plasma Biotal. As was
expected, thermal change can transform one form of calcium phosphate to another,
Raman microscopy and XRD was used to perform a comparative study of major types
of calcium phosphate. EDS revealed that elements present (in wt.%) in the feedstock
material and thus thereby the calculation of the Ca/P ratio for HA powders.
The research also discussed about the Raman microscopic and XRD results of other
major forms of CaP, which included ATCP, BTCP and ACP. Rietveld analysis of both
types of HA powders provided information regarding the lattice parameters of unit cells.
The lattice parameter a is slightly bigger for HA SA but c is smaller compared to the
standard. The lattice parameter (a and c) for HA PB yielded the same value as the
standard.
A coating from CaP and Ti-6Al-4V alloy was achieved on austenitic stainless steel
substrates using a closed-loop optical feedback direct material deposition system.
Experimental results show that the surface profile greatly depends on power. In our
experimental range, the optimum power level to produce minimum roughness was
1000 W. Porosity decreased with the increase in power and the minimum amount of
porosity was achieved at 1,500 W power.
The generated single-layer coating had two distinct layers; top layer was ceramic in
nature. From the research, it is clear that the major factors (traverse speed, power and
gas flow rate) studied in this experiment is affected surface morphology. Traverse
speed significantly changed surface morphology, compared to power. The reason is
that traverse speed has more effect on the distribution of heat and cooling rate. It was
observed that gas flow rate is contributes toward a structured and crystalline-looking
surface.
Chapter 9. CONCLUSION AND FUTURE SCOPE
Muhammad Rakib Mansur Page 162
The study reveals that generated coating has two distinct layers; the top layer is
ceramic in nature and the bottom is HAZ. Diffusion of titanium into iron occurred in the
HAZ. The change of traverse speed and power ultimately changes specific energy,
which is responsible for the change in surface morphology and microstructure. The
higher the specific energy, the more changes were observed on the coating
microstructure in terms of crust height, aspect ratio, angle of the coating and depth of
HAZ. Nozzle gas flow rate contributed towards a structured and crystalline-looking
surface at the top.
Inside the crust of the coating wt. % of calcium, phosphorous, oxygen, and aluminium
varied with specific energy. Ca/P ratio was also dependent on specific energy.
Significant change in the Ca/P ratio occurred with the change in traverse speed (from
run 01 to run 02). The Ca/P ratio (1.66) nearest to that of human bone was obtained for
experimental run 02, with traverse speed 300 mm/min and power 1500 W. The
concentration profile of titanium inside iron pointed towards a non-steady state
diffusion. The diffusion coefficient of Ti inside iron was determined. The diffusion trend
showed linear variation with specific energy. The microhardness of the HAZ was
determined, and the top section of the coating yielded more hardness than the bottom
section.
A composite material was made from HA and Ti-6Al-4V using DMD. The
characterization of the material covered microstructural (porosity, pore size and shape),
chemical (elemental organization, phases and Ca/P ratio) and mechanical
(microhardness, fracture toughness and elastic modulus) perspective. The composite
was compared with a single-layer composite coating (made from HA and Ti-6Al-4V)
and a pure HA coating. The bottom of the composite was less porous than the top. The
crust (top section) of the composite was ceramic in nature and was more porous
compared to the crust of the single layer coating, deposited using the same
experimental parameters. The Ca/P ratio of the composite material was higher
compared to its single layer counterpart. The composite contained more amorphous
content compared to pure HA coating. Vickers micro-hardness and fracture toughness
of the composite reduced with the increase in depth (greater on top compared to
bottom). Vickers micro-hardness of the multilayer composite was greater compared to
the single layer. The elastic modulus of the composite material was found to be slightly
higher than the pure Ti-6Al-4V alloy.
Two techniques were used to determine the temperature evolved during laser
processing of the substrate. One was a non-contact type, where two-colour pyrometer
Chapter 9. CONCLUSION AND FUTURE SCOPE
Muhammad Rakib Mansur Page 163
was used to measure temperature. The technique was adopted for Nd:YAG laser
processing. The other technique was a contact type, where thermocouples were used
to measure temperature underneath the substrate; this technique was applied for CO2
laser-assisted DMD processing.
Two mathematical models were used to evaluate surface temperature for Nd:YAG
laser processing the Ashby–Easterling and the Cline–Anthony model. Experimental
results showed that Ashby–Easterling model predicted surface temperature well
compared to Cline–Anthony model.
For CO2 laser-assisted deposition, an indirect approach was taken because of the
contact method. It was not possible to measure temperature by placing thermocouples
directly underneath the laser, because of the high possibility of melting (especially with
high specific energy). The thermocouples were placed underneath the substrate at a
suitable distance so that temperatures could be measured without destroying the
thermocouples. Then temperature was estimated using a transient heat transfer
equation and the Ashby–Easterling model.
For estimating surface temperature, the Ashby–Easterling model yielded a good result
for both cases. It was possible to determine absorptivity of the substrate by trial and
error method using the Ashby-Easterling model and experimentally obtained
temperature values. The estimation yielded absorptivity value 0.8 for Nd:YAG and 0.35
for CO2 laser. The reason behind this difference is the wavelength associated with
laser types. Nd:YAG and CO2 continuous wave lasers have wavelengths 1.06 µm and
10.6 µm, respectively. For steel, reflectivity increases with the increase in wavelength.
This means absorptivity decreases with the increase in wavelength.
Relationships were established, for both types of laser, for temperature and cooling
rate with power and traverse speed. This assisted the estimation of temperature and
cooling rate within the processing window. From the experiment, it is evident that the
Ashby and Easterling model can be used for predicting temperature and cooling rate
beyond this experimental window, using the calculated absorptivity values.
It was found that the microstructural parameters, such as height and width of the
coating, is dependent on temperature for CO2 and Nd:YAG laser-assisted coating.
Microhardness is dependent on cooling rate. Greater hardness is achieved above a
certain threshold of cooling rate. Above the threshold, the change in microhardness is
less drastic.
Chapter 9. CONCLUSION AND FUTURE SCOPE
Muhammad Rakib Mansur Page 164
9.2 Contribution to new knowledge The following discoveries from the present study are considered to be novel:
Deposition of composite coatings (from HA and Ti-6Al-4V) on austenitic
stainless steel by using laser-assisted deposition technique.
Development of a composite material (from HA and Ti-6Al-4V) which can be
used as a potential bone-graft application.
Microstructural, chemical and mechanical characterization of the developed
composite coating and material.
Determination and evaluation of temperature and cooling rate for different
process parameters.
Establishment of relationships between different parameters (microstructural,
mechanical) with temperature and cooling rate for the developed composite
coatings.
9.3 Recommendations for further study In this section, several recommendations are put forward to extend the present
research for a better understanding of the composite coating and material (derived
from HA and Ti-6Al-4V). The future scope of this research includes the following
aspects:
Study of wettability, surface charge and surface energy to evaluate the
biological response of the coating.
In vitro and in vivo evaluation of cytotoxicity and genotoxicity of the coated
implants.
Study of the corrosion behavior of the coating and the composite.
Evaluation of the coatings apatite formation capability of the coatings, if treated
inside a simulated body fluid (SBF).
Study of the fatigue behavior of the coated prosthetics and grafts made from the
composite.
Assessment of adhesion strength of the coating and the crust.
Chapter 9. REFERENCES
Muhammad Rakib Mansur Page 165
10. REFERENCES
Chapter 9. REFERENCES
Muhammad Rakib Mansur Page 166
1. Leon, B. and J.A. Jansen, eds. Thin Calcium Phosphate Coatings for Medical Implants. 2009 ed. 2009, Springer science: New York 30-31.
2. Olivier, V., N. Faucheux, and P. Hardouin, Biomaterial challenges and approaches to stem cell use in bone reconstructive surgery. Drug Discovery Today, 2004. 9(18): p. 803-811.
3. Paital, S.R. and N.B. Dahotre, Calcium phosphate coatings for bio-implant applications: Materials, performance factors, and methodologies. Materials Science and Engineering R: Reports, 2009. 66(1-3): p. 1-70.
4. Kokubo, T., H.-M. Kim, and M. Kawashita, Novel bioactive materials with different mechanical properties. Biomaterials, 2003. 24(13): p. 2161-2175.
5. Leyens, C. and M. Peters, Titanium and Titanium Alloys. 1st ed. 2003: WILEY-VCH 513.
6. Long, M. and H.J. Rack, Titanium alloys in total joint replacement—a materials science perspective. Biomaterials, 1998. 19(18): p. 1621-1639.
7. Ratner, B.D., Biomaterials Science: An introduction to materials in medicine. 2 ed. 2004: Academic Press.
8. Rae, T., The toxicity of metals used in orthopaedic prostheses. An experimental study using cultured human synovial fibroblasts. Journal of Bone and Joint Surgery - Series B, 1981. 63(3): p. 435-440.
9. Velasco-Ortega, E., A. Jos, A.M. Cameán, J. Pato-Mourelo, and J.J. Segura-Egea, In vitro evaluation of cytotoxicity and genotoxicity of a commercial titanium alloy for dental implantology. Mutation Research/Genetic Toxicology and Environmental Mutagenesis, 2010. 702(1): p. 17-23.
10. Faria, A.C.L., A.L. Rosa, R.C.S. Rodrigues, and R.F. Ribeiro, In vitro cytotoxicity of dental alloys and cpTi obtained by casting. Journal of Biomedical Materials Research - Part B Applied Biomaterials, 2008. 85(2): p. 504-508.
11. Yamaguchi, K., H. Konishi, S. Hara, and Y. Motomura, Biocompatibility studies of titanium-based alloy pedicle screw and rod system: histological aspects. The Spine Journal, 2001. 1(4): p. 260-268.
12. Ratner, B.D., Biomaterials science : an introduction to materials in medicine 1996: San Diego : Academic Press
13. Hench, L.L., Bioceramics. Journal of the American Ceramic Society, 1998. 81(7): p. 1705-1727.
14. Khor, K.A., Y.W. Gu, C.H. Quek, and P. Cheang, Plasma spraying of functionally graded hydroxyapatite/Ti-6Al-4V coatings. Surface and Coatings Technology, 2003. 168(2-3): p. 195-201.
15. Chou, B.Y. and E. Chang, Plasma-sprayed zirconia bond coat as an intermediate layer for hydroxyapatite coating on titanium alloy substrate. Journal of Materials Science: Materials in Medicine, 2002. 13(6): p. 589-595.
16. Chou, B.Y. and E. Chang, Plasma-sprayed hydroxyapatite coating on titanium alloy with ZrO2 second phase and ZrO2 intermediate layer. Surface and Coatings Technology, 2002. 153(1): p. 84-92.
17. Gross, K.A. and C.C. Berndt, Thermal spraying of hydroxyapatite for bioceramic applications. Key Engineering Materials, 1991. 53-55: p. 124-129.
18. Guipont, V., M. Espanol, F. Borit, N. Llorca-Isern, M. Jeandin, K.A. Khor, and P. Cheang, High-pressure plasma spraying of hydroxyapatite powders. Materials Science and Engineering A, 2002. 325(1-2): p. 9-18.
19. Heimann, R.B., Thermal spraying of biomaterials. Surface and Coatings Technology, 2006. 201(5): p. 2012-2019.
20. Choudhuri, A., P.S. Mohanty, and J. Karthikeyan. Bio-ceramic composite coatings by cold spray technology. in International Thermal Spray Conference, ITSC 2009. 2009. Las Vegas, NV.
21. Sun, L., C.C. Berndt, and K.A. Gross, Hydroyapatite/polymer composite flame-sprayed coatings for orthopedic applications. Journal of Biomaterials Science, Polymer Edition, 2002. 13(9): p. 977-990.
Chapter 9. REFERENCES
Muhammad Rakib Mansur Page 167
22. Lima, R.S., K.A. Khor, H. Li, P. Cheang, and B.R. Marple, HVOF spraying of nanostructured hydroxyapatite for biomedical applications. Materials Science and Engineering A, 2005. 396(1-2): p. 181-187.
23. Chu, C., P. Lin, Y. Dong, X. Xue, J. Zhu, and Z. Yin, Fabrication and characterization of hydroxyapatite reinforced with 20 vol % Ti particles for use as hard tissue replacement. Journal of Materials Science: Materials in Medicine, 2002. 13(10): p. 985-992.
24. Sato, M., R. Tu, and T. Goto, Preparation of hydroxyapatite and calcium phosphate films by MOCVD. Materials Transactions, 2007. 48(12): p. 3149-3153.
25. Liu, X., P.K. Chu, and C. Ding, Formation of apatite on hydrogenated amorphous silicon (a-Si:H) film deposited by plasma-enhanced chemical vapor deposition. Materials Chemistry and Physics, 2007. 101(1): p. 124-128.
26. Cavalli, M., G. Gnappi, A. Montenero, D. Bersani, P.P. Lottici, S. Kaciulis, G. Mattogno, and M. Fini, Hydroxy- and fluorapatite films on Ti alloy substrates: Sol-gel preparation and characterization. Journal of Materials Science, 2001. 36(13): p. 3253-3260.
27. Oktar, F.N., Hydroxyapatite-TiO2 composites. Materials Letters, 2006. 60(17-18): p. 2207-2210.
28. Comesaña, R., F. Lusquiños, J. del Val, T. Malot, M. López-Álvarez, A. Riveiro, F. Quintero, M. Boutinguiza, P. Aubry, A. De Carlos, and J. Pou, Calcium phosphate grafts produced by rapid prototyping based on laser cladding. Journal of the European Ceramic Society, 2011. 31(1-2): p. 29-41.
29. Lusquiños, F., J. Pou, J.L. Arias, M. Boutinguiza, B. Léon, M. Pérez-Amor, F.C.M. Driessens, J.C. Merry, I. Gibson, S. Best, and W. Bonfield, Production of calcium phosphate coatings on Ti6Al4V obtained by Nd:Yttrium-aluminum-garnet laser cladding. Journal of Applied Physics, 2001. 90(8): p. 4231-4236.
30. Wang, D.G., C.Z. Chen, J. Ma, and G. Zhang, In situ synthesis of hydroxyapatite coating by laser cladding. Colloids and Surfaces B: Biointerfaces, 2008. 66(2): p. 155-162.
31. Chen, Y., C. Gan, T. Zhang, G. Yu, P. Bai, and A. Kaplan, Laser-surface-alloyed carbon nanotubes reinforced hydroxyapatite composite coatings. Applied Physics Letters, 2005. 86(25): p. 1-3.
32. Chen, Y., Y.Q. Zhang, T.H. Zhang, C.H. Gan, C.Y. Zheng, and G. Yu, Carbon nanotube reinforced hydroxyapatite composite coatings produced through laser surface alloying. Carbon, 2006. 44(1): p. 37-45.
33. Arias, J.L., M.B. Mayor, J. Pou, Y. Leng, B. León, and M. Pérez-Amor, Micro- and nano-testing of calcium phosphate coatings produced by pulsed laser deposition. Biomaterials, 2003. 24(20): p. 3403-3408.
34. Arias, J.L., M.B. Mayor, F.J. García-Sanz, J. Pou, B. León, M. Pérez-Amor, and J.C. Knowles, Structural analysis of calcium phosphate coatings produced by pulsed laser deposition at different water-vapour pressures. Journal of Materials Science: Materials in Medicine, 1997. 8(12): p. 873-876.
35. Mazumder, J., D. Dutta, N. Kikuchi, and A. Ghosh, Closed loop direct metal deposition: Art to Part. Optics and Lasers in Engineering, 2000. 34(4-6): p. 397-414.
36. Mazumder, J., A. Schifferer, and J. Choi, Direct materials deposition: Designed macro and microstructure. Materials Research Innovations, 1999. 3(3): p. 118-131.
37. Kar, A. and J. Mazumder, One-dimensional diffusion model for extended solid solution in laser cladding. Journal of Applied Physics, 1987. 61(7): p. 2645-2655.
38. Hofmeister, W., M. Griffith, M. Ensz, and J. Smugeresky, Solidification in direct metal deposition by LENS processing. JOM, 2001. 53(9): p. 30-34.
39. Rosenthal, D. The theory of moving sources of heat and its application to metal treatments. 1946. ASME.
40. Dykhuizen, R. and D. Dobranich, Cooling rates in the LENS process. Sandia National Laboratories Internal Report, 1998.
Chapter 9. REFERENCES
Muhammad Rakib Mansur Page 168
41. Ueda, T., K. Yamada, and K. Nakayama, Temperature of Work Materials Irradiated with CO2 Laser. CIRP Annals - Manufacturing Technology, 1997. 46(1): p. 117-122.
42. Smurov, I. and M. Doubenskaia, Temperature monitoring by optical methods in laser processing, in Laser-Assisted Fabrication of Materials. 2013, Springer. p. 375-422.
43. Suchanek, W. and M. Yoshimura, Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants. Journal of Materials Research, 1998. 13(1): p. 94-117.
44. Berndt, C.C., G.N. Haddad, A.J.D. Farmer, and K.A. Gross, Thermal spraying for bioceramic applications. Materials Forum, 1990. 14: p. 161-173.
45. Levingstone, T.J., Optimisation of plasma sprayed hydroxyapatite coatings in School of Mechanical and Manufacturing Engineering. 2008, Dublin City University: Dublin.
46. Boskey, A.L., Mineralization of bones and teeth. Elements, 2007. 3(6): p. 385-391. 47. Liu, H., H. Yazici, C. Ergun, T.J. Webster, and H. Bermek, An in vitro evaluation of
the Ca/P ratio for the cytocompatibility of nano-to-micron particulate calcium phosphates for bone regeneration. Acta Biomaterialia, 2008. 4(5): p. 1472-1479.
48. Ahmed, I. and T.L. Bergman, Optimization of plasma spray processing parameters for deposition of nanostructured powders for coating formation. Journal of Fluids Engineering, Transactions of the ASME, 2006. 128(2): p. 394-401.
49. Clarke, B., Normal bone anatomy and physiology. Clinical journal of the American Society of Nephrology, 2008. 3(Supplement 3): p. S131-S139.
50. Pan, Y. and M.E. Fleet, Compositions of the apatite-group minerals: substitution mechanisms and controlling factors. Reviews in Mineralogy and Geochemistry, 2002. 48(1): p. 13-49.
51. Joon, B.P. and J.B. Park, Biomaterials : an introduction, ed. R.S. Lakes. 2007, New York: New York : Springer.
52. Combes, C. and C. Rey, Amorphous calcium phosphates: Synthesis, properties and uses in biomaterials. Acta Biomaterialia, 2010. 6(9): p. 3362-3378.
53. Gross, K.A., C.C. Berndt, P. Stephens, and R. Dinnebier, Oxyapatite in hydroxyapatite coatings. Journal of Materials Science, 1998. 33(15): p. 3985-3991.
54. Gross, K.A., C.C. Berndt, and V.J. Iacono, Variability of Hydroxyapatite-Coated Dental Implants. International Journal of Oral and Maxillofacial Implants, 1998. 13(5): p. 601-610.
55. Tsui, Y.C., C. Doyle, and T.W. Clyne, Plasma sprayed hydroxyapatite coatings on titanium substrates Part 2: optimisation of coating properties. Biomaterials, 1998. 19(22): p. 2031-2043.
56. Liao, C.J., F.H. Lin, K.S. Chen, and J.S. Sun, Thermal decomposition and reconstitution of hydroxyapatite in air atmosphere. Biomaterials, 1999. 20(19): p. 1807-1813.
57. Pawlowski, L., The Science and Engineering of Thermal Spray Coatings. Second ed. 2008: John Wiley & Sons, Ltd.
58. Carter, C.B. and M.G. Norton, Ceramic Materials Science and Engineering. 2007: Springer.
59. MatWeb. T-6Al-4V annealed bar. 2013 [cited 2013; Available from: http://www.matweb.com/index.aspx.
60. Ohara, S., S. Komura, and T. Takeda, Magnetic Properties of Pseudo-Iron Fe1-x(Cr0.5Ni0.5)x Ternary Alloys. J. Phys. Soc., 1973. Vol.34: p. 1472-1476.
61. Deryabin, A.V. and V.I. Rimlyand, On the nature of magnetic volume phenomena in invar alloys. Phys. Met. Metallogr., 1982. Vol.54(3): p. p 185-187.
62. Honeycombe, R.W.K. and P. Hancock, eds. Steels Microstructure and properties. 1st ed. 1992.
63. Inox, E., ed. Pickling and Passivating Stainless Steel. 2nd ed. 2007. 64. Shih, C.C., C.M. Shih, Y.Y. Su, L.H.J. Su, M.S. Chang, and S.J. Lin, Effect of surface
oxide properties on corrosion resistance of 316L stainless steel for biomedical applications. Corrosion Science, 2004. 46(2): p. 427-441.
Chapter 9. REFERENCES
Muhammad Rakib Mansur Page 169
65. MatWeb. AISI Type 304L Austenitic Stainless Steel. 2013 [cited 2013; Available from: http://www.matweb.com/index.aspx.
66. MatWeb. AISI Type 316L Austenitic Stainless Steel. 2013; Available from: http://www.matweb.com/index.aspx.
67. Raghavan, V., The Cr-Fe-Ti (Chromium-Iron-Titanium) System. 1987, Indian Inst. Met.: Phase Diagrams Ternary Iron Alloys. p. 43-54.
68. Metco, S. 2010; Available from: http://www.sulzermetco.com/en/desktopdefault.aspx/tabid-4021/7684_read-17371/.
69. Davis, J.R., ed. Handbook of Thermal Spray Technology. 2005, ASM international. 70. Steen, W.M. and J. Mazumder, Laser material processing 4th ed ed. 2010: London ;
Dordrecht ; Heidelberg ; New York : Springer 71. Roy, M., B.V. Krishna, A. Bandyopadhya, and S. Bose, Laser processing of bioactive
tricalcium phosphate coating on titanium for load-bearing implants. Acta Biomaterialia, 2007. 4(2008): p. 324-333.
72. Bini, R.A., M.L. Santos, E.A. Filho, R.F.C. Marques, and A.C. Guastaldi, Apatite coatings onto titanium surfaces submitted to laser ablation with different energy densities. Surface and Coatings Technology, 2009. 204(4): p. 399-403.
73. Yang, Y., S.R. Paital, and N.B. Dahotre, Effects of SiO2 substitution on wettability of laser deposited Ca-P biocoating on Ti-6Al-4V. Journal of Materials Science: Materials in Medicine, 2010. 21(9): p. 2511-2521.
74. Kurella, A. and N.B. Dahotre, Laser induced hierarchical calcium phosphate structures. Acta Biomaterialia, 2006. 2(6): p. 677-683.
75. Zheng, M., D. Fan, X.K. Li, W.F. Li, Q.B. Liu, and J.B. Zhang, Microstructure and osteoblast response of gradient bioceramic coating on titanium alloy fabricated by laser cladding. Applied Surface Science, 2008. 255(2): p. 426-428.
76. Paital, S.R., K. Balani, A. Agarwal, and N.B. Dahotre, Fabrication and evaluation of a pulse laser-induced Ca–P coating on a Ti alloy for bioapplication. Biomedical Materials, 2009. 4(1): p. 015009.
77. Paital, S.R. and N.B. Dahotre, Laser surface treatment for porous and textured Ca–P bio-ceramic coating on Ti–6Al–4V. Biomedical Materials, 2007. 2(4): p. 274.
78. Paital, S.R. and N.B. Dahotre, Review of laser based biomimetic and bioactive Ca-P coatings. Materials Science and Technology, 2008. 24(9): p. 1144-1161.
79. Merkus, H.G., Particle size measurements. 2008: Dordrecht : Springer 80. Cox, M.R. and M. Budhu, A practical approach to grain shape quantification.
Engineering Geology, 2008. 96(1–2): p. 1-16. 81. Almeida‐Prieto, S., J. Blanco‐Méndez, and F.J. Otero‐Espinar, Image analysis of the
shape of granulated powder grains. Journal of pharmaceutical sciences, 2004. 93(3): p. 621-634.
82. Amiss, J.M., J.M. Amiss, F.D. Jones, and H.H. Ryffel, Machinery's Handbook Guide 27. 2004: Industrial Press Inc.
83. Sun, L., C.C. Berndt, K.A. Khor, H.N. Cheang, and K.A. Gross, Surface characteristics and dissolution behavior of plasma-sprayed hydroxyapatite coating. Journal of Biomedical Materials Research, 2002. 62(2): p. 228-236.
84. Gualtieri, A., A guided training exercise of quantitative phase analysis using EXPGUI. GSAS Tutorials and Examples, 2003.
85. Larson, A.C. and R.B. Von Dreele, GSAS. General Structure Analysis System. LANSCE, MS-H805, Los Alamos, New Mexico, 1994.
86. Toby, B.H., EXPGUI, a graphical user interface for GSAS. Journal of Applied Crystallography, 2001. 34(2): p. 210-213.
87. Connolly, J.R., Introduction quantitative X-ray diffraction methods. 2012: EPS400. 88. Knowles, J.C., K. Gross, C.C. Berndt, and W. Bonfield, Structural changes of
thermally sprayed hydroxyapatite investigated by Rietveld analysis. Biomaterials, 1996. 17(6): p. 639-645.
89. Donwns, R.T. and M. Hall-Wallace. The American Mineralogist Crystal Structure Database. 2003; Available from: http://rruff.geo.arizona.edu/AMS/amcsd.php.
Chapter 9. REFERENCES
Muhammad Rakib Mansur Page 170
90. Downs, R.T. and M. Hall-Wallace. American Mineralogist. 2003; Available from: http://rruff.geo.arizona.edu/AMS/amcsd.php.
91. Elliott, J.C., P.E. Mackie, and R.A. Young, Monoclinic hydroxyapatite. Science, 1973. 180(4090): p. 1055-1057.
92. Yashima, M., A. Sakai, T. Kamiyama, and A. Hoshikawa, Crystal structure analysis of β-tricalcium phosphate Ca 3(PO 4) 2 by neutron powder diffraction. Journal of Solid State Chemistry, 2003. 175(2): p. 272-277.
93. (ICDD), I.C.D.D., Powder Diffraction Files (PDF-2). 2012. 94. Gross, K.A., C.C. Berndt, and H. Herman, Amorphous phase formation in plasma-
sprayed hydroxyapatite coatings. Journal of Biomedical Materials Research, 1998. 39(3): p. 407-414.
95. Sun, L., C.C. Berndt, and C.P. Grey, Phase, structural and microstructural investigations of plasma sprayed hydroxyapatite coatings. Materials Science and Engineering A, 2003. 360(1-2): p. 70-84.
96. Khor, K.A., Y.W. Gu, D. Pan, and P. Cheang, Microstructure and mechanical properties of plasma sprayed HA/YSZ/ Ti-6Al-4V composite coatings. Biomaterials, 2004. 25(18): p. 4009-4017.
97. Yang, Y., C.M. Agrawal, K.H. Kim, H. Martin, K. Schulz, J.D. Bumgardner, and J.L. Ong, Characterization and Dissolution Behavior of Sputtered Calcium Phosphate Coatings After Different Postdeposition Heat Treatment Temperatures. Journal of Oral Implantology, 2003. 29(6): p. 270-277.
98. PerkinElmer, I. Introduction to Raman Spectroscopy Top 20 questions answered. 2007 [cited 2012; Available from: http://www.perkinelmer.com/CMSResources/Images/4674565MAN_Raman20Questions.pdf.
99. Smith, E. and G. Dent, Modern Raman Spectroscopy - A Practical Approach. 2005, Chichester: John Wiley & Sons Ltd.
100. Gross, K.A., M. Phillips, B. Ben-Nissan, and C.C. Berndt, Identification of the amorphous phase in plasma sprayed apatite coatings, in 11th International Symposium on Ceramics in Medicine, R.Z. LeGeros and J.P. LeGeros, Editors. 1998, World Scientific Publishing: NewYork.
101. d'Haese, R., L. Pawlowski, M. Bigan, R. Jaworski, and M. Martel, Phase evolution of hydroxapatite coatings suspension plasma sprayed using variable parameters in simulated body fluid. Surface and Coatings Technology, 2010. 204(8): p. 1236-1246.
102. Wen, J., Y. Leng, J. Chen, and C. Zhang, Chemical gradient in plasma-sprayed HA coatings. Biomaterials, 2000. 21(13): p. 1339-1343.
103. Tsuda, H. and J. Arends, Orientational micro-Raman spectroscopy on hydroxyapatite single crystals and human enamel crystallites. Journal of Dental Research, 1994. 73(11): p. 1703-1710.
104. Callister, W.D., Materials science and engineering: an introduction. 8 ed. 2010: John Wiley & Sons, Inc.
105. Hench, L.L. and J.R. Jones, eds. Biomaterials, artificial organs and tissue engineering 2005, Boca Raton, Calif. : CRC Press ; Cambridge : Woodhead
106. Gross, K.A., S. Saber-Samandari, and K.S. Heemann, Evaluation of commercial implants with nanoindentation defines future development needs for hydroxyapatite coatings. Journal of Biomedical Materials Research - Part B Applied Biomaterials, 2010. 93(1): p. 1-8.
107. Smallman, R.E. and R.J. Bishop, Modern Physical Metallurgy and Materials Engineering. 6th ed. 1999: Reed Educational and Professional Publishing Ltd.
108. Malzbender, J., J.M.J. den Toonder, A.R. Balkenende, and G. de With, Measuring mechanical properties of coatings: a methodology applied to nano-particle-filled sol-gel coatings on glass. Materials Science and Engineering: R: Reports, 2002. 36(2-3): p. 47-103.
Chapter 9. REFERENCES
Muhammad Rakib Mansur Page 171
109. Leigh, S.H., C.K. Lin, and C.C. Berndt, Elastic response of thermal spray deposits under indentation tests. Journal of the American Ceramic Society, 1997. 80(8): p. 2093-2099.
110. Kweh, S.W.K., K.A. Khor, and P. Cheang, Plasma-sprayed hydroxyapatite (HA) coatings with flame-spheroidized feedstock: Microstructure and mechanical properties. Biomaterials, 2000. 21(12): p. 1223-1234.
111. Lawn, B.R. and A.G. Evans, A model for crack initiation in elastic/plastic indentation fields. Journal of Materials Science, 1977. 12(11): p. 2195-2199.
112. Ponton, C.B. and R.D. Rawlings, Vickers indentation fracture toughness test Part 1 Review of literature and formulation of standardised indentation toughness equations. Materials Science and Technology, 1989. 5(9): p. 865-872.
113. Behnamghader, A., R.N. Shirazi, A. Iost, and D. Najjar, Surface Cracking and Degradation of Dense Hydroxyapatite through Vickers Microindentation Testing. Applied Mechanics and Materials, 2011. 66: p. 614-619.
114. Zhang, C., Y. Leng, and J. Chen, Elastic and plastic behavior of plasma-sprayed hydroxyapatite coatings on a Ti-6Al-4V substrate. Biomaterials, 2001. 22(11): p. 1357-1363.
115. Saber-Samandari, S. and K.A. Gross, Micromechanical properties of single crystal hydroxyapatite by nanoindentation. Acta Biomaterialia, 2009. 5(6): p. 2206-2212.
116. Saber-Samandari, S. and K.A. Gross, Nanoindentation on the surface of thermally sprayed coatings. Surface and Coatings Technology, 2009. 203(23): p. 3516-3520.
117. Saber-Samandari, S. and K.A. Gross, Nanoindentation reveals mechanical properties within thermally sprayed hydroxyapatite coatings. Surface and Coatings Technology, 2009. 203(12): p. 1660-1664.
118. Saber-Samandari, S. and K.A. Gross, Effect of angled indentation on mechanical properties. Journal of the European Ceramic Society, 2009. 29(12): p. 2461-2467.
119. Saber-Samandari, S., C.C. Berndt, and K.A. Gross, Selection of the implant and coating materials for optimized performance by means of nanoindentation. Acta Biomaterialia. In Press, Corrected Proof.
120. Kweh, S.W.K., K.A. Khor, and P. Cheang, An in vitro investigation of plasma sprayed hydroxyapatite (HA) coatings produced with flame-spheroidized feedstock. Biomaterials, 2002. 23(3): p. 775-785.
121. Berndt, C.C. and C.K. Lin, Measurement of adhesion for thermally sprayed materials. Journal of Adhesion Science and Technology, 1993. 7(12 pt 2): p. 1235-1264.
122. García-Sanz, F.J., M.B. Mayor, J.L. Arias, J. Pou, B. León, and M. Pérez-Amor, Hydroxyapatite coatings: A comparative study between plasma-spray and pulsed laser deposition techniques. Journal of Materials Science: Materials in Medicine, 1997. 8(12): p. 861-865.
123. Montavon, G., C. Coddet, C.C. Berndt, and S.H. Leigh, Microstructural Index to Quantify Thermal Spray Deposit Microstructures Using Image Analysis. Journal of Thermal Spray Technology, 1998. 7(2): p. 229-241.
124. Chang, E., W.J. Chang, B.C. Wang, and C.Y. Yang, Plasma spraying of zirconia-reinforced hydroxyapatite composite coatings on titanium Part I Phase, microstructure and bonding strength. Journal of Materials Science: Materials in Medicine, 1997. 8(4): p. 193-200.
125. Demnati, I., M. Parco, D. Grossin, I. Fagoaga, C. Drouet, G. Barykin, C. Combes, I. Braceras, S. Goncalves, and C. Rey, Hydroxyapatite coating on titanium by a low energy plasma spraying mini-gun. Surface and Coatings Technology, 2012. 206(8-9): p. 2346-2353.
126. Balbinotti, P., E. Gemelli, G. Buerger, S.A. De Lima, J. De Jesus, N.H.A. Camargo, V.A.R. Henriques, and G.D. De Almeida Soares, Microstructure development on sintered Ti/HA biocomposites produced by powder metallurgy. Materials Research, 2011. 14(3): p. 384-393.
Chapter 9. REFERENCES
Muhammad Rakib Mansur Page 172
127. Ji, H., C.B. Ponton, and P.M. Marquis, Microstructural characterization of hydroxyapatite coating on titanium. Journal of Materials Science: Materials in Medicine, 1992. 3(4): p. 283-287.
128. Wang, Z., A. Kulkarni, S. Deshpande, T. Nakamura, and H. Herman, Effects of pores and interfaces on effective properties of plasma sprayed zirconia coatings. Acta Materialia, 2003. 51(18): p. 5319-5334.
129. Sevostianov, I., M. Kachanov, J. Ruud, P. Lorraine, and M. Dubois, Quantitative characterization of microstructures of plasma-sprayed coatings and their conductive and elastic properties. Materials Science and Engineering A, 2004. 386(1-2): p. 164-174.
130. Leigh, S.H. and C.C. Berndt, Modelling of elastic constants of plasma spray deposits with ellipsoid-shaped voids. Acta Materialia, 1999. 47(5): p. 1575-1586.
131. Leigh, S.H. and C.C. Berndt, Quantitative evaluation of void distributions within a plasma-sprayed ceramic. Journal of the American Ceramic Society, 1999. 82(1): p. 17-21.
132. Jadhav, A.D., N.P. Padture, E.H. Jordan, M. Gell, P. Miranzo, and E.R. Fuller Jr, Low-thermal-conductivity plasma-sprayed thermal barrier coatings with engineered microstructures. Acta Materialia, 2006. 54(12): p. 3343-3349.
133. Wang, B.C., T.M. Lee, E. Chang, and C.Y. Yang, Effect of coating thickness on the shear strength and failure mode of plasma sprayed hydroxyapatite coatings to bone. Biomedical Engineering - Applications, Basis and Communications, 1992. 4(6): p. 605-609.
134. Wang, B.C., T.M. Lee, E. Chang, and C.Y. Yang, The shear strength and the failure mode of plasma-sprayed hydroxyapatite coating to bone: The effect of coating thickness. Journal of Biomedical Materials Research, 1993. 27(10): p. 1315-1327.
135. Svehla, M., P. Morberg, W. Bruce, B. Zicat, and W.R. Walsh, The effect of substrate roughness and hydroxyapatite coating thickness on implant shear strength. Journal of Arthroplasty, 2002. 17(3): p. 304-311.
136. Tietz, U., The Engineering of Bio-Composites via Microstructural Modelling and Performance Simulation. 2008.
137. Rasband, W., ImageJ; US National Institutes of Health: Bethesda, MD, 1997-2006. There is no corresponding record for this reference, 2004.
138. Collins, T.J., ImageJ for microscopy. Biotechniques, 2007. 43(1 Suppl): p. 25-30. 139. Grazulis, S., D. Chateigner, R.T. Downs, A. Yokochi, M. Quirós, L. Lutterotti, E.
Manakova, J. Butkus, P. Moeck, and A. Le Bail, Crystallography Open Database-an open-access collection of crystal structures. Journal of Applied Crystallography, 2009. 42(4): p. 726-729.
140. Wen, H.B., J.R. de Wijn, F.Z. Cui, and K. de Groot, Preparation of bioactive Ti6Al4V surfaces by a simple method. Biomaterials, 1998. 19(1–3): p. 215-221.
141. Hasçalık, A. and U. Çaydaş, Electrical discharge machining of titanium alloy (Ti–6Al–4V). Applied Surface Science, 2007. 253(22): p. 9007-9016.
142. Chakraborty, J., M.K. Sinha, and D. Basu, Biomolecular Template‐Induced Biomimetic Coating of Hydroxyapatite on an SS 316 L Substrate. Journal of the American Ceramic Society, 2007. 90(4): p. 1258-1261.
143. Corporation, O., OriginPro 9.0. 2013, OriginLab Corporation. p. 64 bit. 144. Prevéy, P.S., X-ray diffraction characterization of crystallinity and phase composition
in plasma-sprayed hydroxyapatite coatings. Journal of thermal spray technology, 2000. 9(3): p. 369-376.
145. Chu, C., J. Zhu, Z. Yin, and P. Lin, Structure optimization and properties of hydroxyapatite-Ti symmetrical functionally graded biomaterial. Materials Science and Engineering A, 2001. 316(1-2): p. 205-210.
146. Chu, C., J. Zhu, Z. Yin, and P. Lin, Optimal design and fabrication of hydroxyapatite-Ti asymmetrical functionally graded biomaterial. Materials Science and Engineering A, 2003. 348(1-2): p. 244-250.
Chapter 9. REFERENCES
Muhammad Rakib Mansur Page 173
147. Oktar, F.N., S. Agathopoulos, L.S. Ozyegin, O. Gunduz, N. Demirkol, Y. Bozkurt, and S. Salman, Mechanical properties of bovine hydroxyapatite (BHA) composites doped with SiO2, MgO, Al2O3, and ZrO2. Journal of Materials Science: Materials in Medicine, 2007. 18(11): p. 2137-2143.
148. Mazumder, J. and H.L. Robert. Past present and future of art to part by Direct Metal Deposition. in PICALO 2004 - 1st Pacific International Conference on Applications of Laser and Optics. 2004. Melbourne, VIC.
149. Lowe, H. and C. Spindloe, White light interferometric profilometry of surface structured glass for high power laser microtargets. Central Laser Facility annual report 2006/2007: 7. Target fabrication, 2007.
150. Thivillon, L., P. Bertrand, B. Laget, and I. Smurov, Potential of direct metal deposition technology for manufacturing thick functionally graded coatings and parts for reactors components. Journal of Nuclear Materials, 2009. 385(2): p. 236-241.
151. ASTM, Standard Practice for Microetching Metals and Alloys, in Standard Practice for Microetching Metals and Alloys. 1999, ASTM: 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, USA. p. 21.
152. Matthew J. Donachie, J., Titanium: A Technical Guide. 2000: ASM International. 153. Matusiewicz, P., W. Ratuszek, and A. Zielińska-Lipiec, Recrystallization of ferrite in
spheroidite of Fe-0.67%C steel. Archives of Metallurgy and Materials, 2011. 56(1): p. 63-69.
154. Carter, G.F. and D.E. Paul, Materials Science & Engineering. 2011: ASM International.
155. Handbook, A., Metallography and microstructures. Edited by GF Vander Voort, ASM Intenational, 2004. 9.
156. Khanna, A.S., Introduction to high temperature oxidation and corrosion. 2002: ASM International.
157. Younger, E.M. and M.W. Chapman, Morbidity at bone graft donor sites. Journal of orthopaedic trauma, 1989. 3(3): p. 192-195.
158. Schwartz, C.E., J.F. Martha, P. Kowalski, D.A. Wang, R. Bode, L. Li, and D.H. Kim, Prospective evaluation of chronic pain associated with posterior autologous iliac crest bone graft harvest and its effect on postoperative outcome. Health Qual Life Outcomes, 2009. 7: p. 49.
159. ASTM, Standard Test Method for Microindentation Hardness of Materials. 2002, ASTM International: ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, USA. p. 24.
160. Evans, A. and E. Charles, Fracture toughness determinations by indentation. Journal of the American Ceramic society, 1976. 59(7‐8): p. 371-372.
161. Eagar, T.W. and N.S. Tsai, Temperature fields produced by traveling distributed heat sources. Welding Journal (Miami, Fla), 1983. 62(12): p. 346-355.
162. Ashby, M.F. and K.E. Easterling, The transformation hardening of steel surfaces by laser beams-I. Hypo-eutectoid steels. Acta Metallurgica, 1984. 32(11): p. 1935-1937,1939-1948.
163. Cline, H.E. and T.R. Anthony, Heat treating and melting material with a scanning laser or electron beam. Journal of Applied Physics, 1977. 48(9): p. 3895-3900.
164. Manca, O., B. Morrone, and V. Naso, Quasi-steady state 3 dimensional temperature distribution induced by a moving circular Gaussian hear source in a finite depth solid. International Journal of Heat and Mass Transfer, 1995. 38(7): p. 1305-1315.
165. Han, L., K. Phatak, and F. Liou, Modeling of laser cladding with powder injection. Metallurgical and Materials transactions B, 2004. 35(6): p. 1139-1150.
166. Jendrzejewski, R., I. Kreja, and G. Śliwiński, Temperature distribution in laser-clad multi-layers. Materials Science and Engineering: A, 2004. 379(1–2): p. 313-320.
167. Mackwood, A.P. and R.C. Crafer, Thermal modelling of laser welding and related processes: a literature review. Optics & Laser Technology, 2005. 37(2): p. 99-115.
168. Holman, J., Heat transfer. 2002, McGraw-Hill.
Chapter 9. REFERENCES
Muhammad Rakib Mansur Page 174
169. Ahsan, M.N., A.J. Pinkerton, R.J. Moat, and J. Shackleton, A comparative study of laser direct metal deposition characteristics using gas and plasma-atomized Ti-6Al-4V powders. Materials Science and Engineering A, 2011. 528(25-26): p. 7648-7657.
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 175
11. APPENDIX
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 176
11.1 Appendix 1 Image analysis results of HA procured from Sigma Aldrich is presented in histograms below
to show the frequency distribution of different microstructural parameters that includes feret
diameter, aspect ratio, roundness, circularity and solidity.
Figure 11-1 : Histograms presenting different microstructural observations for HA procured
from Sigma Aldrich.
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 177
Image analysis results of HA procured from Plasma Biotal is presented in histograms below
to show the frequency distribution of different microstructural parameters that includes feret
diameter, aspect ratio, roundness, circularity and solidity.
Figure 11-2 : Histograms presenting different microstructural observations for HA procured
from Plasma Biotal.
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 178
Image analysis results of Ti-6Al-V powders procured from TLS Technik is presented in
histograms below to show the frequency distribution of different microstructural parameters
that includes feret diameter, aspect ratio, roundness, circularity and solidity.
Figure 11-3 : Histograms presenting different microstructural observations for Ti-6Al-4V
powders procured from TLS Technik.
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 179
XRD spectrum of alpha tri-calcium phosphate (ATCP) powder procured from Plasma Biotal
is presented in Figure 11-4.
Figure 11-4 : XRD spectrum of ATCP.
20 25 30 35 40 45 50 55 60 65 70 75 80 85 900
50
100
150
200
250
300
Inte
nsity
(Cou
nts)
Two theta (2)
ATCP
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 180
XRD spectrum of beta tri-calcium phosphate (BTCP) powder procured from Plasma Biotal is
presented in Figure 11-5.
Figure 11-5 : XRD Spectrum of BTCP.
20 25 30 35 40 45 50 55 60 65 70 75 80 85 900
50
100
150
200
250
300
350
400
450
500
In
tens
ity (C
ount
s)
Two theta (2)
BTCP
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 181
The optical micrographs of the top and the bottom section of the composite made from HA
and Ti-6Al-4V are presented in Figure 11-6. Microstructural details of the coating are
discussed in chapter 6 at section 6.3.1.
Figure 11-6 : Optical micrograph of composite made from HA and Ti-6Al-4V (a) Top section
(b) Bottom section
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 182
The EDS spectrum of the top surface of the composite coating is presented in Figure 11-7.
The associated discussion is in chapter 5 at section 5.3.5.
Figure 11-7 : EDS spectrum of HA and Ti-6Al-4V composite coating on SS (AISI 316L).
The Ra and Rq values determined to measure surface roughness of the composite coating
using a stylus type profilometer and presented in the tables below. The analysis is described
in chapter 5 at section 5.3.1.
Table 11-1 : Table containing roughness average data for composite coating (from run 01 to
run 10) along with mean, standard deviation and standard error.
Ra Run 01
Run 02
Run 03
Run 04
Run 05
Run 06
Run 07
Run 08
Run 09
Run 10
1 9.37 8.13 7.22 4.69 8.29 7.9 3.16 7.2 6.86 11
2 9.41 7.98 6.31 6.92 8.57 8.36 3.48 7.83 5.54 10.1
3 10.05 9.29 7.57 6.81 7.08 10.4 3.73 7.47 7.94 10.8
4 7.37 13.3 7.92 6.78 6.72 6.78 4.37 8.15 8.77 11.7
5 6.49 7.78 7.57 5.54 5.59 5.5 3.23 5.27 6.23 9.72
6 11.9 8.04 7.92 7.38 8.52 8.24 3.25 7.21 9.1 11.9
7 7.77 8.26 7.74 6.59 8.56 5.13 4.95 4.95 6.93 12.2
8 8.55 10.4 6.97 5.36 6.57 6.3 5.55 3.92 5.61 12.7
9 11.2 7.97 8.43 8.46 4.67 7.12 4.6 3.95 5.76 12.5
10 8.04 7.91 5.97 7.48 6.5 6.94 3.88 6.07 7.04 9.62
Mean 9.02 8.91 7.36 6.60 7.11 7.27 4.02 6.20 6.98 11.22
Standard deviation 1.71 1.75 0.76 1.12 1.36 1.54 0.82 1.59 1.28 1.14
Standard error 0.54 0.55 0.24 0.35 0.43 0.49 0.26 0.50 0.40 0.36
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 183
Table 11-2 : Table containing RMS roughness (Rq) data for composite coating (from run 01
to run 10) along with mean, standard deviation and standard error.
Run No Run 01
Run 02
Run 03
Run 04
Run 05
Run 06
Run 07
Run 08
Run 09
Run 10
1 11.2 10.9 8.89 5.53 11.5 9.17 4.29 9.66 8.41 18.5
2 11.3 9.52 8.59 8.98 10.5 9.95 4.54 10.7 6.47 13.8
3 12.8 11.7 10 8.55 8.5 13.2 4.69 10 9.5 12.8
4 9.43 16.4 9.66 8.43 8.95 8.92 5.56 11.6 11.2 13.6
5 8.29 9.2 10.1 7.48 7.55 6.79 4.65 6.36 7.24 15.1
6 14 11 9.85 10.1 10.4 10 4.4 9.96 11.7 12.4
7 10.1 10.5 9.99 8.11 11.4 6.82 6.62 6.39 8.14 15.5
8 11 12.2 8.56 6.71 7.75 8.06 7.58 5.32 6.61 15.9
9 13.2 9.9 10.3 11.2 5.8 8.58 6.05 4.82 6.99 14.5
10 10.7 9.66 7.09 9.36 8.63 8.27 4.74 7.81 8.6 14.3
Mean 11.2 11.1 9.303 8.445 9.098 8.976 5.312 8.262 8.486 14.64
Standard dev 1.66 1.99 0.96 1.55 1.74 1.76 1.05 2.30 1.74 1.67
Standard Error 0.52 0.63 0.30 0.49 0.55 0.55 0.33 0.73 0.55 0.53
Figure 11-8 : Linear variation of HAZ area with the specific energy.
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 184
The EDS result for elemental weight percentage along the cross section of the multilayer
composite coating (from top to bottom) is presented in Table 11-3. The analysis is described
in chapter 6 at section 6.3.2.
Table 11-3 : EDS data of HA and Ti-6Al-4V multilayer composite cross section (from Top to
bottom).
Distance (mm) Top to Bottom
Al (wt %)
P (wt %)
Ti (wt %)
V (wt %)
Cr (wt %)
Fe (wt %)
Ni (wt %)
2.9 4.38 4.87 65.63 2.46 4.19 14.85 1.54
2.6 4.41 4.9 68.99 3.03 4.01 14.66 0
2.3 3.74 4.32 67.67 2.77 4.06 14.99 2.45
2 3.75 4.2 68.38 2.86 3.89 16.92 0
1.7 4.3 1.02 53.04 3.06 4.74 27.65 6.19
1.4 5.13 0.73 38.2 3.46 10.08 37.3 5.1
1.1 1.7 0.83 52.12 2.61 7.47 31.18 4.09
0.8 0.95 0.97 49.83 1.77 7.82 33.66 4.23
0.5 2.13 1.99 29.26 1.73 12.14 46.81 5.94
0.2 1.99 2.09 29.34 2.14 11.5 44.32 8.61
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 185
Vickers micro-hardness and fracture toughness data of multilayer composite coating
produced from HA and Ti-6Al-4V is presented below. The associated discussion is in
chapter 6 at section 6.3.3.
Table 11-4 : Table contains Vickers micro-hardness and fracture toughness data for
composite produced from HA and Ti-6Al-4V.
Ser Load (gf)
Diagonal 2a (µm)
Crack length 2c (µm)
Hardenss Hv
Fracture
toughness Kc
(Mpa.√m)
Top
1 500 32.8 48.3 861.8 3.40
2 500 32.7 49.1 867.1 3.32
3 500 32.6 51.6 872.4 3.08
4 500 32.8 48.7 861.8 3.36
5 500 33.1 44.1 846.2 3.90
Mean
32.80 48.36 861.86 3.41
Std. Deviation
0.17 2.42 8.76 0.27
Std. Error 0.07 1.08 3.92 0.12
Middle
1 500 33.7 55.3 816.4 2.78
2 500 34.4 56.6 783.5 2.68
3 500 34.4 53.1 783.5 2.95
4 500 34.4 47.3 783.5 3.51
5 500 34.6 51.5 774.4 3.09
Mean
34.30 52.76 788.26 3.00
Std. Deviation
0.31 3.25 14.50 0.29
Std. error
0.14 1.45 6.48 0.13
Bottom
1 500 35.7 68.5 727.5 2.01
2 500 34.9 57.6 761.2 2.61
3 500 35.7 62.7 727.5 2.30
4 500 35 60.5 756.8 2.43
5 500 35.5 69.2 735.7 1.98
Mean
35.36 63.70 741.74 2.27
Std. Deviation
0.34 4.51 14.47 0.24
Std. error 0.15 2.02 6.47 0.11
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 186
Vickers micro-hardness and fracture toughness data of pure HA coating is presented below.
The associated discussion is in chapter 6 at section 6.3.5.
Table 11-5 : Table contains Vickers micro-hardness and fracture toughness data for pure HA
coating.
Ser Load (gf)
Diagonal 2a (µm)
Crack length 2c (µm)
Hardenss (Hv)
Fracture
toughness Kc
(Mpa.√m)
1.00 100.00 25.60 33.60 282.90 2.18
2.00 100.00 25.50 38.60 285.10 1.34
3.00 100.00 25.60 37.60 282.90 1.46
4.00 100.00 27.10 42.50 252.50 1.10
5.00 100.00 27.10 45.20 252.50 0.94
6.00 100.00 26.00 39.00 274.30 1.33
7.00 100.00 25.10 34.40 294.30 1.90
8.00 100.00 25.60 35.50 282.90 1.77
9.00 100.00 26.10 44.40 272.20 0.95
Mean 25.97 38.98 275.51 1.44
Std. Deviation 0.66 4.01 13.66 0.41
Std. Error 0.22 1.34 4.55 0.14
Chapter 10. APPENDIX
Muhammad Rakib Mansur Page 187
The EDS result for elemental atomic percentage at the top section of the single layer and
multilayer composite coating is presented in Table 11-6. The analysis is described in chapter
6 at section 6.3.4.
Table 11-6 : Elements (at%) present on the top section of the single layer composite coating
and multilayer composite.
Al C Ca Cr Cu Fe Ni O P S Si Ti V Zn
Single layer (at%)
1.79 31.81 4.55 0.16 0.15 1 0 49.71 2.74 0.51 0.74 6.31 0 0.54
Multilayer Composie
(at%) 3.89 7.79 9.01 0.95 0 3.53 0.57 44.12 1.65 0 0.34 27.22 0.93 0