Synthesis and sintering of hydroxyapatite–zirconia composites

Synthesis and sintering of hydroxyapatite–zirconia composites

Materials Science and Engineering C 20 (2002) 187 – 193 www.elsevier.com/locate/msec Synthesis and sintering of hydroxyapatite–zirconia composites R...

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Materials Science and Engineering C 20 (2002) 187 – 193 www.elsevier.com/locate/msec

Synthesis and sintering of hydroxyapatite–zirconia composites R. Ramachandra Rao, T.S. Kannan* Materials Science Division, National Aerospace Laboratories, Bangalore 560 017, India

Abstract Hydroxyapatite (HA) has been synthesised in presence of 10 – 30 wt.% of m-ZrO2 by solid state reaction between tricalcium phosphate (TCP) and Ca(OH)2 at 1000 jC for 8 h. The m-ZrO2 was partly converted into t-ZrO2 by partial consumption of CaO which in turn resulted in a mixture of h-TCP and HA. On sintering these HA – h-TCP – ZrO2 composite powders at 1100 – 1400 jC for 2 h, the HA is further decomposed into h-TCP and CaO. The CaO so produced reacts further with m-ZrO2/t-ZrO2 generating a mixture of t-ZrO2 and CaZrO3 in different proportions. These various phases formed interfere with the sinterability of the composites due to their differential shrinkages leading to a overall reduced density as compared to that of pure HA. The composites show a T-onset of decomposition at around 1150 jC and a 40% HA yield was obtained at the highest sintering temperature of 1400 jC. The products were subjected to XRD for phase analysis and the microstructural features were studied by SEM. D 2002 Published by Elsevier Science B.V. Keywords: Hydroxyapatite; Hydroxyapatite – zirconia; Composites; Sintering

1. Introduction Synthetic hydroxyapatite [HA; Ca10(PO4)6(OH)2], being chemically similar to the inorganic constituent of bone mineral and owing to its excellent biocompatibility, has been studied extensively as a bone replacement material [1 – 4]. The use of synthetic HA is, however, limited to low load bearing applications due to its inferior mechanical properties (especially fracture toughness) compared to cortical bone [2,5 –7]. This is attributable to the minor compositional and microstructural differences between biological apatite (HCA) and synthetic HA [3]. Synthetic HA is more isotropic with a larger grain size than the biological apatite. Further, bone is a composite of an organic (collagen) and an inorganic (biological apatite) constituent. Thus, structural tailoring of synthetic HA along with compositional matching is needed for achieving better mechanical properties. Hence, additions of second-phase ceramic materials to the HA matrix to obtain products with enhanced strength and toughness has been an interesting subject matter of research in recent years [8– 11]. The addition of ZrO2 in the form of particles [12 – 19] or fibers [20 – 23] to HA matrix has drawn much attention due to its biocompatibility coupled with the tendency to enhance the *

Corresponding author. Tel.: +91-80-508-6141; fax: +91-80-527-0098. E-mail addresses: [email protected] (R. Ramachandra Rao), [email protected], [email protected] (T.S. Kannan). 0928-4931/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 9 2 8 - 4 9 3 1 ( 0 2 ) 0 0 0 3 1 - 0

mechanical properties of HA [19,24,25]. Wu and Yeh [12] found that the sinterability of HA –20 wt.% ZrO2 (PSZ; 3 mol% Y2O3-doped) composite was affected by the differential shrinkage between the various component phases. An increase in the calcination temperature of ZrO2 powder and/ or decrease in that of HA improved the sinterability of the product. HA and cubic ZrO2 are the major phases in the compacts having high sintered density, whereas a- and hTCP and CaZrO3 are the major phases in compacts with low sintered densities. Nagarajan and Rao [13] have prepared HA composites with CeO2- and Y2O3-stabilized ZrO2 as well as with pure m-ZrO2 by both sintering at 1400 jC and by HIPping at 1450 jC (140 MPa pressure) in argon atmosphere. They found that in most of the sintered or HIPped samples, HA decomposes virtually completely into h-TCP and CaO and the latter in turn completely dissolves in ZrO2 and stabilizes in its cubic/tetragonal form. The composites possessed higher fracture toughness values than that of pure HA coupled with good biocompatibility behaviour. Vaidhyanathan et al. [14] demonstrated that the implants made of the ZrO2 –HA composite materials [13] are highly biocompatible with no adverse reactions when implanted in rabbit mandibles. Mansur et al. [15] developed ZrO2 –calcium phosphate composites which showed improved strength and toughness with good biocompatibility. Heimann and Vu [16] have shown that addition of CaO to HA –ZrO2 composite mixtures shifts the chemical equilibirum of the product from TCP and tetra calcium phosphate (TTCP) towards HA making it more

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stable. Surplus CaO will be effectively fixed by ZrO2 acting as a sink for Ca2 + ions resulting in the formation of either tZrO2 or CaZrO3. Very recently, Adolfsson et al. [17] showed that HA – ZrO2 composites without any detectable decomposition of HA could be prepared by hot isostatic pressing at 1200 jC, while sintering in air results in decomposition of HA at temperatures < 950 jC, which was attributed to the vacancies formed by release of structural water. Studies by Ruys et al. [19 – 22] on the chemical compatibility of HA with various particulate [19] and fibrous [20 –22] additives under different sintering atmospheres have shown that while significant decomposition of HA occurs under ambient air as well as under argon atmosphere, the flowing H2O(g)/O2 mixtures or hydrothermal sintering conditions could eliminate or reduce the decomposition of HA or increase the Tonset of decomposition. However, the moist atmosphere had a detrimental effect on the sinterability in most of the cases. In the present study, an attempt has been made to synthesise HA by a solid state reaction between its precursor materials with simultaneous addition of m-ZrO2 and the thermal stability and sinterability of HA – ZrO2 composite systems have been studied.

2. Experimental procedure Commercially available calcium phosphate tribasic (TCP), Ca(OH)2 and m-ZrO2 were used in the present study. The source and other characteristics of the precursors are listed in Table 1. The TCP and Ca(OH)2 powders taken in 3:2 molar ratios were mixed with m-ZrO2 powders in varying proportions [0, 10, 20 and 30 wt.% with respect to TCP + Ca(OH)2 mixtures] in deionised water using magnetic stirring for 15 – 20 min. The slurry (40 –50 wt.%) was milled in polythene bottles using Al2O3 balls as milling media for about 16 h to yield homogeneous mixing of the various constituents. The slip was then cast into plaster molds to generate discs of dimensions 25 mm u  10 mm. The green discs were dried at 80 jC for 12 h and then heat treated at 1000 jC for 8 h to achieve the solid state reaction between the precursors. The heat treated discs were then crushed into powders and subsequently characterised by X-

Table 1 Characteristics of the powders used in the study Powder type

TCP Ca(OH)2

ZrO2

a

Supplier/purity

Rolex, Mumbai, India LR grade S.D. Fine chemicals, Boisar, LR grade, Assay 90% Loba-Chemie Indoaustranal Mumbai, LR grade, Assay 97%

After calcination at 800 jC/8 h.

Particle size (Am)

Phases (by XRD)

d10

d50

d90

1.3

7.3

22.0







h-TCPa HA Ca(OH)2

1.90

8.29

19.0

m-ZrO2

ray diffraction (XRD; X-ray diffractometer, PM9002, M/s Philips, Holland) for the constituent phases present. The powders of different compositions were granulated with 2% polyvinyl alcohol (PVA; M/s S.D. Fine Chemicals, India) and uniaxially pressed into discs (18 mm u  5 mm) under 80 MPa pressure and sintered in the temperature range of 1100 – 1400 jC for 2 h. The densities of the sintered products were obtained from weight and dimensional measurements and their phase compositions were determined from their XRD spectra. Some typical sintered discs (1300 jC/2 h) were polished successively with 600 grit SiC emery and 1 Am Al2O3 slurry and etched with 0.1 M acetic acid for 1 min. The microstructural features of the polished and etched surfaces were obtained by scanning electron microscopy (SEM; Model 440, Leo Electron microscopy, Cambridge, UK).

3. Results and discussion 3.1. Synthesis of HA containing ZrO2 additives Pure hydroxyapatite (HA) powder as revealed by XRD, has been synthesised by solid state reaction between TCP and Ca(OH)2 taken in 3:2 molar ratio at 1000 jC for 8 h. The studies on synthesis, thermal stability and sintering of pure HA and its biphasic mixtures with h-tricalcium phosphate (h-TCP) has been presented elsewhere [26,27]. In the present study, the powder products obtained by solid state reactions at 1000 jC for 8 h between TCP and Ca(OH)2 mixtures (3:2 molar ratio) in presence of 10, 20 and 30 wt.% of m-ZrO2 additions are designated as 10ZHA, 20ZHA and 30ZHA, respectively. The XRD spectra for HA and ZHA samples are presented in Fig. 1a– d. The relative percentages of HA and h-TCP phases present in the powder products were calculated from the relative intensities of most intense peaks of HA (d = 0.2814 nm) and h-TCP (d = 0.288 nm) in the respective XRD spectra and are presented in Table 2, with the major and secondary phases present in the products. In the case of pure HA, only a small amount (4%) of hTCP is observed at 1000 jC while m-ZrO2 addition resulted in an increased amount of h-TCP with decreased HA/hTCP ratio. In 10ZHA and 20ZHA samples, m-ZrO2 is transformed almost completely to t-ZrO2 (with minor retention of m-ZrO2), while in 30ZHA sample with higher level of m-ZrO 2 additions considerable amount of m-ZrO 2 appears along with t-ZrO2. The formation of t-ZrO2 by the solid solution of calcium in ZrO2 lattice reduces the availability of calcium required for the formation of stoichiometric HA thereby leading to the formation of an increased amount of h-TCP. On the contrary, the XRD spectra of ZHA samples heat treated to 900 jC for 8 h (not presented) shows the retainment of m-ZrO2 phase and formation of only trace amounts of t-ZrO2 at all compositions. These results are in accordance with the phase diagram for ZrO2 –CaO systems [28].

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Table 2 Relative percentage of HA, h-TCP and the type of major and secondary phases present in the synthesised composite powders (by XRD analysis) Powder Powder m-ZrO2 no. identity added (wt.%)a

Percentage

1 2 3 4

96 84 84 82

HA – 10ZHA 10 20ZHA 20 30ZHA 30 a

HA h-TCP 4 16 16 18

Major phases (XRD)

Secondary phases (XRD)

HA HA, t-ZrO2 HA, t-ZrO2 HA, t-ZrO2

h-TCP h-TCP, m-ZrO2 h-TCP, m-ZrO2 h-TCP, m-ZrO2

With respect to 3:2 molar mixture of TCP and Ca(OH)2.

3.2. Sintering and characterisation of HA –ZrO2 composites The densification and thermal stability of HA and ZHA composites were studied by sintering them at 1100 – 1400 jC for 2 h. The various crystallographic phases formed after sintering are evident from their XRD spectra presented in Figs. 2 – 4 and the evolution of these phases as a function of temperature are presented in Fig. 5a –c by considering the relative intensities of various phases present in the respective XRD spectra. The percentage HA yield (or % HA retained) with respect to h-TCP in the products, calculated by using peak height ratios for the major HA (d = 0.2814 nm) and h-TCP (d = 0.288 nm) peaks, are presented in Fig. 6 as a function of temperature. The density values calculated for HA and ZHA samples without considering the formation of secondary phases like TCP and calcium zirconate (CaZrO3) are depicted in Fig. 7. The SEM of the polished and etched samples of ZHA are presented in Fig. 8. From the XRD results, it can be observed that at 1100 jC, ZrO2 has been completely stabilised in the tetragonal form at all levels of additions. While the percentage HA yield at 1100 jC for 10ZHA and 20ZHA samples are of the same order as that obtained at 1000 jC, a slight reduction in yield is observed for 30ZHA samples with a concomitant increase in h-TCP. At temperatures of 1150 jC and above, a continued reduction in the yield of HA phase and an increased yield of h-TCP is observed. This is attributable to the fact that in ZrO2-doped samples, HA is thermally decomposed into hTCP [Ca3(PO4)2], CaO and water vapour. The CaO so released stabilises the m-ZrO2 by solid solution resulting in the formation of t-ZrO2 [13]. With release of excess CaO by further decomposition of HA, the solubility of Ca in ZrO2 exceeds the maximum solid solution range and CaZrO3 is formed in preference to t-ZrO2 [12,16].

Fig. 1. XRD spectra of (a) HA, (b) 10ZHA, (c) 20ZHA and (d) 30ZHA. Phases identified: D = HA, . = h-TCP, z = m-ZrO2 and o = t-ZrO2.

Ca10 ðPO4 Þ6 ðOHÞ2 ! 3Ca3 ðPO4 Þ2 þ CaO þ H2 O

ð1Þ

m  ZrO2 þ CaO ! Ca  doped t  ZrO2

ð2Þ

Ca  doped t  ZrO2 þ CaO ! CaZrO3

ð3Þ

In agreement with the above hypothesis, the 10ZHA samples (lower content of ZrO2) show the presence of only

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Fig. 3. XRD spectra of 20ZHA sample sintered at (a) 1100, (b) 1150, (c) 1200 and (d) 1300 jC for 2 h. Phases identified: D = HA, . = h-TCP, o = t-ZrO2 and E = CaZrO3.

Fig. 2. XRD spectra of 10ZHA sample sintered at (a) 1100, (b) 1150, (c) 1200, (d) 1300 and (e) 1400 jC for 2 h. Phases identified: D = HA, . = hTCP, 5 = a-TCP, o = t-ZrO2 and E = CaZrO3.

CaZrO3 at higher temperatures ( > 1200 jC) while the 20ZHA and 30ZHA (higher contents of ZrO2) samples show t-ZrO2 phase in increasing amounts with decreasing amounts of CaZrO3 at all corresponding temperatures as clearly seen in Fig. 5. Further, the decomposition of HA and stabilisation of ZrO2 is found to be accelerated in the range 1100 –1200 jC, but found to stagnate in the temperature

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(1150– 1400 jC) as well as those reported by Heimann and Vu [16] for HA – ZrO2 mixtures containing CaO sintered in the range 1000 –1300 jC.

Fig. 4. XRD spectra of 30ZHA sample sintered at (a) 1100, (b) 1150, (c) 1200, (d) 1300 and (e) 1400 jC for 2 h. Phases identified: D = HA, . = hTCP, 5 = a-TCP, o = t-ZrO2 and E = CaZrO3.

range of 1200 –1300 jC. Above 1300 jC, the significant change observed is the appearance of a-TCP as an additional phase. The formation of CaZrO3 is in correlation with the results reported by Wu and Yeh [12] for 20 wt.% ZrO2 containing HA composite sintered at various temperatures

Fig. 5. Phase changes in ZHA samples as a function of temperature: (a) 10ZHA, (b) 20ZHA and (c) 30ZHA. Phase identification: A = HA, B = hTCP, C = t-ZrO2, D = CaZrO3 and E = a-TCP.

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powder additions as well as with those reported by Ruys et al. [23] for ZrO2 fibre additions. The T-onset of decomposition found around 1100 –1150 jC for our samples are very much higher as compared to 750 jC observed by Ruys et al. [11] for 10 vol.% ZrO2 powder additions and 850– 1040 jC for ZrO2 fibre additions [23]. Thus, the above

Fig. 6. Percentage of HA with respect to h-TCP in ZHA samples as a function of temperature.

The HA yield of 73%, 44% and 52% for 10ZHA, 20ZHA and 30ZHA samples sintered at 1200 jC obtained in this study are higher than the values of i38% reported by Ruys et al. [19] for 20 vol.% ZrO2 particle-doped HA sintered at 1200 jC for 1 h in ambient (air) atmosphere and for 10 vol.% addition of ZrO2 fibers sintered at 1200 jC for 1 h under 10 atm. of argon. On the contrary, those researchers [23] have shown 90% HA yield by changing the processing conditions from wet ball milling to dry vibro milling for a 10 vol.% addition of ZrO2 short fibers. The differential shrinkage between the various phases during sintering results in reduced densification of the products. The ZHA samples show percentage theoretical density in the range of 62 – 76% as compared to 70 – 96% for monolithic HA sintered in the temperature range 1100 – 1400 jC. The degree of densification observed in our studies are in good agreement with those reported by Wu and Yeh [12] for ZrO2

Fig. 7. Sintered density of HA and ZHA samples as a function of temperature.

Fig. 8. SEM of polished and etched surfaces of ZHA samples sintered at 1300 jC for 2 h: (a) 10ZHA, (b) 20ZHA and (c) 30ZHA: ‘‘ p ’’ indicates ZrO2 and CaZrO3 phases.

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ZrO2-doped HA products are at par with those reported in the literature with respect to the percentage HA yield, the Tonset of decomposition and the densification achieved at various sintering temperatures. From the energy dispersive X-ray analysis (EDAX) on the SEM of the polished and etched samples of ZHA in Fig. 8a– c, it is found that the light phases are ZrO2 or CaZrO3, which are segregated at the grain boundaries of grey coloured HA matrix phase. In ZHA samples, the major decomposition product of HA is found to be h-TCP, while only a minor amount of aTCP appears at 1400 jC. These results are in agreement with those reported by Nagarajan and Rao [13] and Ruys et al. [19 – 21]. They reported the formation of h-TCP predominantly with traces of a-TCP. The predominant formation of h-TCP in the sintered products could be explained by the structural synergistic effects of HA possessing an hexagonal crystal structure similar to that of h-TCP [13].

4. Conclusions The synthesis of HA by solid state reaction at 1000 jC/8 h between TCP and Ca(OH)2 is influenced by addition of ZrO2. As CaO is partially taken up by m-ZrO2 to form tZrO2, the percentage of HA yield is reduced with the simultaneous appearance of h-TCP phase. Further, on sintering these composite powders at higher temperatures (1100 – 1400 jC/2 h), HA decomposes into h-TCP and CaO, while t-ZrO2 and CaZrO3 are formed by solid solution of CaO in ZrO2. The formation of these various phases interferes with the sinterability of the composites due to differential shrinkage of the phases leading to overall reduced density as compared to that of pure HA. However, the higher values for percentage HA yield, the T-onset of decomposition and the densification at various temperatures achieved for ZrO2-doped HA composites in the present study (as compared to the values reported in the literature) leads to a very significant observation that, high-temperature solid state reaction route results in more dense and more thermally stable HA –ZrO2 composite products than those obtained by other (e.g. coprecipitation) routes.

Acknowledgements The authors are thankful to Dr. (Mrs.) Kalyani Vijayan and Dr. Vasudevan Iyer for XRD spectra, Dr. T. A. Bhaskaran and Mrs Kalavathy for SEM studies and Miss N. Shobha for experimental assistance.

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