fluoridation and sintering of hydroxyapatite material and their mechanical properties

190 Vol.29 No.1 Muhammad Asif et al: Fluoridation and Sintering of Hydroxyapatite Materi...

Fluoridation and Sintering of Hydroxyapatite Material and Their Mechanical Properties

Muhammad Asif , FU Zhengyi*, WANG Weimin, WANG Hao, TAN Tiening, Shahzad Ahmad Khan

(State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China)

Abstract: In the current work hydroxyapatite Ca10(PO4)6·OH2 (HA) was sintered with the addition of 3 wt% aluminum isopropoxide (C9H21AlO3) powder and 3 wt % Te� on powder (-C2F2-). Sample was prepared by following sol-gel technique. Obtained pellets of samples were sintered. For investigation of effects of temperature on microstructures and mechanical properties the samples were sintered at various temperatures. For studying the phase composition, microstructures and elemental analysis the sintered samples were characterized by X-rays diffraction (XRD), scanning electron microscopy(SEM) and energy dispersive X-rays diffraction(EDAX) respectively. After sintering the samples mechanical properties, i e, grains size, apparent density, Vickers hardness, bending strength and compressive strength were found to be 2.14-18.76 m, 1.523 6- 0.752 g/cm3, 3.60-0.600 GPa and bending strength 33.265 8-14.900 MPa , 75-33 MPa, respectively. As a result of sintering � uoridated composite material was obtained.

Key words: � uoridated hydroxyapatite; pressureless sintering; mechanical properties; hydroxyapatite materials

©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2014(Received: Feb. 19, 2013; Accepted: Oct. 29, 2013)

Muhammad Asif : Ph D student; E-mail: [email protected]*Corresponding author: FU Zhengyi( ): Prof.; Ph D;

E-mail address: [email protected]

DOI 10.1007/s11595-014-0891-x

1 Introduction

Hydroxyapatite (HA) is one of the most suitable and interesting biomaterials widely used in artificial bone substitution because it is highly bioactive, nontoxic, non-inflammatory, non-immunogecity[1,2]. Calcium hydroxyapatite ( Ca10(PO4)6·(OH)2 is the most important biomaterial widely used for dental, orthopedics and maxillofacial applications because of its biocompatibility .The surface of hydroxyapatite is reactive and can directly attach to the bone through chemical bonding[2,3]. The composition and structure of hydroxyapatite is similar to apatite of human’s skeleton system. It is most suitable for substitution and reconstruction of bone[4]. Different approaches have been practiced extensively to improve the mechanical properties of HA, i e, by adding the dopants (metals, ceramics, polymers). Likewise different kinds of

hydroxyapatite composite materials have been prepared by controlling the microstructures through novel approach of sintering techniques[5].

Draw back and restriction of HA is due to its nature of brittleness ,relative poor mechanical properties , especially its low fracture toughness .So due to these drawbacks the use of HA is limited in load-bearing applications[6,7]. Sintering technique has key role in improving the mechanical properties due to action of certain mechanisms, i e, surface diffusion, condensation by evaporation, volume diffusion and grain boundary diffusion[8-10].

The mechanical properties and microstructure of HA ceramics is highly dependent on original starting material’s properties such as crystallinity, agglomeration, stoichiometry and other substituents. HA decomposes into alpha tri calcium phosphate -TCP and beta calcium phosphate -TCP when it is sintered at high temperature more than 1 300 during a long time[9]. However changing the microstructure of HA, substituent can prevent the decompositions of HA during pressureless sintering, i e, fluorination of HA. Fluoroapatite [F-HA, Ca10 (PO4)6F2] has higher thermal stability and retains it’s structures at higher sintering

Journal of Wuhan University of Technology-Mater. Sci. Ed. Feb.2014 191

temperature than pure HA [11]. Hydroxyapatite of natural bones contains

traces of elements such as Zn, K, F, Mg, Na, CO3 etc. These elements can be substituted into lattice of synthetic hydroxyapatite. Fluorides study provides clear evidences that Fluoride activates osteoblasts and improves the rate of minerals deposition in osteogenesis. Fluoride which exists in animal’s teeth and bones shows resistance against dissolution[12]. Due to resistance against dissolution � uoroapatite F HA is preferred over pure hydroxyapatite HA in biomedical applications[13,14]. It is found that the incorporation of fluoride ions into Hydroxyapatite HA structure considerably increases the resistance of hydroxyapatite to biodegradation and thermal decomposition as well[15].

The aim of current work is exploring the effects of sintering temperature on morphology and � uoridation of HA by using Te� on powder ( C2F2 )X. The use of organometallic compounds aluminum isoproproxide contains polymer part which furnishes agglomeration of crystals formation in sol-gel solution and metallic part aluminum (Al) of this organometallic compound will develop the mechanical properties of resulting sintered HA. Moreover, aluminum oxide compounds induce bone formation[12].This current experimental investigation of Teflon and aluminum isopropoxide interaction with hydroxyapatite is new research work.

2 Experimental

2.1 Raw material and method The s t a r t i ng r aw ma te r i a l cons i s t ed o f

HA powder, polymer Teflon (solid), polymer, aluminiumisopropoxide (C9H21AlO3), N-propanol, diethyl ether and H2O2. In the current experiment samples were prepared by sol-gel technique and sintering alternatively.2.2 Sol-gel process

Sol-gel is novel techniques for obtaining homogenous mixture of the solution and solution is heated uniformly at low temperature. More over in sol-gel liquid media facilitates the accomplishment of certain chemical reactions. In current experiment starting materials reactants were hydroxyapatite powder Teflon powder and aluminum isopropoxide C9H21Al2O3 powder. Basically 94 wt% of HA was dissolved in 500 mL of distilled water stirred at 60 for one hour and 3 wt% Te� on powder was dissolved in 50 mL of diethyl ether separately. Teflon solution

was stirred with magnetic stirrer for 2 hours at 60 . Aluminumisopropoxide 3 wt% was dissolved in 50 mL of n-propanol while stirring solution with magnetic stirrer at 60 for one hour. All these three solutions were prepared separately. Afterwards entire solutions were mixed together and stirred with magnetic stirrer. Then drops of hydrogen peroxide were added to solution while stirring. The obtained mixed solution was stirred vigorously at 60 for 2 hours .For obtaining fine crystals the obtained solution was kept for 24 hours at room temperature. After this the solution was filtered and washed with distilled water and ethyl alcohol for several times. The obtained sample powder was dried at 90 for 12 hours. Then powder was made into pellets of 12 mm dimension each. The sample pellets were sintered at various temperatures.2.3 Sintering

The sample pellets were sintered in inert media (N2) gas in pressureless sintering furnace. The samples were sintered at various temperatures. Heating rate from room temperature to 900 was fixed at 3

/ min and that from 900 to above temperature was fixed at 5 /min. The aim of using inert media nitrogen gas while sintering the specimens was to avoid the reaction of free atmospheric oxygen and allow the samples reactants elements to fully react themselves. 2.4 Polishing the pellets

Sintered pellets were grinded to make their surface very smooth for polishing. After grinding pellets were polished by using 0.3 micrometer and then 0.25 micrometer polishing machines before the measurement of Vickers hardness.2.5 Characterization

Bulk density of the sintered specimens was measured by Archimedes rule. The phase compositions of specimens were con� rmed by using X-ray diffraction (XRD) (RigakuUltima II with Cu K irradiation, Japan). For elemental analysis the specimen’s were characterized by energy dispersive X-rays spectroscopy (EDAX). The specific area over the samples surface was chosen randomly of each sample and analyzed. For this elemental analysis the sintered and polished specimens were cemented on copper stubs using graphite paint. The microstructures of sintered samples such as grain sizes were measured by scanning electron microscopy (SEM). For mechanical properties such as bending strength, the specimens were machined to bar shape of dimensions 2×4×12 mm3. For measuring out compressive strength the specimens were prepared to bar shape 2 × 3× 7 mm3 dimensions. Bending strength


and compressive strength were determined in testing centre of Wuhan University of Technology.

For Vickers hardness specimens were ground and polished. Hardness of the specimens was measured with Vickers indenter at load of 1 000 g.

3 Results and discussions

3.1 XRD

Fig.1 shows diffractogram of the samples sintered at various temperatures. While sintering the samples various temperatures ranges were chosen such as the sample (a) was sintered at 1 200 , (b) at 1 250 , (c) 1 300 , (d) 1 350 , (e) 1 375 , (f) 1 400 , (g) 1 430 and sample (h) 1 460 . The samples were scanned at the scanning rate of 100 /min at 2 degree in the range of ten to eighty degrees. The peaks of different intensities were obtained. At the most bottom of diffractogram shortest peaks are results of sample that was prepared by sol-gel method and dried at 100 . The intensity of this green sample peaks is very weak because sample is not crystalline well and sample has polymers also as shown in Fig.1. Others above all peaks are stronger because when sample was sintered at higher temperature, water molecules and gases were discharged and � ne crystals formed. Larger peaks at 32o prove the crystalline phase of fluoridated hydroxyapatite (F-HA) crystals. These peaks were studied by using X-pert software and peaks reference code numbers were noted. The code no. 86-0740, 71-0880, 38-1429 , 02-1189, 65-0535 correlate with compounds Ca3(PO4)OH, Ca5(PO4)3F, Ca3Al2O6,

AlF3 and CaF2 respectively. The major strong peak is obtained at 32o which is con� rmation of major phase of � uoridated hydroxyapatite.3.2 Thermo gravimetric analysis (TGA)

Fig.2. corresponds to thermo gravimetric analysis (TGA). The weight loss is the function of temperature under the controlled conditions. Sample of weight 14.76 mg was analyzed at 1 300 . The first weight lost (2.76 % ) is found at 300 which is due to removal of lattice water and 2nd weight loss (2.76 % ) is due to discharge of hydroxyl ions till 500 . The third weight loss of 6.67 % is due to certain carbonates decomposition[16,17]. While stirring water solution may absorb some carbon dioxide from air and for certain carbonates. During sol-gel formation solution absorbs CO2 from atmosphere and carbonated Apatite is developed. For polymers investigations polymers are completely burnt at 600 [18]. At 600 or at higher temperature polymers are decomposed and changed into gaseous molecules CO2 and are discharged. Above 600 weight loss is 3.43% . At higher temperature above 800 weight loss is 3.34 %.3.3 Scanning electron microscopy (SEM)

In Fig.3 SEM micrographs show grains sizes of the samples that were sintered at various temperatures. The samples were sintered at various temperatures such as sample (a) was sintered at 1 200 , (b) 1 250 , (c) 1 300 , (d) 1 350 , (e) 1 375 , (f) 1 400 , (g) 1 430 , and sample (h) 1 460 . After sintering their grains sizes were measured. The sample (a) average grains size was 2.14 m, sample (b) 1.28 m, (c) 7.29 m, (d) 7. 46 m, (e) 8.41 m, (f) 10.58 m, (g) 13.72 m and sample (h) 18.76 m respectively. The sample (a) sintered at 1 200 has average grain size 2.14 m and the sample (b) sintered at 1 250 has mean grain size 1.28 m .The sample (b) grains size is smaller than the sample (a) sintered at 1 200 because sample (a) is not sintered completely and their grains boundaries are not clear also as shown


in Fig.4.(a) and (b). The sample sintered at highest temperature 1 460 has obtained maximum grain size 18.76 m. By sintering the samples gradually at higher temperature sample grain size has increased gradually. When hydroxyapatite is sintered at higher temperatures, their grains become rod like as have been reported[19]. This increase in grain size causes in decrease of density. So as results of sintering at higher temperatures, fracture toughness and Vickers hardness are decreased[19,20]. Our sample that was sintered at 1 250 has minimum grain size. The sample sintered at 1 250 has satisfactory highest Vickers hardness 6.3

GPa with bending and compressive strength 33.265 8 MPa, 75.1331 MPa respectively.3.4 Energy dispersive X-rays (EDAX)

Elemental analysis was studied by energy dispersive X-rays spectroscopy as shown in Fig.5. The peaks are labelled with corresponding elements. This analysis provides the determination of elements of crystals. This analysis peaks have confirmed the presence of elements oxygen, fluoride, phosphorous and calcium. 3.5 Relationship of temperatures mecha-

nical properties Mechanical properties are highly dependent on

sintering temperatures. When the sample was sintered at 1 250 the grains size of the sample was 2.14 m but when the sample was sintered at 1 250 the grains size was decreased at little extent.

By sintering the sample at higher temperature the grains sizes was increased. When the sample was sintered at 1 460 the maximum average grains size was found to be 18.76 m. The graphical line representing grains size has reached at high altitude as shown in Fig.6.

By increasing the grains size the density of the sample was decreased. When the sample was sintered at 1 250 the density was found to be 2.746 2 g/cm3. While increasing the sintering temperature the density was deceased and by sintering the sample at 1 460 the achieved density of the sample was


found to be 1.244 3 g/cm3. The slope of density has reached at lower point steadily as shown in Fig.7. By sintering the sample at temperature higher than 1 300

hydroxyapatite decomposes into, -TCP to -TCP which also causes decrease in density as reported[21,22].

The mechanical properties such as compressive strength, bending strength, Vickers hardness are highly dependent on grains sizes, shapes and gains arrangements also. Fracture and crack of the ceramics is highly dependent on grain size and grain arrangements. The crack is Trans granular because great contribution of crack resistance is related to crossing of grain boundaries. The increase of grains

sizes causes the decrease of fracture toughness[20]. When the sample (a) was sintered at 1 200 the bending strength and Vickers harness were found to be 4.94 MPa and 3.6 GPa respectively. By sintering the sample (b) at 1 250 Vickers hardness and bending strength were found to be maximum 33.265 8 MPa and 6.3 GPa because grains sizes were found smallest. When sintering temperatures increased, the bending strength and Vickers hardness decreased. The graphs slopes of temperatures Vs bending strength and Vickers hardness have reached from climax to bottom as shown in Fig.8 and Fig.9. Increase in grains sizes causes decrease in Vickers hardness as reported[23].

Compressive strength was also found to increase with increasing sintering temperature. Maximum compressive strength was found to be 75.1331 MPa when the sample was sintered at 1 250 . By increasing the sintering temperature compressive strength was also going on decreasing. When the sample was sintered at higher temperature 1 460 the compressive strength was decreased to 33.616 6 MPa.3.6 Chemical reactions and explanations


Fluoridated hydroxyapatite composite material was obtained as a result of sintering. The starting materials were hydroxyapatite powder, Te� on powder and aluminum isopropoxide.

The effects of various sintering temperature on morphology of grains and mechanical properties were studied. When the sample was sintered at 1 200 the average grains size was found to be 2.14

m. After sintering the sample at highest temperature 1 460 the average grains size is found to be 18.76

m. Sintering temperatures has greatly affected the grains morphology. On increasing the sintering temperature gradually the grains sizes have increased. On increasing the sintering temperature smaller grains have merged their boundaries and developed into larger rod like structures. With sintering at 700 hexagonal prismatic HA crystals have been observed having an equivalent diameter of about 0.50-1.67 m and length ranging from 5.00 to 13.00 m reported by Ref.[24]. This change in morphology has changed the densities of samples. When the samples were sintered at 1 250 the density of the sample was found to be 2.746 2 g/cm3 with average grains size 1.28 m and Vickers hardness 6.3 GPa. By increasing the sintering temperature the density, bending strength and Vickers decreased. When the grains have smaller sizes they have higher densities. On increasing the sizes of the grains spaces among the grains become larger and larger which may have caused lowering in densities of samples. Moreover hardness and bending strength depend upon the arrangements of grains as well. In the current experiment the use of Teflon powder is new. Teflon has not been used with hydroxyapatite up till now. But certain other polymers have been used with hydroxyapatite by some researchers. While crystallization of preparing sol-gel solution calcium Ca2+ and PO4 of HA are nucleated on organic polymers[1]. Additionally water molecules are condensed between small size particles .This process leads to formation of chemical bonds and accelerates the agglomeration[25]. Our experiment TGA results show 15.64 wt% weight loss on sintering the sample at high temperature . This weight loss is due to the removal of polymers[26]. When Teflon solution was stirred with hydroxyapatite Teflon molecules may have associated with hydroxyapatite. Sintering the sample was accomplished in inert media nitrogen gas in absence of free oxygen. In the absence of oxygen sample’s enter atomic chemical reactions were carried out. On sintering the sample, polymeric parts of aluminum isopropoxide and Teflon were decomposed

while fluorides were not completely discharged by sintering. Obtained sintered samples were not free from � uorides as has been con� rmed from XRD.

But what is the exact chemical reaction and exact interaction of Te� on with hydroxyapatite is still questionable. This needs more detail work in future. As a result of sintering fluoridated hydroxyapatite was obtained which is con� rmed from XRD as shown in Fig.1. Moreover some fluorides have reacted with calcium of apatite and have formed calcium fluoride. Aluminum isopropoxide C9H21AlO3 contains aluminum. On sintering at higher temperature polymeric part of this compound has decomposed into carbon dioxide while leaving aluminum oxide. Some � uorides have reacted with aluminum and have formed aluminum tri� uoride AlF3. Some aluminum has reacted with calcium ions of hydroxyapatite and oxygen and has formed calcium aluminum oxide Ca3Al2O3. As a result of sintering � uoridated hydroxyapatite composite material has been obtained.

4 Conclusions

In current work fluoridated hydroxyapatite composite biomaterial was synthesized with the addition of polymers aluminiumisopropoxide powder 3 wt% and Te� on powder 3 wt%. The polymers templates with hydroxyapatite powder were sintered at various temperatures from 1 200 to 1 460 . After sintering at 1 460 the grain size was found to increase to 18.76 m with apparent density 1.198 8 g/cm3. Increasing the sintering temperature affected the grains sizes and mechanical properties. On increasing the sintering temperature grain size were found to increase steadily which caused decrease in apparent density and mechanical properties. On sintering the sample at 1 250 the mechanical properties such as bending strength, Vickers hardness and compressive strength were found to be maximum. While sintered at 1 250 the sample obtained maximum enhanced bending strength, Vickers hardness and compressive strength which were 33.265 8 MPa, 6.3 GPa and 75.133 1 MPa respectively. It is suggested that this obtained � uoridated composite biomaterial can be used in orthopedics.

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fluoridation and sintering of hydroxyapatite material and their mechanical properties

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