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A study of phase stability and mechanical properties of hydroxylapatite–nanosize α-alumina composites
Materials Science and Engineering C 27 (2007) 421 – 425 www.elsevier.com/locate/msec
A study of phase stability and mechanical properties of hydroxylapatite–nanosize α-alumina composites Zafer Evis a,⁎, Robert H. Doremus b a
b
Middle East Technical University, Engineering Sciences, Ankara, 06531, Turkey Rensselaer Polytechnic Institute, Materials Science and Engineering, Troy, NY, 12180, USA Available online 21 June 2006
Abstract Hydroxylapatite (HA)–nanosize alumina composites were synthesized to study their phase stability and mechanical properties. To make these composites, nanosize α-Al2O3 powder was used because of its better sinterability and densification as compared to nanosize γ-Al2O3. The composites were air sintered without pressure and hot pressed in vacuum at 1100 °C and 1200 °C. In the composites, HA decomposed to tricalcium phosphate faster after the air sintering than hot pressing. Moreover, hexagonal unit cell volume of HA left in the composites showed that there was more decomposition of HA after the air sintering than hot pressing. It also showed that HA in the composites was OH− and Ca2+ deficient. As the amount of alumina increased, sinterability considerably decreased. Hot pressing at 1200 °C resulted in better mechanical properties (μ-hardness and fracture toughness) than the hot pressing at 1100 °C. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydroxylapatite; Nanosize alumina; Hot pressing; X-ray diffraction
1. Introduction Hydroxylapatite (HA, Ca10(PO4)6(OH)2) is mainly used as a hard tissue implant because of its excellent biocompatibility in the human body [1]. A HA implant can only be used in low impact applications, e.g. in the middle ear, because it is brittle. Composites of HA with alumina can combine the advantages of HAs biocompatibility and higher strength of alumina. When mixtures of HA and alumina powders are sintered in air from 1100 °C to 1400 °C, HA decomposes and reacts to form unwanted second phases [2]. Two approaches can overcome this problem: composites can be sintered in a water environment, which minimizes the removal of OH− from HA, or a more stable HA can be synthesized by substituting small amounts of impurities in HA or by increasing the Ca/P ratio in HA [3]. HA composites made with micron-size alumina powder have been studied previously [2,4,5]. The alumina increases the decomposition rate of HA to tri-calcium phosphate (TCP, Ca3 ⁎ Corresponding author. E-mail address: [email protected] (Z. Evis). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.05.028
(PO4)2) at temperatures above 1150 °C [2,4], probably because of the formation of calcium aluminates (e.g. CaAl2O4) by the following reaction: Ca10 ðPO4 Þ6 ðOHÞ2 þ Al2 O3 →3Ca3 ðPO4 Þ2 þ CaAl2 O4 þ H2 O
ð1Þ
Al2O3 completely reacts with HA at temperatures above 1300 °C. However, this reaction can be inhibited by adding a few vol.% of CaF2 to the powder mixture [3]. Then the HA–alumina composites can be sintered to high density at temperatures of 1450 °C and above. The fluoride ions apparently substitute for OH− in the HA, which reduces its tendency to decompose [3,6]. Sintering the mixed powders at intermediate temperatures [5] or using Al2O3 platelets instead of equi-axed powders [7] also increased density and improved the mechanical properties of HA–Al2O3 composites. HA–alumina composites showed improved flexural strength over monolithic HA [5,8,9]. Kim et al. [5] obtained a flexure strength of 255 MPa for 40-vol.% Al2O3–HA composite after 1200 °C–2 h hot pressing under 30 MPa pressure and Li et al.
422
Z. Evis, R.H. Doremus / Materials Science and Engineering C 27 (2007) 421–425
racterized by X-ray diffraction and mechanical testing methods (μ-hardness and fracture toughness). 2. Experimental procedures HA was synthesized by a precipitation method by mixing reagent grade calcium nitrate and di-ammonium hydrogen phosphate solutions in the alkaline pH region [11].
Fig. 1. Amount of densification in 10, 25, and 40 wt.% α-n-Al2O3–HA composites.
[9] obtained a flexure strength of 250 MPa for 30-vol.% αAl2O3–HA composite after 1275 °C hot isostatic pressing under 200 MPa pressure as compared with the usual strength of sintered HA of about 100 MPa [10]. The purpose of the present study was to synthesize and characterize composites of HA and nano-sized alumina powder. These composites were air sintered and hot pressed and cha-
Fig. 2. XRD spectra of 10, 25, and 40 wt.% α-n-Al2O3–HA composites air sintered at 1100 °C and 1200 °C.
Fig. 3. XRD results of 25 wt.% α-n-Al2O3–HA composites air sintered (A) and hot pressed (B) at 1100 °C and 1200 °C.
Z. Evis, R.H. Doremus / Materials Science and Engineering C 27 (2007) 421–425
Fig. 4. XRD spectra of 10, 25, and 40 wt.% α-n-Al2O3–HA composites hot pressed at 1100 °C and 1200 °C.
423
Fig. 5. Vickers μ-hardness of 10, 25, and 40 wt.% α-n-Al2O3–HA composites hot pressed at 1100 °C and 1200 °C.
each phase present in the structure. The volume of the unit cells for the hexagonal HA was calculated by: Nanosize (n)-Al2O3 powder (48 nm size, Nanophase Technologies Inc., Burr Ridge, IL) was mixed with HA powder. The compositions of the ceramics used in this study were 10, 25, and 40 wt.% α-n-Al2O3 and the matrix was HA. The as received γ-nAl2O3 was transformed to α-n-Al2O3 by heat treatment in a platinum crucible at 1300 °C for 10 min before mixing with HA. Dried HA particles were ground to ≤75 μm (−200 mesh) powder using a mortar and pestle, and calcined at 500 °C for 1 h. The calcined HA and Al2O3 powders were mixed by ball milling for 24 h. The composites were air sintered at 1100 °C and 1200 °C for 1 h. Moreover, they were also hot pressed under 60 MPa pressure in vacuum at 1100 °C and 1200 °C in a high temperature-high vacuum furnace (Thermal Technology Inc., Concord, NH). X-ray diffraction (XRD) was performed on the samples with a Cu-Kα radiation at 50 kV/30 mA and each sample was scanned from 20° to 50° in 2θ with a scanning speed of 1°/min using a XDS-2000 Scintag Inc diffractometer. The positions of the diffraction lines were compared with JCPDS files to identify
V ¼ 2:589ða2 Þc
The density of a sample was calculated by dividing the weight by its volume. The theoretical density of the composites (components a and b) was calculated by the following formula. Densityðg=cm3 Þ ¼
Hot pressed composites Composites sintered in air
ID
Temperature (°C)
ΔV (Å3)
HA (JCPDS# 9-432) 25 wt.% α-n-Al2O3 25 wt.% α-n-Al2O3 25 wt.% α-n-Al2O3 25 wt.% α-n-Al2O3
1100 1100 1200 1100 1200
0.0 9.5 12.4 27.8 –
WðaþbÞ
Wa qa
þ Wq b
ð3Þ
b
A JOEL (JSM-840) scanning electron microscope (SEM) at a voltage of 20 kV was used to examine the samples. Samples were etched with a 0.15 M lactic acid for 10 s and then rinsed in water and dried. The micro (μ)-hardness of the samples was measured with a Vickers μ-hardness tester. A diamond indenter with a 200 g (or 300 g) load was pressed into a sample. Approximately 20 measurements were performed on each sample. The hardness for each sample was calculated by: HV ¼ 0:0018544
Table 1 Lattice parameters of HA present in the 25 wt.% α-n-Al2O3 - HA composites
ð2Þ
P d2
ð4Þ
where HV: Vickers hardness (GPa), P (N): applied load (g)*9.806 / 1000, d (mm): diagonal indent length (μm) / 1000. The fracture toughness (K1c) of the composites was estimated from the Vickers μ-hardness test. The Palmqvist equation was used to determine the fracture toughness [12]: ! 0:6 0:4 H 4E a K1c ¼ 0:0354 ð5Þ 4 /0:6 ðc aÞ0:5
424
Z. Evis, R.H. Doremus / Materials Science and Engineering C 27 (2007) 421–425
where, H: hardness, E: Young's modulus, Φ: the coefficient related to the material constraint (Φ ≅ 3). 3. Results and discussion To make HA and n-Al2O3 composites, α-n-Al2O3 powder was used because of its better sinterability and densification as compared to γ-n-Al2O3 [13]. Firstly, the composites were sintered in air without pressure at 1100 °C and 1200 °C. Densification of the HA-α-n-Al2O3 composites was improved slightly when the sintering temperature increased from 1100 °C to 1200 °C, as shown in Fig. 1. The porosity increased substantially as the amount of alumina increased. To reduce porosity, these samples were hot pressed at 60 MPa in vacuum (Fig. 1). Hot pressing resulted in better densification than air sintering. The XRD spectra of air sintered HA-α-n-Al2O3 composites are shown in Fig. 2. When the sintering temperature was increased from 1100 °C to 1200 °C, the stability of the HA phase in the composites decreased dramatically. In all the composites (10, 25, 40 wt.% α-n-Al2O3–HA), the HA phase decomposed almost completely to α-TCP when the sintering temperature was increased from 1100 °C to 1200 °C, as can be seen from the main TCP peak at 30.7°. In addition to α-TCP, calcium aluminates (CaAl2O4, Ca2Al2O5) were observed in these composites (Figs. 2
Fig. 6. SEM micrographs of α-n-Al2O3–HA composites hot pressed at 1200 °C: A) 10wt.% α-n-Al2O3–HA, B) 40 wt.% α-n-Al2O3–HA.
Fig. 7. K1c of 10, 25, and 40 wt.% α-n-Al2O3–HA composites hot pressed at 1100 °C and 1200 °C.
and 3). No CaO peak was observed in any of the composites. We believe CaO formed by transformation of HA to α-TCP, and then reacted with α-n-Al2O3 to form calcium aluminates. The XRD spectra of hot pressed HA-α-n-Al2O3 composites are shown in Fig. 4. When the sintering temperature was increased from 1100 °C to 1200 °C, the stability of the HA in the composites decreased slightly. In 10 and 25 wt.% α-n-Al2O3 composites, HA was stable after the hot pressing at 1100 °C and 1200 °C. In 40 wt.% α-n-Al2O3 composites, the HA phase decomposed partially to α-TCP after the hot pressing at 1100 °C and 1200 °C. When the wt.% α-n-Al2O3 increased from 10 to 40, transformation from HA to TCP increased. The XRD results of air sintered and hot pressed 25 wt.% α-nAl2O3 composites are presented in Fig. 3 to show the effect of air sintering and hot pressing on phase transformations. HA decomposed to TCP faster after the air sintering. In Table 1, hexagonal unit cell volumes of the HA that remained in the composites after the sinterings are presented for 25 wt.% α-n-Al2O3 composite. After the air sintering at 1100 °C, hexagonal unit cell volume of the HA changed in a high rate which suggested that the untransformed HAwas Ca2+ and OH− deficient. Moreover, there was no HA left after the air sintering at 1200 °C. When the composites were hot pressed, there was not a significant change in the unit cell volume of the HA. Alumina appears to catalyze the decomposition of HA (reaction 1) especially after the air sintering with the formation of calcium aluminates. The solid solubility in Al2O3 is limited because of its close-packed structure, so that the formation of the aluminates probably takes place on the surface of the alumina particles. The removal of calcium must be compensated by the formation of oxygen vacancies, leading to a relaxation of the HA structure and its decomposition to TCP. The suppression of decomposition of HA during sintering by these mechanisms
Z. Evis, R.H. Doremus / Materials Science and Engineering C 27 (2007) 421–425
reduces the amount of water formed by decomposition of HA (reaction 1). Vickers μ-hardness results of 10, 25 and 40 wt.% α-n-Al2O3– HA composites, which were hot pressed at 1100 °C and 1200 °C, are presented in Fig. 5. For 10 and 25 wt.% α-n-Al2O3–HA composites, the μ-hardness of the composites improved when the sintering temperature was increased from 1100 °C to 1200 °C. 40 wt.% α-n-Al2O3 showed poor sinterability and low hardness compared to 10 and 25 wt.% α-n-Al2O3 because of its higher porosity; α-n-Al2O3 phase requires higher sintering temperatures than 1100 °C or 1200 °C for a complete densification. Amount of porosity in the composites is presented in Fig. 1. There were porosities up to 20.4 % in the composites, which resulted in poor μ-hardness values. SEM micrographs of 10 and 40 wt.% α-nAl2O3–HA composites, which were hot pressed at 1200 °C, are presented in Fig. 6. K1c of the hot pressed composites was determined by comparison of the length of the cracks and the size of the indentation formed after Vickers μ-hardness testing. K1c of the 10, 25 and 40 wt.% α-n-Al2O3–HA composites are presented in Fig. 7. A maximum of 2 MPaMm of K1c was observed for 40 wt. % α-n-Al2O3 composite. 4. Conclusion Hot pressing in vacuum resulted in improved densification of the composites. HA decomposed to second phases faster in air
425
sintering than in hot pressing, which was verified by the hexagonal unit cell volume of HA left in the composites. As the amount of alumina increased, sinterability considerably decreased. Hot pressing at 1200 °C resulted in better mechanical properties (μ-hardness and fracture toughness) than the hot pressing at 1100 °C. References [1] R.H. Doremus, J. Mater. Sci. 27 (1992) 285. [2] H. Ji, P.M. Marquis, Biomaterials 13–11 (1992) 744. [3] H. Kim, Y. Koh, S. Seo, H. Kim, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 1072 (2003) 1. [4] H. Ji, P.M. Marquis, J. Mater. Sci. 28 (1993) 1941. [5] S. Kim, Y. Kong, I. Lee, H. Kim, J. Mater. Sci., Mater. Med. 13 (2002) 307. [6] E. Adolfsson, M. Nygren, L. Hermansson, J. Am. Ceram. Soc. 82–10 (1999) 2909. [7] S. Gautier, E. Champion, D. Bernache-Assollant, J. Mater. Sci., Mater. Med. 10 (1999) 533. [8] J. Choi, Y. Kong, H. Kim, I. Lee, J. Am. Ceram. Soc. 81–7 (1998) 1743. [9] J. Li, B. Fartash, L. Hermansson, Biomaterials 16 (1995) 417. [10] M.B. Thomas, R.H. Doremus, Am. Ceram. Soc. Bull. 60–2 (1981) 258. [11] M. Jarcho, C.H. Bolen, M.B. Thomas, J. Babock, J.F. Kay, R.H. Doremus, J. Mater. Sci. 11 (1976) 2027. [12] A. Slosarczyk, J. Bialoskorski, J. Mater. Sci., Mater. Med. 9 (1998) 103. [13] S. Chang, R.H. Doremus, L.S. Schadler, R.W. Siegel, Inter. J. Appl. Ceram. Technol. 1 (2004) 172.
A study of phase stability and mechanical properties of hydroxylapatite–nanosize α-alumina composites Zafer Evis a,⁎, Robert H. Doremus b a
b
Middle East Technical University, Engineering Sciences, Ankara, 06531, Turkey Rensselaer Polytechnic Institute, Materials Science and Engineering, Troy, NY, 12180, USA Available online 21 June 2006
Abstract Hydroxylapatite (HA)–nanosize alumina composites were synthesized to study their phase stability and mechanical properties. To make these composites, nanosize α-Al2O3 powder was used because of its better sinterability and densification as compared to nanosize γ-Al2O3. The composites were air sintered without pressure and hot pressed in vacuum at 1100 °C and 1200 °C. In the composites, HA decomposed to tricalcium phosphate faster after the air sintering than hot pressing. Moreover, hexagonal unit cell volume of HA left in the composites showed that there was more decomposition of HA after the air sintering than hot pressing. It also showed that HA in the composites was OH− and Ca2+ deficient. As the amount of alumina increased, sinterability considerably decreased. Hot pressing at 1200 °C resulted in better mechanical properties (μ-hardness and fracture toughness) than the hot pressing at 1100 °C. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydroxylapatite; Nanosize alumina; Hot pressing; X-ray diffraction
1. Introduction Hydroxylapatite (HA, Ca10(PO4)6(OH)2) is mainly used as a hard tissue implant because of its excellent biocompatibility in the human body [1]. A HA implant can only be used in low impact applications, e.g. in the middle ear, because it is brittle. Composites of HA with alumina can combine the advantages of HAs biocompatibility and higher strength of alumina. When mixtures of HA and alumina powders are sintered in air from 1100 °C to 1400 °C, HA decomposes and reacts to form unwanted second phases [2]. Two approaches can overcome this problem: composites can be sintered in a water environment, which minimizes the removal of OH− from HA, or a more stable HA can be synthesized by substituting small amounts of impurities in HA or by increasing the Ca/P ratio in HA [3]. HA composites made with micron-size alumina powder have been studied previously [2,4,5]. The alumina increases the decomposition rate of HA to tri-calcium phosphate (TCP, Ca3 ⁎ Corresponding author. E-mail address: [email protected] (Z. Evis). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.05.028
(PO4)2) at temperatures above 1150 °C [2,4], probably because of the formation of calcium aluminates (e.g. CaAl2O4) by the following reaction: Ca10 ðPO4 Þ6 ðOHÞ2 þ Al2 O3 →3Ca3 ðPO4 Þ2 þ CaAl2 O4 þ H2 O
ð1Þ
Al2O3 completely reacts with HA at temperatures above 1300 °C. However, this reaction can be inhibited by adding a few vol.% of CaF2 to the powder mixture [3]. Then the HA–alumina composites can be sintered to high density at temperatures of 1450 °C and above. The fluoride ions apparently substitute for OH− in the HA, which reduces its tendency to decompose [3,6]. Sintering the mixed powders at intermediate temperatures [5] or using Al2O3 platelets instead of equi-axed powders [7] also increased density and improved the mechanical properties of HA–Al2O3 composites. HA–alumina composites showed improved flexural strength over monolithic HA [5,8,9]. Kim et al. [5] obtained a flexure strength of 255 MPa for 40-vol.% Al2O3–HA composite after 1200 °C–2 h hot pressing under 30 MPa pressure and Li et al.
422
Z. Evis, R.H. Doremus / Materials Science and Engineering C 27 (2007) 421–425
racterized by X-ray diffraction and mechanical testing methods (μ-hardness and fracture toughness). 2. Experimental procedures HA was synthesized by a precipitation method by mixing reagent grade calcium nitrate and di-ammonium hydrogen phosphate solutions in the alkaline pH region [11].
Fig. 1. Amount of densification in 10, 25, and 40 wt.% α-n-Al2O3–HA composites.
[9] obtained a flexure strength of 250 MPa for 30-vol.% αAl2O3–HA composite after 1275 °C hot isostatic pressing under 200 MPa pressure as compared with the usual strength of sintered HA of about 100 MPa [10]. The purpose of the present study was to synthesize and characterize composites of HA and nano-sized alumina powder. These composites were air sintered and hot pressed and cha-
Fig. 2. XRD spectra of 10, 25, and 40 wt.% α-n-Al2O3–HA composites air sintered at 1100 °C and 1200 °C.
Fig. 3. XRD results of 25 wt.% α-n-Al2O3–HA composites air sintered (A) and hot pressed (B) at 1100 °C and 1200 °C.
Z. Evis, R.H. Doremus / Materials Science and Engineering C 27 (2007) 421–425
Fig. 4. XRD spectra of 10, 25, and 40 wt.% α-n-Al2O3–HA composites hot pressed at 1100 °C and 1200 °C.
423
Fig. 5. Vickers μ-hardness of 10, 25, and 40 wt.% α-n-Al2O3–HA composites hot pressed at 1100 °C and 1200 °C.
each phase present in the structure. The volume of the unit cells for the hexagonal HA was calculated by: Nanosize (n)-Al2O3 powder (48 nm size, Nanophase Technologies Inc., Burr Ridge, IL) was mixed with HA powder. The compositions of the ceramics used in this study were 10, 25, and 40 wt.% α-n-Al2O3 and the matrix was HA. The as received γ-nAl2O3 was transformed to α-n-Al2O3 by heat treatment in a platinum crucible at 1300 °C for 10 min before mixing with HA. Dried HA particles were ground to ≤75 μm (−200 mesh) powder using a mortar and pestle, and calcined at 500 °C for 1 h. The calcined HA and Al2O3 powders were mixed by ball milling for 24 h. The composites were air sintered at 1100 °C and 1200 °C for 1 h. Moreover, they were also hot pressed under 60 MPa pressure in vacuum at 1100 °C and 1200 °C in a high temperature-high vacuum furnace (Thermal Technology Inc., Concord, NH). X-ray diffraction (XRD) was performed on the samples with a Cu-Kα radiation at 50 kV/30 mA and each sample was scanned from 20° to 50° in 2θ with a scanning speed of 1°/min using a XDS-2000 Scintag Inc diffractometer. The positions of the diffraction lines were compared with JCPDS files to identify
V ¼ 2:589ða2 Þc
The density of a sample was calculated by dividing the weight by its volume. The theoretical density of the composites (components a and b) was calculated by the following formula. Densityðg=cm3 Þ ¼
Hot pressed composites Composites sintered in air
ID
Temperature (°C)
ΔV (Å3)
HA (JCPDS# 9-432) 25 wt.% α-n-Al2O3 25 wt.% α-n-Al2O3 25 wt.% α-n-Al2O3 25 wt.% α-n-Al2O3
1100 1100 1200 1100 1200
0.0 9.5 12.4 27.8 –
WðaþbÞ
Wa qa
þ Wq b
ð3Þ
b
A JOEL (JSM-840) scanning electron microscope (SEM) at a voltage of 20 kV was used to examine the samples. Samples were etched with a 0.15 M lactic acid for 10 s and then rinsed in water and dried. The micro (μ)-hardness of the samples was measured with a Vickers μ-hardness tester. A diamond indenter with a 200 g (or 300 g) load was pressed into a sample. Approximately 20 measurements were performed on each sample. The hardness for each sample was calculated by: HV ¼ 0:0018544
Table 1 Lattice parameters of HA present in the 25 wt.% α-n-Al2O3 - HA composites
ð2Þ
P d2
ð4Þ
where HV: Vickers hardness (GPa), P (N): applied load (g)*9.806 / 1000, d (mm): diagonal indent length (μm) / 1000. The fracture toughness (K1c) of the composites was estimated from the Vickers μ-hardness test. The Palmqvist equation was used to determine the fracture toughness [12]: ! 0:6 0:4 H 4E a K1c ¼ 0:0354 ð5Þ 4 /0:6 ðc aÞ0:5
424
Z. Evis, R.H. Doremus / Materials Science and Engineering C 27 (2007) 421–425
where, H: hardness, E: Young's modulus, Φ: the coefficient related to the material constraint (Φ ≅ 3). 3. Results and discussion To make HA and n-Al2O3 composites, α-n-Al2O3 powder was used because of its better sinterability and densification as compared to γ-n-Al2O3 [13]. Firstly, the composites were sintered in air without pressure at 1100 °C and 1200 °C. Densification of the HA-α-n-Al2O3 composites was improved slightly when the sintering temperature increased from 1100 °C to 1200 °C, as shown in Fig. 1. The porosity increased substantially as the amount of alumina increased. To reduce porosity, these samples were hot pressed at 60 MPa in vacuum (Fig. 1). Hot pressing resulted in better densification than air sintering. The XRD spectra of air sintered HA-α-n-Al2O3 composites are shown in Fig. 2. When the sintering temperature was increased from 1100 °C to 1200 °C, the stability of the HA phase in the composites decreased dramatically. In all the composites (10, 25, 40 wt.% α-n-Al2O3–HA), the HA phase decomposed almost completely to α-TCP when the sintering temperature was increased from 1100 °C to 1200 °C, as can be seen from the main TCP peak at 30.7°. In addition to α-TCP, calcium aluminates (CaAl2O4, Ca2Al2O5) were observed in these composites (Figs. 2
Fig. 6. SEM micrographs of α-n-Al2O3–HA composites hot pressed at 1200 °C: A) 10wt.% α-n-Al2O3–HA, B) 40 wt.% α-n-Al2O3–HA.
Fig. 7. K1c of 10, 25, and 40 wt.% α-n-Al2O3–HA composites hot pressed at 1100 °C and 1200 °C.
and 3). No CaO peak was observed in any of the composites. We believe CaO formed by transformation of HA to α-TCP, and then reacted with α-n-Al2O3 to form calcium aluminates. The XRD spectra of hot pressed HA-α-n-Al2O3 composites are shown in Fig. 4. When the sintering temperature was increased from 1100 °C to 1200 °C, the stability of the HA in the composites decreased slightly. In 10 and 25 wt.% α-n-Al2O3 composites, HA was stable after the hot pressing at 1100 °C and 1200 °C. In 40 wt.% α-n-Al2O3 composites, the HA phase decomposed partially to α-TCP after the hot pressing at 1100 °C and 1200 °C. When the wt.% α-n-Al2O3 increased from 10 to 40, transformation from HA to TCP increased. The XRD results of air sintered and hot pressed 25 wt.% α-nAl2O3 composites are presented in Fig. 3 to show the effect of air sintering and hot pressing on phase transformations. HA decomposed to TCP faster after the air sintering. In Table 1, hexagonal unit cell volumes of the HA that remained in the composites after the sinterings are presented for 25 wt.% α-n-Al2O3 composite. After the air sintering at 1100 °C, hexagonal unit cell volume of the HA changed in a high rate which suggested that the untransformed HAwas Ca2+ and OH− deficient. Moreover, there was no HA left after the air sintering at 1200 °C. When the composites were hot pressed, there was not a significant change in the unit cell volume of the HA. Alumina appears to catalyze the decomposition of HA (reaction 1) especially after the air sintering with the formation of calcium aluminates. The solid solubility in Al2O3 is limited because of its close-packed structure, so that the formation of the aluminates probably takes place on the surface of the alumina particles. The removal of calcium must be compensated by the formation of oxygen vacancies, leading to a relaxation of the HA structure and its decomposition to TCP. The suppression of decomposition of HA during sintering by these mechanisms
Z. Evis, R.H. Doremus / Materials Science and Engineering C 27 (2007) 421–425
reduces the amount of water formed by decomposition of HA (reaction 1). Vickers μ-hardness results of 10, 25 and 40 wt.% α-n-Al2O3– HA composites, which were hot pressed at 1100 °C and 1200 °C, are presented in Fig. 5. For 10 and 25 wt.% α-n-Al2O3–HA composites, the μ-hardness of the composites improved when the sintering temperature was increased from 1100 °C to 1200 °C. 40 wt.% α-n-Al2O3 showed poor sinterability and low hardness compared to 10 and 25 wt.% α-n-Al2O3 because of its higher porosity; α-n-Al2O3 phase requires higher sintering temperatures than 1100 °C or 1200 °C for a complete densification. Amount of porosity in the composites is presented in Fig. 1. There were porosities up to 20.4 % in the composites, which resulted in poor μ-hardness values. SEM micrographs of 10 and 40 wt.% α-nAl2O3–HA composites, which were hot pressed at 1200 °C, are presented in Fig. 6. K1c of the hot pressed composites was determined by comparison of the length of the cracks and the size of the indentation formed after Vickers μ-hardness testing. K1c of the 10, 25 and 40 wt.% α-n-Al2O3–HA composites are presented in Fig. 7. A maximum of 2 MPaMm of K1c was observed for 40 wt. % α-n-Al2O3 composite. 4. Conclusion Hot pressing in vacuum resulted in improved densification of the composites. HA decomposed to second phases faster in air
425
sintering than in hot pressing, which was verified by the hexagonal unit cell volume of HA left in the composites. As the amount of alumina increased, sinterability considerably decreased. Hot pressing at 1200 °C resulted in better mechanical properties (μ-hardness and fracture toughness) than the hot pressing at 1100 °C. References [1] R.H. Doremus, J. Mater. Sci. 27 (1992) 285. [2] H. Ji, P.M. Marquis, Biomaterials 13–11 (1992) 744. [3] H. Kim, Y. Koh, S. Seo, H. Kim, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 1072 (2003) 1. [4] H. Ji, P.M. Marquis, J. Mater. Sci. 28 (1993) 1941. [5] S. Kim, Y. Kong, I. Lee, H. Kim, J. Mater. Sci., Mater. Med. 13 (2002) 307. [6] E. Adolfsson, M. Nygren, L. Hermansson, J. Am. Ceram. Soc. 82–10 (1999) 2909. [7] S. Gautier, E. Champion, D. Bernache-Assollant, J. Mater. Sci., Mater. Med. 10 (1999) 533. [8] J. Choi, Y. Kong, H. Kim, I. Lee, J. Am. Ceram. Soc. 81–7 (1998) 1743. [9] J. Li, B. Fartash, L. Hermansson, Biomaterials 16 (1995) 417. [10] M.B. Thomas, R.H. Doremus, Am. Ceram. Soc. Bull. 60–2 (1981) 258. [11] M. Jarcho, C.H. Bolen, M.B. Thomas, J. Babock, J.F. Kay, R.H. Doremus, J. Mater. Sci. 11 (1976) 2027. [12] A. Slosarczyk, J. Bialoskorski, J. Mater. Sci., Mater. Med. 9 (1998) 103. [13] S. Chang, R.H. Doremus, L.S. Schadler, R.W. Siegel, Inter. J. Appl. Ceram. Technol. 1 (2004) 172.