deposition and characterization of a coating from calcium ... · long-cherished dream. i am very...

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

.

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2014

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.

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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-

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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

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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

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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

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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

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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

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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

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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

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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



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-



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



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



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.



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.



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 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



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


2. REVIEW OF MATERIALS, METHODS AND CHARACTERIZATION TECHNIQUES



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



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.



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.



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.



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



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)



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.



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.



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,



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



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.



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.



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.



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.



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



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



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.



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



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.



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



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.



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.



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.



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



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.



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.



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



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.



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:



∑ ∑ ∑

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.



∑ | ( )

( )| 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.



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



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



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



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.



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.



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.



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.



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]



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



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



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



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



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.



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]:



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.



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



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.



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



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



∑ ( )

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



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



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


3. ANALYSIS OF FEEDSTOCK AND DIFFERENT TYPES OF CALCIUM PHOSPHATES



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.



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.



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.



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).



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].



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.



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).



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.



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.



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].



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.



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



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



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].



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



Figure 3-13 : Rietveld analysis of HA powder procured from Plasma biotal.

Figure 3-14 : Rietveld analysis of HA powder procured from Sigma Aldrich.



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


4. DEPOSITION OF CALCIUM PHOSPHATES AND Ti-6Al-4V ON STAINLESS STEEL



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.



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:



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.



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



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.



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



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.



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


5. CHARACTERIZATION OF CALCIUM PHOSPHATE AND Ti-6Al-4V COMPOSITE COATING



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.



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.



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



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



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 .



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.



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



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



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)


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



(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).



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.



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).



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.



(

√ )

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)


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 - -



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.



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.



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



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.



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



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


6. CALCIUM PHOSPHATE AND Ti-6Al-4V MULTILAYER COMPOSITE



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



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.



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



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



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.



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



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



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



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.



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%.



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.



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



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



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


7. TEMPERATURE AND COOLING RATE



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



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



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.



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



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.



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.



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



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.



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:



( )

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.



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.



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).



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.



Figure 7-7 : (a) Variation of temperature with power and traverse speed (b) variation of

cooling rate with power and traverse speed.



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.



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



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.



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



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



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.



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).



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



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



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


8. EFFECT OF TEMPERATURE AND COOLING RATE



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



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.



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.



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.



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)


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.



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.



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


9. CONCLUSION AND FUTURE SCOPE



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.



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



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.



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.

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Chapter 10. APPENDIX


11. APPENDIX



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.



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.



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.



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



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



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



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



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.



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



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



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



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

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