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fluoroapatite composites
Materials Science and Engineering C 20 (2002) 111 – 115 www.elsevier.com/locate/msec
Functionally graded tricalcium phosphate/f luoroapatite composites Lydia Helena Wong, Betty Tio, Xigeng Miao* School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore
Abstract This study was conducted to prepare fluorine-substituted apatite (FA)/beta-tricalcium phosphate (h-TCP) composites to combine the biostability of FA with the bioresorbability of h-TCP. The FA was prepared for the first time by mixing hydroxyapatite (HA) with aluminium fluoride (coded as AF). On the other hand, the h-TCP was prepared from a mixture of HA and CaHPO4 (calcium hydrogen phosphate). The dissolution behaviour of the composites was tested using a 10-wt.% citric acid solution. Scanning electron microscopy (SEM) indicated that h-TCP dissolved in the citric acid much faster than FA, resulting in macropores among the FA matrix. In addition to bulk FA/h-TCP composites, novel functionally gradient FA/h-TCP composites were also prepared by varying the particle size and the volume content of the h-TCP granules. The functionally gradient FA/h-TCP composites could be used to design implants. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Fluoroapatite; Hydroxyapatite; Tricalcium phosphate; Ceramic composites; Functionally gradient materials
1. Introduction Hydroxyapatite (HA) is an extensively studied biomaterial. One of the disadvantages of HA is its low thermal stability. It is well known that plasma-sprayed HA ceramics tend to contain tricalcium phosphates. HA ceramics prepared by other sintering methods also have impurity tricalcium phosphate (TCP) phases [1,2]. Even TCP-free HA ceramics are not always stoichiometric, causing the problem of grain boundary degradation in a biological environment and resulting in failure near the subsurface of an implant rather than on the implant –bone interface [3]. Fluorine is an essential trace element required for normal dental and skeletal development. Fluorine-substituted apatite (FA) has higher chemical and thermal stability than HA. Unlike HA, which normally transforms to TCP at above 1300 jC, fluoroapatite has shown phase stability even at higher temperatures [4]. The chemical stability of fluoroapatite is also reflected in the fact that fluorine-substituted apatites are less soluble than fluorine-free synthetic and biological apatites. FA does not degrade, whereas HA degrades severely in a biological environment [5]. In addition to the chemical and thermal stability, FA is also biocompatible and has a certain level of bioactivity.
*
Corresponding author. Tel.: +65-7904260; fax: +65-7909081. E-mail address: [email protected] (X. Miao).
On the other hand, beta-tricalcium phosphate (h-TCP) is a well-known bioresorbable ceramic. The advantage of hTCP is that it can be replaced by growing bone upon implantation. Thus, h-TCP has been widely used for bone repair. However, either HA, FA, or h-TCP has limitations if used individually. It is known that h-TCP dissolves too fast, whereas FA has too low a bioactivity compared to HA. If FA/h-TCP composites are prepared and implanted, then the h-TCP component can enhance early-stage bone ingrowth and bone bonding, whereas the remaining porous FA component can achieve long-term fixation of an implant. The concept of a functionally gradient material (FGM) has been increasingly used for biomaterial design. For example, HA/glass FGM layers were coated on titanium alloy (Ti –6Al – 4V) substrates [6]. The design of the FGM layers and the use of the glass were for achieving strong bonding between the FGM layered coatings and the substrates. In addition, HA/alpha-tricalcium phosphate (a-TCP) functionally gradient bioceramics were prepared by spreading a diamond powder on the surface of an HA powder compact, which was then fired at 1280 jC under reduced pressure [7]. In this case, the a-TCP phase could result in fast bone bonding, but the size of a-TCP phase was not controlled and was too fine to allow bone ingrowth. In line with the development of the above dense FGMs are porous FGMs. Various routes have been used for the formation of structures with graded micro- and macroporosity. Becker and Bolton [8] investigated the possibility of
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producing graded porous layered acetabular cups, where a porous surface layer was supported by a dense core. Werner et al. [9] prepared multilayer structures of hydroxyapatite with graded and interconnected macroporosity for optimal bone ingrowth. A recent development was reflected by the work done by Oh et al. [10], who prepared TCP – TiO2 (titania) biocomposites consisting of a TiO2 –TCP mixed layer on a TiO2 core. The TCP – TiO2 biocomposites were dense after sintering but gradually became porous on implantation. The purpose of this study was to prepare bulk FA/h-TCP composites and dense functionally gradient FA/h-TCP composites to finally prepare implants with a structure of a graded porous layer on a dense core. The FA for the matrices of the composites was prepared for the first time from a mixture of HA and aluminium fluoride. The particle size and the volume content of the h-TCP granules were so controlled that graded and interconnected pores could be formed for bone ingrowth and bone bonding.
2. Materials and methods 2.1. Preparation Hydroxyapatite [HA; Ca10(PO4)6(OH)2] powder (E. Merck, D-6100 Darmstadt, Germany) was the main starting material used. Aluminium fluoride (AlF33H2O) powder (coded as AF) and nature-occurring topaz powder (Al2SiO4 [F0.75, (OH)0.25]2; coded as SF) were used as additives. Different amounts of AF and SF additives were individually mixed with the HA powder by ball-free milling in distilled water containing glycerol (Sigma-Aldrich) as a binder, with the solid loading of the suspensions being about 40 wt.%. To ensure homogeneous mixing, the slurries of the mixed powders were stirred and dried on a hot plate. The dried powder was then crushed into fine powder using a ceramic mortar. Some of the powder was then pressed at 100 MPa and sintered in air at 1300 jC for 2 h. The remaining mixed powder of HA and AF was stored for later use. These sintered samples were used to study the effects of the additives on the thermal and chemical stability of the sintered ceramics. To prepare h-TCP granules, calcium hydrogen-phosphate (CaHPO4; coded as CHP) was thoroughly mixed with HA in the weight ratio of CHP/HA = 25:75. A small amount of glyceryl binder was also added into the powder mixture and green compacts of the mixed powder were formed at 100 MPa. The green compacts were then crushed into fine agglomerates and fired at 1150 jC for 2 h. The fired h-TCP granules were classified using sieves into four different particle sizes, namely, 150 –300, 106– 150, 75– 106, and < 75 Am. To produce a fluorine-substituted apatite (FA) matrix powder for the FA/h-TCP composites, some of the previously mixed powder of HA and AF was pressed at 100 MPa, crushed, and calcined at 850 jC for 1 h. After
calcination, four sizes (i.e. 150 –300, 106 – 150, 75– 106, and < 75 Am) of the FA granules were obtained by sieving. Four FA/h-TCP composites were prepared with the following combinations of phase content and particle size: (1) 50 wt.% FA (150 – 300 Am) + 50 wt.% h-TCP (150 – 300 Am); (2) 60 wt.% FA (106– 150 Am) + 40 wt.% h-TCP (106 – 150 Am); (3) 70 wt.% FA (75 – 106 Am) + 30 wt.% h-TCP (75 – 106 Am); (4) 80 wt.% FA ( < 75 Am) + 20 wt.% h-TCP ( < 75 Am) (coded as FA + 20 TCP, as an example). Finally, functionally gradient FA/h-TCP composites were prepared. As shown in Fig. 1, one of the graded composites was in the shape of a disk and contained four layers of the abovementioned four compositions, each layer being about 1 mm thick. The thickness of about 1 mm for each of the green compact layer was achieved by using a predetermined amount of mixed powder, which resulted in the targeted thickness after being pressed uniaxially in a steel die of 20 mm in diameter under the pressure of 100 MPa. The other graded composite was also in the shape of a disk but contained two sets of the abovementioned four composite layers, each layer being 0.5 mm thick controlled by using a certain amount of the mixed powder. Both the bulk FA/hTCP composites and the functionally graded (gradient) FA/ h-TCP composites were formed at 100 MPa and sintered at 1300 jC for 2 h. 2.2. Characterisation For X-ray diffraction (XRD) phase analysis on a Shimadzu XRD-6000 diffractometer, the fine grade h-TCP powder ( < 75 Am) was pressed into the cavity of a sample holder. Other sintered samples such as AF added HA, SF added HA, and pure HA were polished before XRD examination. Monochromic CuKa radiation was used with the operating conditions of 40 kV and 20 mA. The XRD data were collected over the 2h angle range of 20j– 40j at a speed of 4j/min. On the other hand, some sintered samples were cut into halves for SEM observation. The two sets of the halves were then cold mounted with a resin, followed by polishing to a mirror finish. One set of polished halves was further subjected to corrosion test in a 10 wt.% citric acid
Fig. 1. Schematic diagram showing the arrangement of the FA/h-TCP composite layers. (a) Non-symmetric functionally gradient material (FGM); (b) symmetric FGM.
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solution for 2 h. The corroded surfaces were then cleaned with running distilled water and ethanol solution before being dried in an oven (55 jC) for a few hours. Finally, the as-polished and the corroded surfaces were carbon coated before examination on a JSM-5310 scanning electron microscope.
3. Results and discussion 3.1. Effect of additives on microstructure After sintering at 1300 jC for 2 h, pure HA samples were bluish in color, AF added HA samples were slightly bluish, and SF added HA samples were white. The shrinkage of AF added HA samples was similar to that of the pure HA samples, whereas the SF added HA samples exhibited only a small shrinkage. SEM on the polished surfaces revealed large amounts of micropores in the SF added HA samples, whereas only small amounts of pores were visible in the pure HA and the AF added HA samples. The different colors suggest chemical reactions taking place during the sintering, and the high porosity of the SF added HA samples indicates the poor densification during the sintering. Pure HA samples resulted in expected HA phase (Fig. 2). On the other hand, AF added HA resulted in XRD patterns similar to those of the HA (Fig. 2). The phase in the AF added HA sample was loosely referred to FA phase due to the color change observed, the fluorine element in the AF additive, and the chemical stability of the AF added HA samples (to be seen later). Since AF is a chemical compound that can decompose into vapors (AF3/AlOF) at f 700 jC, much lower than the sintering temperature of 1300 jC, the vapor could enter the host HA crystal structure to form fluorine-substituted HA (i.e. FA). In addition, the decomposition of AF could be completed well before the densification process. Thus, the AF addition to HA was able to achieve stable FA phase and high sinterability.
Fig. 2. XRD results of pure HA and AF added HA (AFHA) samples.
Fig. 3. XRD results of pure HA, synthetic TCP, and SF added HA (SFHA) samples.
On the other hand, as shown in Fig. 3, the SF addition to HA caused the formation of h-TCP phase in addition to an FAV phase. FAV should have the same Bravais lattice as the previously said FA, although the chemical composition of FAV should be slightly different from that of FA. As mentioned before, the SF was a natural topaz, having the composition of Al2SiO4[F0.75, (OH)0.25]2. On heating to temperatures higher than 1000 jC, the natural topaz decomposes to form a crystalline phase and various vapors [11], that is, 6Al2 SiO4 ½F0:75 , ðOHÞ0:25 2 ! 2ð3Al2 O3 2SiO2 Þ # ðmulliteÞ þ 2SiF4 z þ HFz þ H2 Oz
ð1Þ
Based on this fact of SF, one can imagine that the evaporation of SF in the samples during the sintering would impair the densification process. The formed porosity in turn was in favor of the decomposition of the HA into hTCP, since the H2O vapor evolving from the HA could escape through the pore channels.
Fig. 4. SEM micrograph of h-TCP granules with particle size 106 – 150 Am.
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Fig. 5. Schematic diagram showing the shape change from green compacts to sintered bodies. (a) Non-symmetric FGM; (b) symmetric FGM.
When 25 g of CaHPO4 (CHP) was mixed with 75 g of HA (Ca10(PO4)6(OH)2) and heated to 1150 jC for 2 h, pure h-TCP phase was formed, as shown in Fig. 3. Thus, the following hypothesis is confirmed: Ca10 ðPO4 Þ6 ðOHÞ2 þ 2:51CaHPO4 ! 4:17Ca3 ðPO4 Þ2 þ 2:225H2 O
ð2Þ
From the above reaction equation, one can see that the weight percentage of CHP/(CHP + HA) for the formation of h-TCP is 25.4 wt.%, which is in agreement with the ratio of CHP/HA = 25:75 used in this study. 3.2. Effect of additives on chemical stability When HA, AF added HA, and SF added HA samples were sintered, polished, and finally immersed in a 10-wt.% citric acid solution for 2 h, it was found that the surface of AF added HA was still quite shining, whereas the surfaces of pure HA and SF added HA samples were quite dim.
Fig. 6. SEM micrograph of a cross section corresponding to the dashed and arrowhead-marked area shown in Fig. 5(b).
Fig. 7. Stereomicroscopic image of the non-symmetric FGM shown in Fig. 5(a).
Thus, the corrosion resistance of AF added HA in the acid was higher than that of the other two samples, as was also confirmed by SEM observation. SEM micrographs indicated that the corroded surface of the AF added HA sample was relatively smooth. On the other hand, the corroded surface of pure HA was very rough and looked like a transgranular fracture surface. The corroded surface of the SF added HA sample was featured with both roughness and porosity. Therefore, it was established that AF added HA sample was more chemically stable than HA and SF added HA samples. The low corrosion resistance of SF added HA sample was attributed to the high porosity and the presence of highly resorbable h-TCP phase in the sample. It should be noted that the acid solution used in this study was more corrosive than the simulated body fluid (SBF) commonly found in other literature. Thus, the acid solution gave an accelerated assessment on the corrosion resistance of the samples. The chemical stability of h-TCP, HA, and FA was found to follow the order of h-TCP < HA < FA, which is in agreement with the well-known order of bioactivity, that is, h-TCP > HA>FA.
Fig. 8. Stereomicroscopic image of the symmetric FGM shown in Fig. 5(b).
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FA/h-TCP composites would become graded porous FA, which would be desirable for both bone ingrowth and bone bonding.
4. Conclusions 1.
2.
3. Fig. 9. SEM micrograph showing the graded porosity formed on the surface of the graded FA/h-TCP composite subjected to citric acid corrosion.
4. 3.3. Functionally gradient FA/h-TCP composites Fig. 4 shows the prepared h-TCP granules with particle size 106 –150 Am, which would be suitable for creating pores in FA/h-TCP implants. The granule size is also within the well-known pore size range of 100 –400 Am suitable for bone ingrowth. Furthermore, the h-TCP granules exhibited open microporosity, which is favorable since porous TCP granules can result in faster dissolution in a biological environment than fully dense TCP. Fig. 5 schematically shows the composite layers of the green disks and the sintered shapes of the functionally gradient FA/h-TCP composites. Figs. 6 – 8 are photographs of the composites indicated in Fig. 5. To be more specific, in Fig. 5(a), the non-symmetrically graded green compact resulted in a distorted shape after sintering. In other words, the side surface was like a cone, whereas the top surface was in the shape of a dimple about 2 mm deep in the center, and the bottom surface was convex with the center protruding 1.5 mm out. On the other hand, Fig. 5(b) shows that a disklike shape with a neck in the middle was formed after sintering. The difference between the sintered shape and the original green shape was due to the differential sintering shrinkage, which was affected by the particle size and the phase content of the h-TCP component. The matrix FA phase and the h-TCP phase were identifiable under SEM based on the difference in brightness and contrast. The content and the size of the h-TCP phase were found to vary gradually to form a functionally gradient material. When the functionally gradient FA/h-TCP composite was polished and subjected to and immersed in the 10 wt.% citric acid solution, the h-TCP phase was dissolved and a functionally gradient porous FA material was formed (Fig. 9). This result suggests that if implanted, the graded
A novel fluorine-substituted apatite (FA) ceramic was produced by mixing aluminium fluoride (AF) with hydroxyapatite (HA), followed by sintering at 1300 jC for 2 h. Beta-tricalcium phosphate (h-TCP) granules were prepared from HA and CaHPO4 (calcium hydrogen phosphate) mixtures through a solid state reaction. FA/h-TCP composites, consisting of a stable FA matrix and a soluble h-TCP phase, were successfully prepared. Functionally gradient FA/h-TCP composites were also prepared with gradients in particle size and phase content of the h-TCP phase. The preferential dissolution of the h-TCP phase would result in functionally gradient porosity for bone ingrowth.
Acknowledgements The authors thank all those laboratory technicians, who were very supportive for the experimental work, especially, Wilson Lim, Shao Rui, and Fabian Seow of the ceramic processing laboratory and Irene Heng and Wang Lee Chin of the advanced materials characterisation laboratory.
References [1] N. Kivrak, A.C. Tas, J. Am. Ceram. Soc. 81 (9) (1998) 2245. [2] O. Gauthier, J.M. Bouler, J. Mater. Sci.: Mater. Med. 10 (1999) 199. [3] J.T. Edwards, J.B. Brunski, H.W. Higuchi, J. Biomed. Mater. Res. 36 (1997) 454. [4] E. Adolfsson, M. Nygren, L. Hermansson, J. Am. Ceram. Soc. 82 (10) (1999) 2909. [5] W.J.A. Dhert, P. Thomsen, C.P.A.T. Klein, K. De Groot, P.M. Rozing, L.E. Ericson, J. Mater. Sci.: Mater. Med. 5 (2) (1994) 59. [6] S. Ban, J. Hasegawa, S. Maruno, Mater. Sci. Forum 308 – 311 (1999) 350. [7] M. Kon, K. Ishikawa, Y. Miyamoto, K. Asaoka, Biomaterials 16 (9) (1995) 709. [8] B.S. Becker, J.D. Bolton, J. Mater. Sci.: Mater. Med. 8 (12) (1997) 793. [9] J.-P. Werner, C. Lathe, P. Greil, in: British Ceramic Proceedings No. 60, the Sixth Conference and Exhibition of the European Ceramic Society, Abstracts, vol. 2, Institute of Ceramics, Stroke-on-Trent, UK, 1990, p. 157. [10] K.-S. Oh, F. Caroff, R. Famery, M.-F. Sigot-Luizard, P. Boch, J. Eur. Ceram. Soc. 18 (1998) 1931. [11] R.A. Day, E.R. Vance, D.J. Cassidy, J. Mater. Res. 10 (1995) 2963.
Functionally graded tricalcium phosphate/f luoroapatite composites Lydia Helena Wong, Betty Tio, Xigeng Miao* School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore
Abstract This study was conducted to prepare fluorine-substituted apatite (FA)/beta-tricalcium phosphate (h-TCP) composites to combine the biostability of FA with the bioresorbability of h-TCP. The FA was prepared for the first time by mixing hydroxyapatite (HA) with aluminium fluoride (coded as AF). On the other hand, the h-TCP was prepared from a mixture of HA and CaHPO4 (calcium hydrogen phosphate). The dissolution behaviour of the composites was tested using a 10-wt.% citric acid solution. Scanning electron microscopy (SEM) indicated that h-TCP dissolved in the citric acid much faster than FA, resulting in macropores among the FA matrix. In addition to bulk FA/h-TCP composites, novel functionally gradient FA/h-TCP composites were also prepared by varying the particle size and the volume content of the h-TCP granules. The functionally gradient FA/h-TCP composites could be used to design implants. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Fluoroapatite; Hydroxyapatite; Tricalcium phosphate; Ceramic composites; Functionally gradient materials
1. Introduction Hydroxyapatite (HA) is an extensively studied biomaterial. One of the disadvantages of HA is its low thermal stability. It is well known that plasma-sprayed HA ceramics tend to contain tricalcium phosphates. HA ceramics prepared by other sintering methods also have impurity tricalcium phosphate (TCP) phases [1,2]. Even TCP-free HA ceramics are not always stoichiometric, causing the problem of grain boundary degradation in a biological environment and resulting in failure near the subsurface of an implant rather than on the implant –bone interface [3]. Fluorine is an essential trace element required for normal dental and skeletal development. Fluorine-substituted apatite (FA) has higher chemical and thermal stability than HA. Unlike HA, which normally transforms to TCP at above 1300 jC, fluoroapatite has shown phase stability even at higher temperatures [4]. The chemical stability of fluoroapatite is also reflected in the fact that fluorine-substituted apatites are less soluble than fluorine-free synthetic and biological apatites. FA does not degrade, whereas HA degrades severely in a biological environment [5]. In addition to the chemical and thermal stability, FA is also biocompatible and has a certain level of bioactivity.
*
Corresponding author. Tel.: +65-7904260; fax: +65-7909081. E-mail address: [email protected] (X. Miao).
On the other hand, beta-tricalcium phosphate (h-TCP) is a well-known bioresorbable ceramic. The advantage of hTCP is that it can be replaced by growing bone upon implantation. Thus, h-TCP has been widely used for bone repair. However, either HA, FA, or h-TCP has limitations if used individually. It is known that h-TCP dissolves too fast, whereas FA has too low a bioactivity compared to HA. If FA/h-TCP composites are prepared and implanted, then the h-TCP component can enhance early-stage bone ingrowth and bone bonding, whereas the remaining porous FA component can achieve long-term fixation of an implant. The concept of a functionally gradient material (FGM) has been increasingly used for biomaterial design. For example, HA/glass FGM layers were coated on titanium alloy (Ti –6Al – 4V) substrates [6]. The design of the FGM layers and the use of the glass were for achieving strong bonding between the FGM layered coatings and the substrates. In addition, HA/alpha-tricalcium phosphate (a-TCP) functionally gradient bioceramics were prepared by spreading a diamond powder on the surface of an HA powder compact, which was then fired at 1280 jC under reduced pressure [7]. In this case, the a-TCP phase could result in fast bone bonding, but the size of a-TCP phase was not controlled and was too fine to allow bone ingrowth. In line with the development of the above dense FGMs are porous FGMs. Various routes have been used for the formation of structures with graded micro- and macroporosity. Becker and Bolton [8] investigated the possibility of
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producing graded porous layered acetabular cups, where a porous surface layer was supported by a dense core. Werner et al. [9] prepared multilayer structures of hydroxyapatite with graded and interconnected macroporosity for optimal bone ingrowth. A recent development was reflected by the work done by Oh et al. [10], who prepared TCP – TiO2 (titania) biocomposites consisting of a TiO2 –TCP mixed layer on a TiO2 core. The TCP – TiO2 biocomposites were dense after sintering but gradually became porous on implantation. The purpose of this study was to prepare bulk FA/h-TCP composites and dense functionally gradient FA/h-TCP composites to finally prepare implants with a structure of a graded porous layer on a dense core. The FA for the matrices of the composites was prepared for the first time from a mixture of HA and aluminium fluoride. The particle size and the volume content of the h-TCP granules were so controlled that graded and interconnected pores could be formed for bone ingrowth and bone bonding.
2. Materials and methods 2.1. Preparation Hydroxyapatite [HA; Ca10(PO4)6(OH)2] powder (E. Merck, D-6100 Darmstadt, Germany) was the main starting material used. Aluminium fluoride (AlF33H2O) powder (coded as AF) and nature-occurring topaz powder (Al2SiO4 [F0.75, (OH)0.25]2; coded as SF) were used as additives. Different amounts of AF and SF additives were individually mixed with the HA powder by ball-free milling in distilled water containing glycerol (Sigma-Aldrich) as a binder, with the solid loading of the suspensions being about 40 wt.%. To ensure homogeneous mixing, the slurries of the mixed powders were stirred and dried on a hot plate. The dried powder was then crushed into fine powder using a ceramic mortar. Some of the powder was then pressed at 100 MPa and sintered in air at 1300 jC for 2 h. The remaining mixed powder of HA and AF was stored for later use. These sintered samples were used to study the effects of the additives on the thermal and chemical stability of the sintered ceramics. To prepare h-TCP granules, calcium hydrogen-phosphate (CaHPO4; coded as CHP) was thoroughly mixed with HA in the weight ratio of CHP/HA = 25:75. A small amount of glyceryl binder was also added into the powder mixture and green compacts of the mixed powder were formed at 100 MPa. The green compacts were then crushed into fine agglomerates and fired at 1150 jC for 2 h. The fired h-TCP granules were classified using sieves into four different particle sizes, namely, 150 –300, 106– 150, 75– 106, and < 75 Am. To produce a fluorine-substituted apatite (FA) matrix powder for the FA/h-TCP composites, some of the previously mixed powder of HA and AF was pressed at 100 MPa, crushed, and calcined at 850 jC for 1 h. After
calcination, four sizes (i.e. 150 –300, 106 – 150, 75– 106, and < 75 Am) of the FA granules were obtained by sieving. Four FA/h-TCP composites were prepared with the following combinations of phase content and particle size: (1) 50 wt.% FA (150 – 300 Am) + 50 wt.% h-TCP (150 – 300 Am); (2) 60 wt.% FA (106– 150 Am) + 40 wt.% h-TCP (106 – 150 Am); (3) 70 wt.% FA (75 – 106 Am) + 30 wt.% h-TCP (75 – 106 Am); (4) 80 wt.% FA ( < 75 Am) + 20 wt.% h-TCP ( < 75 Am) (coded as FA + 20 TCP, as an example). Finally, functionally gradient FA/h-TCP composites were prepared. As shown in Fig. 1, one of the graded composites was in the shape of a disk and contained four layers of the abovementioned four compositions, each layer being about 1 mm thick. The thickness of about 1 mm for each of the green compact layer was achieved by using a predetermined amount of mixed powder, which resulted in the targeted thickness after being pressed uniaxially in a steel die of 20 mm in diameter under the pressure of 100 MPa. The other graded composite was also in the shape of a disk but contained two sets of the abovementioned four composite layers, each layer being 0.5 mm thick controlled by using a certain amount of the mixed powder. Both the bulk FA/hTCP composites and the functionally graded (gradient) FA/ h-TCP composites were formed at 100 MPa and sintered at 1300 jC for 2 h. 2.2. Characterisation For X-ray diffraction (XRD) phase analysis on a Shimadzu XRD-6000 diffractometer, the fine grade h-TCP powder ( < 75 Am) was pressed into the cavity of a sample holder. Other sintered samples such as AF added HA, SF added HA, and pure HA were polished before XRD examination. Monochromic CuKa radiation was used with the operating conditions of 40 kV and 20 mA. The XRD data were collected over the 2h angle range of 20j– 40j at a speed of 4j/min. On the other hand, some sintered samples were cut into halves for SEM observation. The two sets of the halves were then cold mounted with a resin, followed by polishing to a mirror finish. One set of polished halves was further subjected to corrosion test in a 10 wt.% citric acid
Fig. 1. Schematic diagram showing the arrangement of the FA/h-TCP composite layers. (a) Non-symmetric functionally gradient material (FGM); (b) symmetric FGM.
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solution for 2 h. The corroded surfaces were then cleaned with running distilled water and ethanol solution before being dried in an oven (55 jC) for a few hours. Finally, the as-polished and the corroded surfaces were carbon coated before examination on a JSM-5310 scanning electron microscope.
3. Results and discussion 3.1. Effect of additives on microstructure After sintering at 1300 jC for 2 h, pure HA samples were bluish in color, AF added HA samples were slightly bluish, and SF added HA samples were white. The shrinkage of AF added HA samples was similar to that of the pure HA samples, whereas the SF added HA samples exhibited only a small shrinkage. SEM on the polished surfaces revealed large amounts of micropores in the SF added HA samples, whereas only small amounts of pores were visible in the pure HA and the AF added HA samples. The different colors suggest chemical reactions taking place during the sintering, and the high porosity of the SF added HA samples indicates the poor densification during the sintering. Pure HA samples resulted in expected HA phase (Fig. 2). On the other hand, AF added HA resulted in XRD patterns similar to those of the HA (Fig. 2). The phase in the AF added HA sample was loosely referred to FA phase due to the color change observed, the fluorine element in the AF additive, and the chemical stability of the AF added HA samples (to be seen later). Since AF is a chemical compound that can decompose into vapors (AF3/AlOF) at f 700 jC, much lower than the sintering temperature of 1300 jC, the vapor could enter the host HA crystal structure to form fluorine-substituted HA (i.e. FA). In addition, the decomposition of AF could be completed well before the densification process. Thus, the AF addition to HA was able to achieve stable FA phase and high sinterability.
Fig. 2. XRD results of pure HA and AF added HA (AFHA) samples.
Fig. 3. XRD results of pure HA, synthetic TCP, and SF added HA (SFHA) samples.
On the other hand, as shown in Fig. 3, the SF addition to HA caused the formation of h-TCP phase in addition to an FAV phase. FAV should have the same Bravais lattice as the previously said FA, although the chemical composition of FAV should be slightly different from that of FA. As mentioned before, the SF was a natural topaz, having the composition of Al2SiO4[F0.75, (OH)0.25]2. On heating to temperatures higher than 1000 jC, the natural topaz decomposes to form a crystalline phase and various vapors [11], that is, 6Al2 SiO4 ½F0:75 , ðOHÞ0:25 2 ! 2ð3Al2 O3 2SiO2 Þ # ðmulliteÞ þ 2SiF4 z þ HFz þ H2 Oz
ð1Þ
Based on this fact of SF, one can imagine that the evaporation of SF in the samples during the sintering would impair the densification process. The formed porosity in turn was in favor of the decomposition of the HA into hTCP, since the H2O vapor evolving from the HA could escape through the pore channels.
Fig. 4. SEM micrograph of h-TCP granules with particle size 106 – 150 Am.
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Fig. 5. Schematic diagram showing the shape change from green compacts to sintered bodies. (a) Non-symmetric FGM; (b) symmetric FGM.
When 25 g of CaHPO4 (CHP) was mixed with 75 g of HA (Ca10(PO4)6(OH)2) and heated to 1150 jC for 2 h, pure h-TCP phase was formed, as shown in Fig. 3. Thus, the following hypothesis is confirmed: Ca10 ðPO4 Þ6 ðOHÞ2 þ 2:51CaHPO4 ! 4:17Ca3 ðPO4 Þ2 þ 2:225H2 O
ð2Þ
From the above reaction equation, one can see that the weight percentage of CHP/(CHP + HA) for the formation of h-TCP is 25.4 wt.%, which is in agreement with the ratio of CHP/HA = 25:75 used in this study. 3.2. Effect of additives on chemical stability When HA, AF added HA, and SF added HA samples were sintered, polished, and finally immersed in a 10-wt.% citric acid solution for 2 h, it was found that the surface of AF added HA was still quite shining, whereas the surfaces of pure HA and SF added HA samples were quite dim.
Fig. 6. SEM micrograph of a cross section corresponding to the dashed and arrowhead-marked area shown in Fig. 5(b).
Fig. 7. Stereomicroscopic image of the non-symmetric FGM shown in Fig. 5(a).
Thus, the corrosion resistance of AF added HA in the acid was higher than that of the other two samples, as was also confirmed by SEM observation. SEM micrographs indicated that the corroded surface of the AF added HA sample was relatively smooth. On the other hand, the corroded surface of pure HA was very rough and looked like a transgranular fracture surface. The corroded surface of the SF added HA sample was featured with both roughness and porosity. Therefore, it was established that AF added HA sample was more chemically stable than HA and SF added HA samples. The low corrosion resistance of SF added HA sample was attributed to the high porosity and the presence of highly resorbable h-TCP phase in the sample. It should be noted that the acid solution used in this study was more corrosive than the simulated body fluid (SBF) commonly found in other literature. Thus, the acid solution gave an accelerated assessment on the corrosion resistance of the samples. The chemical stability of h-TCP, HA, and FA was found to follow the order of h-TCP < HA < FA, which is in agreement with the well-known order of bioactivity, that is, h-TCP > HA>FA.
Fig. 8. Stereomicroscopic image of the symmetric FGM shown in Fig. 5(b).
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FA/h-TCP composites would become graded porous FA, which would be desirable for both bone ingrowth and bone bonding.
4. Conclusions 1.
2.
3. Fig. 9. SEM micrograph showing the graded porosity formed on the surface of the graded FA/h-TCP composite subjected to citric acid corrosion.
4. 3.3. Functionally gradient FA/h-TCP composites Fig. 4 shows the prepared h-TCP granules with particle size 106 –150 Am, which would be suitable for creating pores in FA/h-TCP implants. The granule size is also within the well-known pore size range of 100 –400 Am suitable for bone ingrowth. Furthermore, the h-TCP granules exhibited open microporosity, which is favorable since porous TCP granules can result in faster dissolution in a biological environment than fully dense TCP. Fig. 5 schematically shows the composite layers of the green disks and the sintered shapes of the functionally gradient FA/h-TCP composites. Figs. 6 – 8 are photographs of the composites indicated in Fig. 5. To be more specific, in Fig. 5(a), the non-symmetrically graded green compact resulted in a distorted shape after sintering. In other words, the side surface was like a cone, whereas the top surface was in the shape of a dimple about 2 mm deep in the center, and the bottom surface was convex with the center protruding 1.5 mm out. On the other hand, Fig. 5(b) shows that a disklike shape with a neck in the middle was formed after sintering. The difference between the sintered shape and the original green shape was due to the differential sintering shrinkage, which was affected by the particle size and the phase content of the h-TCP component. The matrix FA phase and the h-TCP phase were identifiable under SEM based on the difference in brightness and contrast. The content and the size of the h-TCP phase were found to vary gradually to form a functionally gradient material. When the functionally gradient FA/h-TCP composite was polished and subjected to and immersed in the 10 wt.% citric acid solution, the h-TCP phase was dissolved and a functionally gradient porous FA material was formed (Fig. 9). This result suggests that if implanted, the graded
A novel fluorine-substituted apatite (FA) ceramic was produced by mixing aluminium fluoride (AF) with hydroxyapatite (HA), followed by sintering at 1300 jC for 2 h. Beta-tricalcium phosphate (h-TCP) granules were prepared from HA and CaHPO4 (calcium hydrogen phosphate) mixtures through a solid state reaction. FA/h-TCP composites, consisting of a stable FA matrix and a soluble h-TCP phase, were successfully prepared. Functionally gradient FA/h-TCP composites were also prepared with gradients in particle size and phase content of the h-TCP phase. The preferential dissolution of the h-TCP phase would result in functionally gradient porosity for bone ingrowth.
Acknowledgements The authors thank all those laboratory technicians, who were very supportive for the experimental work, especially, Wilson Lim, Shao Rui, and Fabian Seow of the ceramic processing laboratory and Irene Heng and Wang Lee Chin of the advanced materials characterisation laboratory.
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