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Improved thermal and mechanical properties in hydroxyapatite–titanium composites by incorporating silica-coated titanium
Materials Letters 143 (2015) 322–325
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improved thermal and mechanical properties in hydroxyapatite–titanium composites by incorporating silica-coated titanium Hakimeh Wakily a, Ali Dabbagh a,n, Hadijah Abdullah a, Nur Farha Abdul Halim a, Noor Hayaty Abu Kasim a a
Department of Restorative Dentistry, Faculty of Dentistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 19 March 2014 Accepted 19 December 2014 Available online 30 December 2014
Hydroxyapatite–titanium composites are promising materials for fabrication of the load-bearing implants. However, the mechanical properties of these composites are negatively affected by formation of fragile compounds during the high-temperature processing. In this study, titanium particles were coated with a silica layer to decrease the metallic–ceramic interactions at the sintering temperature range. The results indicated the decomposition of hydroxyapatite and oxidation of titanium during the sintering process at 1100 1C. However, the undesired interactions between hydroxyapatite and titanium components were minimized, causing complete removal of calcium titanate and titanium phosphides as well as the formation of stable calcium phosphates in the sintered composite. Consequently, composites containing identical weight ratios of hydroxyapatite and silica-coated titanium exhibited a relatively high Vickers' hardness value comparable to that of titanium–hydroxyapatite composites with a weight ratio of 3:1. Therefore, surface modification of titanium particles using a silica layer could significantly improve the mechanical properties of the obtained composites by increasing their thermal stability during the sintering process. & 2014 Elsevier B.V. All rights reserved.
Keywords: Biomaterials Composite materials Sintering Phase transformation
1. Introduction Hydroxyapatite–titanium (HA–Ti) bio-composites have recently received increasing attention because of offering both biocompatibility and high mechanical strength contributed by HA and Ti, respectively [1–3]. These composites exhibit characteristics such as bio-inertness, low Young’s modulus and high biocompatibility. However, undesirable interaction between HA and Ti components during the sintering process results in composites with poor mechanical characteristics [1,3]. Pure HA is dehydroxylated at approximately 900 1C in air and at 850 1C in a water-free atmosphere, followed by decomposition to tetra calcium phosphate (TTCP: Ca4(PO4)2O) and/or tricalcium phosphate (TCP: Ca3(PO4)2) [3]. However, during the sintering of HA–Ti system, the dehydrated water of HA reacts with Ti ions to yield titanium oxide, thereby accelerating the HA dehydroxylation and decomposition [3]. Based on the experimental parameters, the sintering products may include TCP, TTCP, calcium titanate (CaTiO3), titanium phosphides (TixPy), titanates (TixOy), calcium oxide (CaO), and amorphous phases [4]. Particularly, a sintering process under argon atmosphere
n
Corresponding author. Tel.: þ 603 7967 6451; fax: þ 603 7967 6456. E-mail address: [email protected] (A. Dabbagh).
http://dx.doi.org/10.1016/j.matlet.2014.12.092 0167-577X/& 2014 Elsevier B.V. All rights reserved.
at 1200 1C could completely eliminate the calcium phosphate phases (CaPs) and produce a compact layer comprised of TixPy and CaTiO3 around the titanium/titanate particles [2]. It is well-known that the mechanical behaviour of the HA–Ti system is significantly affected by the microstructure and phase distribution in the sintered composite. Previous studies indicated that some phases such as TixPy and CaTiO3 which form between HA and Ti particles at elevated temperatures, present a fragile behaviour without displaying any yield strength prior to rupture and thus provide a weak interface bonding in the matrix [2]. These fragile phases are subsequently removed during polishing and result in the formation of gaps and pores between Ti particles [2]. One particular approach to address this issue is the incorporation of low-melting-point biocompatible additives as sintering aids to the initial components [5]. However, this method may not eliminate the undesired sintering products especially in composites with high HA/Ti ratios. This study is an attempt to present a new technique for improving the thermal and mechanical properties of HA–Ti composites with high HA/Ti ratios. In this approach, the Ti particles are coated by thin silica layers prior mixing with HA to prevent the undesired metallic– ceramic reactions at increased temperatures. By removing the fragile compounds, the resulting composite may provide improved mechanical strength for load-bearing application.
H. Wakily et al. / Materials Letters 143 (2015) 322–325
323
2. Materials and methods
3. Results and discussion
The composites containing HA and silica-coated titanium particles (HA–STP) were prepared using commercially available pure titanium ( 45 mm), hydroxyapatite ( o45 mm), tetraethylorthosilicate (TEOS), and ammonia (25% v/v) purchased from Sigma-Aldrich. For the synthesis of STPs, 1 g of Ti particles was stirred in 100 mL of ethanol–water solution (10:1), followed by injecting ammonia to achieve a pH of 12.5. Then, 4.67 mL TEOS was added to initiate the reaction. After stirring for 4 h, the mixture was aged for 24 h, rinsed with deionized water, and centrifuged to remove unattached polysiloxane oligomers. The obtained STP particles were then dried at 60 1C for 6 h. The HA–STP composites were fabricated by milling a mixture containing equal weight ratios of HA and STPs using a planetary ball mill (Retch Germany, 200 rpm) with zirconia balls. The obtained powder was then compacted under 450 MPa, calcinated at 850 1C for 2 h, and sintered at 1100 1C for 1 h under argon atmosphere. Similar protocol was applied to produce composites with HA/Ti weight ratios of 1.0 (HA–Ti) and 0.33 (Ti3–HA) as references to investigate the influence of silica on the chemical and mechanical properties of HA–STP composites. The structural morphology of STP powder was evaluated under 3D-surface texture Analyser (Alicona, Infinite Focus). The chemical composition of the composites was investigated using X-ray diffractometry (XRD: Philips PW1840), Fourier transform infrared spectroscopy (FT-IR: Thermoscientific, Nicolet 6700), and energydispersive X-ray spectroscopy (EDX: FEI Quanta 250). The microhardness experiments were also performed using a Vickers hardness tester (HMV-Shimadzu).
The microscopic images of the Ti and STP powders are illustrated in Fig. 1a and b. The silica shells could uniformly cover the Ti particles and decrease their metallic luminescence. This finding was further confirmed by SEM and EDX analyses (Fig. 1c). The XRD patterns of Ti, STP, sintered HA–Ti, and sintered HA–STP specimens are illustrated in Fig. 2. Comparison of the XRD patterns for Ti and STP powders indicated the stability of titanium in basic environment required for preparation of SiO2 coating. In the XRD pattern of the sintered HA–Ti composite, the HA and Ti peaks completely disappeared and TixPy (mostly Ti5P3) and TiCaO3 were the dominant phases. On the contrary in the sintered HA–STP, the undesired interaction between Ti and HA powders was prevented, causing complete removal of CaTiO3 and TixPy. However, the presence of SiO2 could not protect Ti and HA against oxidation and decomposition to CaP, respectively. Due to the similarity of the diffraction peaks of different CaP formulations (e.g. HA, oxyapatites, TTCP, α-TCP, β-TCP) and overshadowing their peaks by strong peaks of Ti2O3, it was difficult to accurately determine the CaP types produced in the composite. It is important to note that the addition of SiO2 resulted in the formation of calcium silicates (mostly CaSiO3), however this compound may exhibit less negative impact on the mechanical properties compared to calcium titanate and titanium phosphides. According to the previous studies, calcium silicate may even show reinforcing influence on the HA matrix [6]. Fig. 3 shows the FT-IR spectra of the pure HA, pure silica, calcinated HA–STP, sintered HA–Ti, and sintered HA–STP composites. In pure HA, strong peaks observed at 963 cm 1, 1021 cm 1, and 1087 cm 1 represented the phosphate (PO34 ) band, while the weak peak at 3571 cm 1 showed the hydroxyl (OH ) group which
Fig. 1. The microscopic images of (a) titanium, and (b) silica-coated titanium; (c) EDX analysis of silica-coated titanium.
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H. Wakily et al. / Materials Letters 143 (2015) 322–325
Fig. 2. XRD analyses of (a) titanium, (b) silica-coated titanium (STP), (c) sintered HA–Ti, and (d) sintered HA–STP.
Fig. 3. FT-IR spectra of (a) hydroxyapatite (HA), (b) silica-coated titanium (STP), (c) calcinated HA–STP, (d) sintered HA–Ti, and (e) sintered HA–STP.
is the characteristic band of the HA structure [7]. In the FT-IR spectrum of the STPs, all the absorption peaks represented the Si–O band. For the calcinated HA–STP, the silane and phosphate peaks
were clearly observed. However, due to a low SiO2/HA ratio, the silane vibration at 1058 cm 1 was overlapped by the phosphate principal peak. The FT-IR spectrum of the sintered HA–Ti indicated the elimination of phosphate peaks due to the complete removal of HA during the sintering process. This spectrum was similar to that of pure CaO [7] where the peaks at 875 cm 1 and 1400– 1600 cm 1 corresponded to the C–O band, while the short peak at 3634 cm 1 represented the Ca–O–H vibration. The carbonate band showed the formation of CaCO3 due to the integration of CaO with CO2 absorbed during handling and sample preparation [7]. Elimination of PO34 bands demonstrated the complete decomposition of CaPs to an amorphous phase and CaO which further resulted in formation of CaTiO3 and CaCO3 [7]. On the contrary, in the FT-IR pattern of HA–STP composite, the phosphate peaks at 940 cm 1 and 970 cm 1 were clearly observed, demonstrating the presence of CaPs. However, the phosphate peaks were different from those of pure HA or oxyapatite (dehydoxylated HA) and fitted with those of β-TCP [8]. The sharp peak at 710 cm 1 also represented the Ti–O band, while the broad peaks at 820 cm 1 and 1060 cm 1 were, respectively, attributed to the Si–O band of amorphous silica and Si–O–Ca vibration, indicating the formation of calcium silicate. Therefore, although the HA decomposition was observed in the HA–STP composites, the addition of silica shell around Ti particles inhibited the decomposition of secondary calcium phosphates to CaO. Fig. 4 compares the Vickers’ hardness and expansion ratio of the HA–STP composite with those of pure Ti, HA–Ti, and Ti3–HA. The mechanical stability of HA–Ti composite was not sufficient for hardness measurement and thus, the hardness data could not be reported. The low hardness of HA–Ti samples was attributed to
H. Wakily et al. / Materials Letters 143 (2015) 322–325
325
4. Conclusion In this study, silica-coated titanium particles were synthesized to minimize the unfavourable inter-phase reactions in the HA–Ti composites. Incorporation of silica-coated titanium resulted in complete removal of CaTiO3 and TixPy phases from the sintered composites. Although the added silica layer decreased the HA thermal stability and produced calcium silicate phases, its negative influences on structural and mechanical properties of the composite were significantly lower than the compounds produced from HA interaction with pure Ti. Moreover, by addition of silica, the secondary calcium phosphates produced from HA decomposition remained stable at sintering temperatures, resulting in less physical stresses during the cooling process. Therefore, coating of Ti particles with inert and thermally stable compounds such as silica could significantly improve the compositional and mechanical properties of HA–Ti composites.
Acknowledgement This research was supported by High-Impact Research MoE Grant, UM.C/625/1/HIR/MOE/DENT/14 from the Ministry of Higher Education Malaysia.
References
Fig. 4. Comparison between the (a) Vickers’ hardnesses, and (b) expansion ratios of sintered HA–STP, HA–Ti, Ti3–HA, and titanium.
the presence of brittle phases (TiCaO3 and TixPy) which require temperatures higher than 1400 1C for densification [9]. Moreover, HA–Ti composites exhibited high expansion values which consequently resulted in low mechanical properties. This expansion is probably due to the significant mismatch between the crystal lattice parameters of the initial and resulting phases, which produces a large quantity of defects such as vacancies, dislocations and stacking faults at particle interfaces, thereby resulting in unpacked and fragile matrix [10]. Instead, the HA–STP composites exhibited a relatively high hardness value which was comparable to that of Ti3–HA. The limited inter-phase interactions in HA–STP composites could result in more stability of the crystal lattice parameters at the interfaces. Moreover, the lattice parameters of the decomposition phases are almost similar to that of the initial phases, producing minimum physical stresses in the HA–STP composite [10].
[1] Xiao XF, Liu RF, Zheng YZ. Hydoxyapatite/titanium composite coating prepared by hydrothermal–electrochemical technique. Mater Lett 2005;59:1660–4. [2] Balbinotti P, Gemelli E, Buerger G, SAd Lima, Jd Jesus, Camargo NHA, et al. Microstructure development on sintered Ti/HA biocomposites produced by powder metallurgy. Mater Res 2011;14:384–93. [3] Arifin A, Sulong AB, Muhamad N, Syarif J, Ramli MI. Material processing of hydroxyapatite and titanium alloy (HA/Ti) composite as implant materials using powder metallurgy: a review. Mater Des 2014;55:165–75. [4] Yang Y, Kim K-H, Agrawal CM, Ong JL. Interaction of hydroxyapatite–titanium at elevated temperature in vacuum environment. Biomaterials 2004;25:2927–32. [5] Ning C, Zhou Y. Correlations between the in vitro and in vivo bioactivity of the Ti/HA composites fabricated by a powder metallurgy method. Acta Biomater 2008;4:1944–52. [6] Kaewsichan L, Riyapana D, Prommajana P, Kaewsrichan J. Effects of sintering temperatures on micro-morphology, mechanical properties, and bioactivity of bone scaffolds containing calcium silicate. Sci Asia 2011;37:240–6. [7] Ye H, Liu XY, Hong H. Characterization of sintered titanium/hydroxyapatite biocomposite using FTIR spectroscopy. J Mater Sci Mater Med 2009;20:843–50. [8] Meejoo S, Maneeprakorn W, Winotai P. Phase and thermal stability of nanocrystalline hydroxyapatite prepared via microwave heating. Thermochim Acta 2006;447:115–20. [9] Taïbi-Benziada L, Mezroua A, Von Der Mühll R. CaTiO3 related materials for resonators. Ceram Silik 2004;48:180–3. [10] Brown W, Epstein EF. Crystallography of tetracalcium phosphate. J Res Nat Bur Stand 1965;69:547–51.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improved thermal and mechanical properties in hydroxyapatite–titanium composites by incorporating silica-coated titanium Hakimeh Wakily a, Ali Dabbagh a,n, Hadijah Abdullah a, Nur Farha Abdul Halim a, Noor Hayaty Abu Kasim a a
Department of Restorative Dentistry, Faculty of Dentistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 19 March 2014 Accepted 19 December 2014 Available online 30 December 2014
Hydroxyapatite–titanium composites are promising materials for fabrication of the load-bearing implants. However, the mechanical properties of these composites are negatively affected by formation of fragile compounds during the high-temperature processing. In this study, titanium particles were coated with a silica layer to decrease the metallic–ceramic interactions at the sintering temperature range. The results indicated the decomposition of hydroxyapatite and oxidation of titanium during the sintering process at 1100 1C. However, the undesired interactions between hydroxyapatite and titanium components were minimized, causing complete removal of calcium titanate and titanium phosphides as well as the formation of stable calcium phosphates in the sintered composite. Consequently, composites containing identical weight ratios of hydroxyapatite and silica-coated titanium exhibited a relatively high Vickers' hardness value comparable to that of titanium–hydroxyapatite composites with a weight ratio of 3:1. Therefore, surface modification of titanium particles using a silica layer could significantly improve the mechanical properties of the obtained composites by increasing their thermal stability during the sintering process. & 2014 Elsevier B.V. All rights reserved.
Keywords: Biomaterials Composite materials Sintering Phase transformation
1. Introduction Hydroxyapatite–titanium (HA–Ti) bio-composites have recently received increasing attention because of offering both biocompatibility and high mechanical strength contributed by HA and Ti, respectively [1–3]. These composites exhibit characteristics such as bio-inertness, low Young’s modulus and high biocompatibility. However, undesirable interaction between HA and Ti components during the sintering process results in composites with poor mechanical characteristics [1,3]. Pure HA is dehydroxylated at approximately 900 1C in air and at 850 1C in a water-free atmosphere, followed by decomposition to tetra calcium phosphate (TTCP: Ca4(PO4)2O) and/or tricalcium phosphate (TCP: Ca3(PO4)2) [3]. However, during the sintering of HA–Ti system, the dehydrated water of HA reacts with Ti ions to yield titanium oxide, thereby accelerating the HA dehydroxylation and decomposition [3]. Based on the experimental parameters, the sintering products may include TCP, TTCP, calcium titanate (CaTiO3), titanium phosphides (TixPy), titanates (TixOy), calcium oxide (CaO), and amorphous phases [4]. Particularly, a sintering process under argon atmosphere
n
Corresponding author. Tel.: þ 603 7967 6451; fax: þ 603 7967 6456. E-mail address: [email protected] (A. Dabbagh).
http://dx.doi.org/10.1016/j.matlet.2014.12.092 0167-577X/& 2014 Elsevier B.V. All rights reserved.
at 1200 1C could completely eliminate the calcium phosphate phases (CaPs) and produce a compact layer comprised of TixPy and CaTiO3 around the titanium/titanate particles [2]. It is well-known that the mechanical behaviour of the HA–Ti system is significantly affected by the microstructure and phase distribution in the sintered composite. Previous studies indicated that some phases such as TixPy and CaTiO3 which form between HA and Ti particles at elevated temperatures, present a fragile behaviour without displaying any yield strength prior to rupture and thus provide a weak interface bonding in the matrix [2]. These fragile phases are subsequently removed during polishing and result in the formation of gaps and pores between Ti particles [2]. One particular approach to address this issue is the incorporation of low-melting-point biocompatible additives as sintering aids to the initial components [5]. However, this method may not eliminate the undesired sintering products especially in composites with high HA/Ti ratios. This study is an attempt to present a new technique for improving the thermal and mechanical properties of HA–Ti composites with high HA/Ti ratios. In this approach, the Ti particles are coated by thin silica layers prior mixing with HA to prevent the undesired metallic– ceramic reactions at increased temperatures. By removing the fragile compounds, the resulting composite may provide improved mechanical strength for load-bearing application.
H. Wakily et al. / Materials Letters 143 (2015) 322–325
323
2. Materials and methods
3. Results and discussion
The composites containing HA and silica-coated titanium particles (HA–STP) were prepared using commercially available pure titanium ( 45 mm), hydroxyapatite ( o45 mm), tetraethylorthosilicate (TEOS), and ammonia (25% v/v) purchased from Sigma-Aldrich. For the synthesis of STPs, 1 g of Ti particles was stirred in 100 mL of ethanol–water solution (10:1), followed by injecting ammonia to achieve a pH of 12.5. Then, 4.67 mL TEOS was added to initiate the reaction. After stirring for 4 h, the mixture was aged for 24 h, rinsed with deionized water, and centrifuged to remove unattached polysiloxane oligomers. The obtained STP particles were then dried at 60 1C for 6 h. The HA–STP composites were fabricated by milling a mixture containing equal weight ratios of HA and STPs using a planetary ball mill (Retch Germany, 200 rpm) with zirconia balls. The obtained powder was then compacted under 450 MPa, calcinated at 850 1C for 2 h, and sintered at 1100 1C for 1 h under argon atmosphere. Similar protocol was applied to produce composites with HA/Ti weight ratios of 1.0 (HA–Ti) and 0.33 (Ti3–HA) as references to investigate the influence of silica on the chemical and mechanical properties of HA–STP composites. The structural morphology of STP powder was evaluated under 3D-surface texture Analyser (Alicona, Infinite Focus). The chemical composition of the composites was investigated using X-ray diffractometry (XRD: Philips PW1840), Fourier transform infrared spectroscopy (FT-IR: Thermoscientific, Nicolet 6700), and energydispersive X-ray spectroscopy (EDX: FEI Quanta 250). The microhardness experiments were also performed using a Vickers hardness tester (HMV-Shimadzu).
The microscopic images of the Ti and STP powders are illustrated in Fig. 1a and b. The silica shells could uniformly cover the Ti particles and decrease their metallic luminescence. This finding was further confirmed by SEM and EDX analyses (Fig. 1c). The XRD patterns of Ti, STP, sintered HA–Ti, and sintered HA–STP specimens are illustrated in Fig. 2. Comparison of the XRD patterns for Ti and STP powders indicated the stability of titanium in basic environment required for preparation of SiO2 coating. In the XRD pattern of the sintered HA–Ti composite, the HA and Ti peaks completely disappeared and TixPy (mostly Ti5P3) and TiCaO3 were the dominant phases. On the contrary in the sintered HA–STP, the undesired interaction between Ti and HA powders was prevented, causing complete removal of CaTiO3 and TixPy. However, the presence of SiO2 could not protect Ti and HA against oxidation and decomposition to CaP, respectively. Due to the similarity of the diffraction peaks of different CaP formulations (e.g. HA, oxyapatites, TTCP, α-TCP, β-TCP) and overshadowing their peaks by strong peaks of Ti2O3, it was difficult to accurately determine the CaP types produced in the composite. It is important to note that the addition of SiO2 resulted in the formation of calcium silicates (mostly CaSiO3), however this compound may exhibit less negative impact on the mechanical properties compared to calcium titanate and titanium phosphides. According to the previous studies, calcium silicate may even show reinforcing influence on the HA matrix [6]. Fig. 3 shows the FT-IR spectra of the pure HA, pure silica, calcinated HA–STP, sintered HA–Ti, and sintered HA–STP composites. In pure HA, strong peaks observed at 963 cm 1, 1021 cm 1, and 1087 cm 1 represented the phosphate (PO34 ) band, while the weak peak at 3571 cm 1 showed the hydroxyl (OH ) group which
Fig. 1. The microscopic images of (a) titanium, and (b) silica-coated titanium; (c) EDX analysis of silica-coated titanium.
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H. Wakily et al. / Materials Letters 143 (2015) 322–325
Fig. 2. XRD analyses of (a) titanium, (b) silica-coated titanium (STP), (c) sintered HA–Ti, and (d) sintered HA–STP.
Fig. 3. FT-IR spectra of (a) hydroxyapatite (HA), (b) silica-coated titanium (STP), (c) calcinated HA–STP, (d) sintered HA–Ti, and (e) sintered HA–STP.
is the characteristic band of the HA structure [7]. In the FT-IR spectrum of the STPs, all the absorption peaks represented the Si–O band. For the calcinated HA–STP, the silane and phosphate peaks
were clearly observed. However, due to a low SiO2/HA ratio, the silane vibration at 1058 cm 1 was overlapped by the phosphate principal peak. The FT-IR spectrum of the sintered HA–Ti indicated the elimination of phosphate peaks due to the complete removal of HA during the sintering process. This spectrum was similar to that of pure CaO [7] where the peaks at 875 cm 1 and 1400– 1600 cm 1 corresponded to the C–O band, while the short peak at 3634 cm 1 represented the Ca–O–H vibration. The carbonate band showed the formation of CaCO3 due to the integration of CaO with CO2 absorbed during handling and sample preparation [7]. Elimination of PO34 bands demonstrated the complete decomposition of CaPs to an amorphous phase and CaO which further resulted in formation of CaTiO3 and CaCO3 [7]. On the contrary, in the FT-IR pattern of HA–STP composite, the phosphate peaks at 940 cm 1 and 970 cm 1 were clearly observed, demonstrating the presence of CaPs. However, the phosphate peaks were different from those of pure HA or oxyapatite (dehydoxylated HA) and fitted with those of β-TCP [8]. The sharp peak at 710 cm 1 also represented the Ti–O band, while the broad peaks at 820 cm 1 and 1060 cm 1 were, respectively, attributed to the Si–O band of amorphous silica and Si–O–Ca vibration, indicating the formation of calcium silicate. Therefore, although the HA decomposition was observed in the HA–STP composites, the addition of silica shell around Ti particles inhibited the decomposition of secondary calcium phosphates to CaO. Fig. 4 compares the Vickers’ hardness and expansion ratio of the HA–STP composite with those of pure Ti, HA–Ti, and Ti3–HA. The mechanical stability of HA–Ti composite was not sufficient for hardness measurement and thus, the hardness data could not be reported. The low hardness of HA–Ti samples was attributed to
H. Wakily et al. / Materials Letters 143 (2015) 322–325
325
4. Conclusion In this study, silica-coated titanium particles were synthesized to minimize the unfavourable inter-phase reactions in the HA–Ti composites. Incorporation of silica-coated titanium resulted in complete removal of CaTiO3 and TixPy phases from the sintered composites. Although the added silica layer decreased the HA thermal stability and produced calcium silicate phases, its negative influences on structural and mechanical properties of the composite were significantly lower than the compounds produced from HA interaction with pure Ti. Moreover, by addition of silica, the secondary calcium phosphates produced from HA decomposition remained stable at sintering temperatures, resulting in less physical stresses during the cooling process. Therefore, coating of Ti particles with inert and thermally stable compounds such as silica could significantly improve the compositional and mechanical properties of HA–Ti composites.
Acknowledgement This research was supported by High-Impact Research MoE Grant, UM.C/625/1/HIR/MOE/DENT/14 from the Ministry of Higher Education Malaysia.
References
Fig. 4. Comparison between the (a) Vickers’ hardnesses, and (b) expansion ratios of sintered HA–STP, HA–Ti, Ti3–HA, and titanium.
the presence of brittle phases (TiCaO3 and TixPy) which require temperatures higher than 1400 1C for densification [9]. Moreover, HA–Ti composites exhibited high expansion values which consequently resulted in low mechanical properties. This expansion is probably due to the significant mismatch between the crystal lattice parameters of the initial and resulting phases, which produces a large quantity of defects such as vacancies, dislocations and stacking faults at particle interfaces, thereby resulting in unpacked and fragile matrix [10]. Instead, the HA–STP composites exhibited a relatively high hardness value which was comparable to that of Ti3–HA. The limited inter-phase interactions in HA–STP composites could result in more stability of the crystal lattice parameters at the interfaces. Moreover, the lattice parameters of the decomposition phases are almost similar to that of the initial phases, producing minimum physical stresses in the HA–STP composite [10].
[1] Xiao XF, Liu RF, Zheng YZ. Hydoxyapatite/titanium composite coating prepared by hydrothermal–electrochemical technique. Mater Lett 2005;59:1660–4. [2] Balbinotti P, Gemelli E, Buerger G, SAd Lima, Jd Jesus, Camargo NHA, et al. Microstructure development on sintered Ti/HA biocomposites produced by powder metallurgy. Mater Res 2011;14:384–93. [3] Arifin A, Sulong AB, Muhamad N, Syarif J, Ramli MI. Material processing of hydroxyapatite and titanium alloy (HA/Ti) composite as implant materials using powder metallurgy: a review. Mater Des 2014;55:165–75. [4] Yang Y, Kim K-H, Agrawal CM, Ong JL. Interaction of hydroxyapatite–titanium at elevated temperature in vacuum environment. Biomaterials 2004;25:2927–32. [5] Ning C, Zhou Y. Correlations between the in vitro and in vivo bioactivity of the Ti/HA composites fabricated by a powder metallurgy method. Acta Biomater 2008;4:1944–52. [6] Kaewsichan L, Riyapana D, Prommajana P, Kaewsrichan J. Effects of sintering temperatures on micro-morphology, mechanical properties, and bioactivity of bone scaffolds containing calcium silicate. Sci Asia 2011;37:240–6. [7] Ye H, Liu XY, Hong H. Characterization of sintered titanium/hydroxyapatite biocomposite using FTIR spectroscopy. J Mater Sci Mater Med 2009;20:843–50. [8] Meejoo S, Maneeprakorn W, Winotai P. Phase and thermal stability of nanocrystalline hydroxyapatite prepared via microwave heating. Thermochim Acta 2006;447:115–20. [9] Taïbi-Benziada L, Mezroua A, Von Der Mühll R. CaTiO3 related materials for resonators. Ceram Silik 2004;48:180–3. [10] Brown W, Epstein EF. Crystallography of tetracalcium phosphate. J Res Nat Bur Stand 1965;69:547–51.