Availableonlineatwww.sciencedirect.cor ScienceDirect materials letters ELSEVIER Materials Letters 61(2007)405-408 www.elseviercom/locate/matlet Microstructure characterization of fibrous HAp-(20 vol% t-zrO2) AlO3-(25 vol. m-ZrO2)composites by multi-pass extrusion process Chi-woo Lee Asit Kumar Gain, Soo-Jae Yim, Byong-Taek Lee a,* School of Advanced Materials Engineering, Kongju National University, 182, Shinkwan-dong, Kongju Ciry, Chun 4-701. South korea epartment of Physiology, School of Medicine, Soonchunhyang University, 366-1, Ssangyoung-dong, Cheonan Cin, C am. 330-090. South korea Received 9 March 2006: accepted 9 April 2006 Available online 11 May 2006 Abstract Core-shell structured HAp-(t-ZrO2)AL,O3(m-zrO2)composites were fabricated using a multi extrusion process. The shell of AlO3-(m-zrO2 phases was selected due to their excellent biocompatibility and mechanical properties and the core was designed with t-zrO2 dispersed in the HAp matrix. The t-ZrO2 and m-ZrO2 particles (<400 nm) were homogeneously dispersed in the HAp and al2O3 phases, respectively. In the hap-(t- ZrO2)core region, a heavy strain field contrast was observed due to the mismatch of their thermal expansion coefficients. The values of relative ensity, bending strength and Vickers hardness of the third pass fibrous HAp(t-ZrO2)Al2O3-(m-ZrO2)composites, which were sintered at 1400C, were about 93%, 169 MPa, and 792 Hv, respectively C 2006 Elsevier B V. All rights reserved. Keywords: Hydroxyapatite; Al2O3-ZrO2 composite; Microstructure; Material properties 1. Introduction excellent material properties such as strength, chemical stability, wear resistance, and biocompatibility [10-12]. Fibrous Al2O3- Hydroxyapatite(HAp)has been widely used as a biomaterial ZrO2 composites have shown improved fracture strength and for bone tissue implantation due to its excellent biocompatibility toughness due to multi-toughening mechanisms such as crack and bioactivity as well as its chemical composition, which is diffraction, microcracking and phase transformation [13, 14] similar to natural bones [1-4]. Unfortunately, HAp cannot be Therefore, Al O3 and ZrO2 ceramics are considered to be one of used for heavy load-bearing parts, implants such as artificial the best reinforcement materials for making HAp composites teeth, or hip replacements due to its low inherent mechanical In this study, fibrous HAp-(20 vol %t-ZrO2 VAIO3-(25 vol% properties, especially its low fracture toughness [5,6]. Several m-ZrO2) composites were fabricated using the multi-pass approaches for improving the mechanical properties of HAp have extrusion process. The microstructure was characterized in detail, been investigated through the control of microstructure or the depending on the sintering temperature using XRD, SEM and development of HAp composites by the addition of a second TEM technique phase [7, 8]. The second phase as reinforcement generally re- quires the following parameters: excellent material properties, 2. Experimental procedure biocompatibility, and no reaction with the HAp matrix. Al2O3 and ZrO ceramics are widely used not only as biomaterials, but To fabricate the HAp-(t-zrO2)Al2O3-(m-ZrO2)composit have industrial applications such as cutting tools and wear-re- HAp(10 um, Strem chemical, USA), AlO3(0.3 um, AKP-50 sistant components [9]. The Al2O3-ZrO2 composites have shown Sumitomo, Japan), t-ZrO2(80 nm, Tosho, Japan), and m-ZrO2 (80 nm, Tosho, Japan) powders were used as starting materials Ethylene vinyl acetate(EVA)(ELVAX 210 and 250, Dupont, Corresponding author. Tel +82 41 850 8677: fax: +82 41 858 2939 USA)and stearic acid(CH3(CH2)16COOH, Daejung Chemicals E-mail address: Ibt(@ kongju ac kr(B -T. Lee). and Metals Co., Korea) were added as binder and lubricant 0167-577X/S- see front matter e 2006 Elsevier B. V. All rights reserved. doi:10.1016/ malet.2006.04.071
Microstructure characterization of fibrous HAp-(20 vol.% t-ZrO2)/ Al2O3-(25 vol.% m-ZrO2) composites by multi-pass extrusion process Chi-woo Lee a , Asit Kumar Gain a , Soo-Jae Yim b , Byong-Taek Lee a,⁎ a School of Advanced Materials Engineering, Kongju National University, 182, Shinkwan-dong, Kongju City, Chungnam, 314-701, South Korea b Department of Physiology, School of Medicine, Soonchunhyang University, 366-1, Ssangyoung-dong, Cheonan City, Chungnam, 330-090, South Korea Received 9 March 2006; accepted 9 April 2006 Available online 11 May 2006 Abstract Core–shell structured HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites were fabricated using a multi extrusion process. The shell of Al2O3-(m-ZrO2) phases was selected due to their excellent biocompatibility and mechanical properties and the core was designed with t-ZrO2 dispersed in the HAp matrix. The t-ZrO2 and m-ZrO2 particles (b400 nm) were homogeneously dispersed in the HAp and Al2O3 phases, respectively. In the HAp-(tZrO2) core region, a heavy strain field contrast was observed due to the mismatch of their thermal expansion coefficients. The values of relative density, bending strength and Vickers hardness of the third pass fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites, which were sintered at 1400 °C, were about 93%, 169 MPa, and 792 Hv, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite; Al2O3–ZrO2 composite; Microstructure; Material properties 1. Introduction Hydroxyapatite (HAp) has been widely used as a biomaterial for bone tissue implantation due to its excellent biocompatibility and bioactivity as well as its chemical composition, which is similar to natural bones [1–4]. Unfortunately, HAp cannot be used for heavy load-bearing parts, implants such as artificial teeth, or hip replacements due to its low inherent mechanical properties, especially its low fracture toughness [5,6]. Several approaches for improving the mechanical properties of HAp have been investigated through the control of microstructure or the development of HAp composites by the addition of a second phase [7,8]. The second phase as reinforcement generally requires the following parameters: excellent material properties, biocompatibility, and no reaction with the HAp matrix. Al2O3 and ZrO2 ceramics are widely used not only as biomaterials, but have industrial applications such as cutting tools and wear-resistant components[9]. The Al2O3–ZrO2 composites have shown excellent material properties such as strength, chemical stability, wear resistance, and biocompatibility [10–12]. Fibrous Al2O3– ZrO2 composites have shown improved fracture strength and toughness due to multi-toughening mechanisms such as crack diffraction, microcracking and phase transformation [13,14]. Therefore, Al2O3 and ZrO2 ceramics are considered to be one of the best reinforcement materials for making HAp composites. In this study, fibrous HAp-(20 vol.% t-ZrO2)/Al2O3-(25 vol.% m-ZrO2) composites were fabricated using the multi-pass extrusion process. The microstructure was characterized in detail, depending on the sintering temperature using XRD, SEM and TEM techniques. 2. Experimental procedure To fabricate the HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites, HAp (10 μm, Strem chemical, USA), Al2O3 (0.3 μm, AKP-50, Sumitomo, Japan), t-ZrO2 (80 nm, Tosho, Japan), and m-ZrO2 (80 nm, Tosho, Japan) powders were used as starting materials. Ethylene vinyl acetate (EVA) (ELVAX 210 and 250, Dupont, USA) and stearic acid (CH3(CH2)16COOH, Daejung Chemicals and Metals Co., Korea) were added as binder and lubricant, Materials Letters 61 (2007) 405–408 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +82 41 850 8677; fax: +82 41 858 2939. E-mail address: lbt@kongju.ac.kr (B.-T. Lee). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.04.071
C. Lee et al. Materials Letters 61(2007)405-408 HAp(t-ZrO Fig. 1. SEM micrographs of 3rd passed(a)cross-sectional HAp-(t-ZrO2)AlO3(m-ZrO2) bodies and ( b),(c), enlarged images sintered at 1200C and 1400C. respectively,(d) longitudinal sectional images. respectively. HAp/t-ZrO2(volume fraction 80: 20)and Al2O3/ scattered electron scanning microscope(BSE-SEM, JEOL-JSM m-zrO2(volume fraction 75: 25) were homogeneously mixed in 5410)and transmission electron microscope (TEM, JEM-2010, ethanol by ball milling using Al2O3 milling media. After JEOL, Japan) techniques. The crystal phases were analyzed by mixing, each composition was dried on a hot plate while being X-ray diffraction (XRD, D/MAX-250, Rigaku, Japan)using stirred. The Al,O3/m-zrO2, EVA, and stearic acid(volume Cu-Ka of 0. 1542. nm. fraction 60: 30: 10) were homogeneously mixed using a shear mixer(Shina Platec Co., Korea). The shear-mixed materials 3. Results and discussion were used to produce a hollow tube or shell by warm pressing To make the core rod, HAp/t-ZrO2 powders, EVA, and stearic Fig. 1 shows SEM micrographs of the 3rd pass fibrous HAp- acid(volume fraction 50: 40: 10)were mixed using a shear (20 voL %o t-ZrO2)Al203-(25 voL %m-ZrO2) composites. In cross- mixer. These core and shell were assembled to make a feedrod section, low magnification images(Fig. 1(a)), the core-shell and extruded in a heated die to make the lst pass filaments, which were about 3.5 mm in diameter. The lst pass filaments mZrO2·HAp, were then cut and loaded into a steel die and extruded to make the 2nd pass filaments. The 3rd pass filaments were produced in the same way, using the 2nd pass filaments. To remove the eVA binder, the burn-out process was carried out in a tube furnace with a heating rate(45C/h)up to 700C for 2 h under a N mosphere [15]. The samples were densified by pressureless sintering at1200°C-1400°for2 h in air The relative density was measured by the Archimedes method in an immersion medium of water. The average Vickers hardness was measured randomly by indenting with a load of 2.5 kg(10 pointssample)in core and shell regions. To prepare the bending test samples, 3.4 mm in diameter, the as sintered bulk samples were cut 35 mm in length and measured by a 4- oint bending method using 5 specimens with a crosshead speed fo I mm/min, using a universal testing machine(Unitech M,R Fig. 2. XRD profiles of raw powders and HAp(t-ZrO2MA1203-(m-z102 and B, Korea). Microstructures were examined using back- composites sintered at(a)1000C(b)1200C and(c)1400C
respectively. HAp/t-ZrO2 (volume fraction 80:20) and Al2O3/ m-ZrO2 (volume fraction 75:25) were homogeneously mixed in ethanol by ball milling using Al2O3 milling media. After mixing, each composition was dried on a hot plate while being stirred. The Al2O3/m-ZrO2, EVA, and stearic acid (volume fraction 60:30:10) were homogeneously mixed using a shear mixer (Shina Platec. Co., Korea). The shear-mixed materials were used to produce a hollow tube or shell by warm pressing. To make the core rod, HAp/t-ZrO2 powders, EVA, and stearic acid (volume fraction 50:40:10) were mixed using a shear mixer. These core and shell were assembled to make a feedrod and extruded in a heated die to make the 1st pass filaments, which were about 3.5 mm in diameter. The 1st pass filaments were then cut and loaded into a steel die and extruded to make the 2nd pass filaments. The 3rd pass filaments were produced in the same way, using the 2nd pass filaments. To remove the EVA binder, the burn-out process was carried out in a tube furnace with a heating rate (45 °C/h) up to 700 °C for 2 h under a N2 atmosphere [15]. The samples were densified by pressureless sintering at 1200 °C–1400 °C for 2 h in air. The relative density was measured by the Archimedes method in an immersion medium of water. The average Vickers hardness was measured randomly by indenting with a load of 2.5 kg (10 points/sample) in core and shell regions. To prepare the bending test samples, 3.4 mm in diameter, the as sintered bulk samples were cut 35 mm in length and measured by a 4- point bending method using 5 specimens with a crosshead speed of 0.1 mm/min, using a universal testing machine (Unitech™, R and B, Korea). Microstructures were examined using backscattered electron scanning microscope (BSE-SEM, JEOL-JSM 5410) and transmission electron microscope (TEM, JEM-2010, JEOL, Japan) techniques. The crystal phases were analyzed by X-ray diffraction (XRD, D/MAX-250, Rigaku, Japan) using Cu–Kα of 0.1542 nm. 3. Results and discussion Fig. 1 shows SEM micrographs of the 3rd pass fibrous HAp- (20 vol.% t-ZrO2)/Al2O3-(25 vol.% m-ZrO2) composites. In crosssection, low magnification images (Fig. 1(a)), the core–shell Fig. 1. SEM micrographs of 3rd passed (a) cross-sectional HAp-(t-ZrO2)/Al2O3-(m-ZrO2) bodies and (b), (c), enlarged images sintered at 1200 °C and 1400 °C., respectively, (d) longitudinal sectional images. Fig. 2. XRD profiles of raw powders and HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites sintered at (a) 1000 °C (b) 1200 °C and (c) 1400 °C. 406 C. Lee et al. / Materials Letters 61 (2007) 405–408
C. Lee et al / Materials Letters 61(2007)405-408 Fig 3. TEM micrographs of HAp-(t-ZrO2)Al2Or-(m-ZrO2)composite sintered at 1400C;(a)core region and (b)shell region. Fig 4. SEM fracture surfaces of 3rd passed HAp-(t-ZrO2VAl2O3-(m-zrO2)bodies sintered at different temperatures; (a)1200C,(b)1400C. microstructure was clearly observed without any processing defects bright and dark contrast(Fig. 3(a)) corresponds to HAp and t-zrO2 such as large cracks or shrinkage cavities. The core was about 35 um in phases, respectively. The fine t-zrO2 phase, less than 400 nm in diameter and the shell was 4.5 um thick. In the enlarged SEM images diameter, was homogeneously dispersed in the HAp matrix On the (b, c), which were sintered at 1200C and 1400C, respectively, the HAp-(t-ZrO2) core and AlO3(m-zrO2) shell regions were clearly observed In the sample sintered at 1200C, HAp-(t-zrO2)and Al2O3- HAp (m-zrO2) regions appeared with a porous microstructure due to the loy sintering temperature. However, at 1400C, the Al2O3-(m-zrO2) shell was comprised of a dense, fine microstructure due to the higher density. On the other hand, the HAp-(t-zrO2)core region showed some porous structure due to decomposition of the HAp phase. Also, the grain size was larger compared with the sample sintered at 1200C. Furthermore the t-ZrO2 and m-ZrO, phases were homogeneously dispersed in the HAp and AlO3 matrices, respectively. In the longitudinal orientation (Fig. 1(d)), the continuous fibrous microstructure was well controlled Fig. 2 shows the XRD profiles of HAp-t-ZrO2VAI2O3-m-zrO2 composites, depending on the sintering temperature. In the samples sintered at(a)1000C and (b), 1200C, a HAp phase was detected well as t-ZrO2, Al2O3, and m-zrO2 phases. However, after sintering at (c)1400C, it was found that most of HAp phase was transformed to B-tricalcium phosphate(B-TCP) Fig 3 shows TEM micrographs of the cross-sections of(a)core and Fig. 5. TEM images of crack propagation of 3rd passed HAp-(t-ZI02)core (b) shell regions of fibrous HAp composite sintered at 1400C. The region sintered at 1400C
microstructure was clearly observed without any processing defects such as large cracks or shrinkage cavities. The core was about 35 μm in diameter and the shell was 4.5 μm thick. In the enlarged SEM images (b, c), which were sintered at 1200 °C and 1400 °C, respectively, the HAp-(t-ZrO2) core and Al2O3-(m-ZrO2) shell regions were clearly observed. In the sample sintered at 1200 °C, HAp-(t-ZrO2) and Al2O3- (m-ZrO2) regions appeared with a porous microstructure due to the low sintering temperature. However, at 1400 °C, the Al2O3-(m-ZrO2) shell was comprised of a dense, fine microstructure due to the higher density. On the other hand, the HAp-(t-ZrO2) core region showed some porous structure due to decomposition of the HAp phase. Also, the grain size was larger compared with the sample sintered at 1200 °C. Furthermore, the t-ZrO2 and m-ZrO2 phases were homogeneously dispersed in the HAp and Al2O3 matrices, respectively. In the longitudinal orientation (Fig. 1(d)), the continuous fibrous microstructure was well controlled with white and gray colors, respectively. Fig. 2 shows the XRD profiles of HAp-(t-ZrO2)/Al2O3-m-ZrO2 composites, depending on the sintering temperature. In the samples sintered at (a) 1000 °C and (b), 1200 °C, a HAp phase was detected as well as t-ZrO2, Al2O3, and m-ZrO2 phases. However, after sintering at (c) 1400 °C, it was found that most of HAp phase was transformed to β-tricalcium phosphate (β-TCP). Fig. 3 shows TEM micrographs of the cross-sections of (a) core and (b) shell regions of fibrous HAp composite sintered at 1400 °C. The bright and dark contrast (Fig. 3(a)) corresponds to HAp and t-ZrO2 phases, respectively. The fine t-ZrO2 phase, less than 400 nm in diameter, was homogeneously dispersed in the HAp matrix. On the Fig. 3. TEM micrographs of HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composite sintered at 1400 °C; (a) core region and (b) shell region. Fig. 4. SEM fracture surfaces of 3rd passed HAp-(t-ZrO2)/Al2O3-(m-ZrO2) bodies sintered at different temperatures; (a) 1200 °C, (b) 1400 °C. Fig. 5. TEM images of crack propagation of 3rd passed HAp-(t-ZrO2) core region sintered at 1400 °C. C. Lee et al. / Materials Letters 61 (2007) 405–408 407
C. Lee et al. /Materials Letters 61(2007)405-408 Table I In the sample sintered at 1200C, the HAp-(t-zrO2) core and Material properties of fibrous HAp-ft-ZrO2VAl203-(m-ZrO2)composites Al 03-(m-zrO2) shell regions had a porous structure. However. depending on the sintering temperature in the sample sintered at 1400C, the Al2O3-(m-zrO2)shell Samples Properties region possessed a dense structure, although the HAp-(t-ZrO2) Shrinkage Relative density Vickers hardness Bending strength core region showed a porous structure due to the formation of B-TCP phase. The fine ZrO2 particles (400 nm in diameter) 1200°18.4583 133±15 72±8 were homogeneously dispersed in the HAp and Al2O3 matrices. 1400°35.65 In the HAp matrix, large amounts of strain field contrast was observed by TEM analysis, which is believed to have enhanced other hand, in the Al2O3-(m-ZrO2)shell region(b), the bright and dark crack deflection and microcracking mechanisms. This was due ontrast corresponds to Al203 and m-ZrO2 phases, respectively. Round to the mismatch of their thermal expansion coefficients.The m-ZrO2 grains less than 400 nm were clearly observed, but in Fig 3(a) values of relative density, Vickers hardness and bending the HAp-(t-ZrO2) core region contains a heavy strain field contrast(as indicated by the inset arrows)which may have formed due to the Strength increased as the sintering temperature increased.The average values of the relative density, hardness and bending Fig. 4 shows SEM fracture surfaces of the 3rd pass fibrous HAp strength of HAp composites sintered at 1400"C were about composites sintered at(a)1200C and(b)1400C. As mentioned in 93%, 792 Hv and 169 MPa, respectively. Fig. 1, the Al2O3-(m-ZrO2)shell region appeared to possess a denser microstructure as the sintering temperatures increased. However, in the Acknowledgement HAp-(t-ZrO2) core region, sintered at 1400 C, many pores were TCP),as shown in Fig. 4(b). Although pull-out behavior was not the Korean Ministry of Science and Technolog. program of clearly observed because of the formation of B-tricalcium phosphate This work was supported by the NRL research found on the fracture surface, a rough surface was observed in the sample sintered at 1400C due to the existence of porosity [16]. This References bservation indicates that the Al2O3(m-ZrO2)may be responsible for the increased crack deflection and the subsequent increase in energy [w. Suchanek, M. Yoshimura, J Mater Res. 13(1998)94-117 required for crack propagatio [2]YK Jun, W.H. Kim, O.K. Kweon, S.H. Hong, Biomaterials 24(2003) Fig. 5 is a TEM micrograph showing the crack propagation, which 3731-3739 was taken from the HAp-(t-zrO2) core region. The crack propagated [3] H.K. Varma, R. Sivakumar, Mater. Lett. 29(1996)57-61 into the HAp matrix with visible crack deflection due to the dispersion [4]C. Benaqqa, J. Chevalier, M. Saadaoui, G. Fantozzi, Biomaterials 26 of the t-zrO2 phase. Many microcracks were also observed (indicated by inset arrows)due to the strong strain field, also observed in Fig 3(a). [5] M.A. Lopes, F.J. Monteiro, J.D. Santos, Biomaterials 20(1999) However, in the monolithic HAp sintered body, the crack was straight, 2085-2090 without any deflection [17 [6]W Suchanek, M. Yashima, M. Kakihana, M. Yashima, Biomaterials 18 Table I shows the relative density, Vickers hardness, and bending 1997)923-933 strength of the 3rd pass fibrous HAp composite depending on the [7K Yoshida, K. Hashimoto, Y. Toda, S Udagawa, T. Kanazawa, J.Eur. Ceram.Soc.26(2005)515-518 sintering temperature. The values of shrinkage, relative density, 8]A,R. Kmita, A Slosarczyk, Z. Paszkiewicz. J. Eur. Ceram Soc. (in press). Vickers hardness and bending strength increased with increasing [9] M.C. Heisel, M. M. Silva, M.T. P. Schmalzried, J Bone Jt. Surg. 85(2003) sintering temperature. In the sample sintered at 1200C, the composite 1366-1379. showed low mechanical properties due to its reduced density. The [10]SG Huang, J. Vleugels, L Li, O.V. Biest, P L. Wang, J. Eur. Ceram Soc. values of relative density, shrinkage, hardness and bending strength of 25(2005)31093115 HAp composites sintered at 1200C were about 83%, 18.45%, 134 Hv [II] M Uo, G. SJoren, A Sundh, F. Watan, M. Bergman, U Lemer, Dent. Mater.19(2003)487-492 the values of relative density, hardness and bending strength of fibrous [12 P. ChristelL, A. Meunier, M. Heller, J.P. Torre,B.Cales, C.N. Peille HAp composites increased remarkably. The average relative density, [13 B.T. Lee, K.H. Kim, J.K. Han, J Mater Res. 19(2004)3234-3241 shrinkage, hardness and bending strength of HAp composites sintered [14 B.T. Lee, D.H. Jang, L.C.Kang. C W.Lee, J.Am. Ceram Soc.88(2005) at 1400 C were about 95%, 35.65%, 967 Hv and 278 MPa. 2874-2878 respectively. [15A.K Gain, B.T. Lee, Mater Sci Eng, A 419(2006)269-275. [16]KS. Blanks, A. Kristoffersson, E. Carlstrom, W.J. Clegg, J. Eur. Ceram. 4. Conclusion Soc,18(1998)1945-1951. [] B.T. Lee, N.Y. Shin, J.K. Han, H.Y. Song, Mater. Sci. Eng, A(2005) (submitted) Fibrous HAp-(t-ZrO2)Al2O3-(m-ZrO2) composites were uccessfully fabricated using the multi-pass extrusion process
other hand, in the Al2O3-(m-ZrO2) shell region (b), the bright and dark contrast corresponds to Al2O3 and m-ZrO2 phases, respectively. Round m-ZrO2 grains less than 400 nm were clearly observed, but in Fig. 3(a) the HAp-(t-ZrO2) core region contains a heavy strain field contrast (as indicated by the inset arrows) which may have formed due to the mismatch of their thermal expansion coefficients. Fig. 4 shows SEM fracture surfaces of the 3rd pass fibrous HAp composites sintered at (a) 1200 °C and (b) 1400 °C. As mentioned in Fig. 1, the Al2O3-(m-ZrO2) shell region appeared to possess a denser microstructure as the sintering temperatures increased. However, in the HAp-(t-ZrO2) core region, sintered at 1400 °C, many pores were clearly observed because of the formation of β-tricalcium phosphate (β-TCP), as shown in Fig. 4(b). Although pull-out behavior was not found on the fracture surface, a rough surface was observed in the sample sintered at 1400 °C due to the existence of porosity [16]. This observation indicates that the Al2O3-(m-ZrO2) may be responsible for the increased crack deflection and the subsequent increase in energy required for crack propagation. Fig. 5 is a TEM micrograph showing the crack propagation, which was taken from the HAp-(t-ZrO2) core region. The crack propagated into the HAp matrix with visible crack deflection due to the dispersion of the t-ZrO2 phase. Many microcracks were also observed (indicated by inset arrows) due to the strong strain field, also observed in Fig. 3(a). However, in the monolithic HAp sintered body, the crack was straight, without any deflection [17]. Table 1 shows the relative density, Vickers hardness, and bending strength of the 3rd pass fibrous HAp composite depending on the sintering temperature. The values of shrinkage, relative density, Vickers hardness and bending strength increased with increasing sintering temperature. In the sample sintered at 1200 °C, the composite showed low mechanical properties due to its reduced density. The values of relative density, shrinkage, hardness and bending strength of HAp composites sintered at 1200 °C were about 83%, 18.45%, 134 Hv and 75 MPa, respectively. However, in the sample sintered at 1400 °C, the values of relative density, hardness and bending strength of fibrous HAp composites increased remarkably. The average relative density, shrinkage, hardness and bending strength of HAp composites sintered at 1400 °C were about 95%, 35.65%, 967 Hv and 278 MPa, respectively. 4. Conclusion Fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites were successfully fabricated using the multi-pass extrusion process. In the sample sintered at 1200 °C, the HAp-(t-ZrO2) core and Al2O3-(m-ZrO2) shell regions had a porous structure. However, in the sample sintered at 1400 °C, the Al2O3-(m-ZrO2) shell region possessed a dense structure, although the HAp-(t-ZrO2) core region showed a porous structure due to the formation of β-TCP phase. The fine ZrO2 particles (∼400 nm in diameter) were homogeneously dispersed in the HAp and Al2O3 matrices. In the HAp matrix, large amounts of strain field contrast was observed by TEM analysis, which is believed to have enhanced crack deflection and microcracking mechanisms. This was due to the mismatch of their thermal expansion coefficients. The values of relative density, Vickers hardness and bending strength increased as the sintering temperature increased. The average values of the relative density, hardness and bending strength of HAp composites sintered at 1400 °C were about 93%, 792 Hv and 169 MPa, respectively. Acknowledgement This work was supported by the NRL research program of the Korean Ministry of Science and Technology. References [1] W. Suchanek, M. Yoshimura, J. Mater. Res. 13 (1998) 94–117. [2] Y.K. Jun, W.H. Kim, O.K. Kweon, S.H. Hong, Biomaterials 24 (2003) 3731–3739. [3] H.K. Varma, R. Sivakumar, Mater. Lett. 29 (1996) 57–61. [4] C. Benaqqa, J. Chevalier, M. Saadaoui, G. Fantozzi, Biomaterials 26 (2005) 6106–6112. [5] M.A. Lopes, F.J. Monteiro, J.D. Santos, Biomaterials 20 (1999) 2085–2090. [6] W. Suchanek, M. Yashima, M. Kakihana, M. Yashima, Biomaterials 18 (1997) 923–933. [7] K. Yoshida, K. Hashimoto, Y. Toda, S. Udagawa, T. Kanazawa, J. Eur. Ceram. Soc. 26 (2005) 515–518. [8] A, R. Kmita, A Slosarczyk, Z. Paszkiewicz. J. Eur. Ceram. Soc. (in press). [9] M.C. Heisel, M.M. Silva, M.T.P. Schmalzried, J. Bone Jt. Surg. 85 (2003) 1366–1379. [10] S.G. Huang, J. Vleugels, L. Li, O.V. Biest, P.L. Wang, J. Eur. Ceram. Soc. 25 (2005) 3109–3115. [11] M. Uo, G. Sjoren, A. Sundh, F. Watari, M. Bergman, U. Lerner, Dent. Mater. 19 (2003) 487–492. [12] P. Christel, A. Meunier, M. Heller, J.P. Torre, B. Cales, C.N. Peille, J. Biomed. Mater. Res. 23 (1989) 45–61. [13] B.T. Lee, K.H. Kim, J.K. Han, J. Mater. Res. 19 (2004) 3234–3241. [14] B.T. Lee, D.H. Jang, I.C. Kang, C.W. Lee, J. Am. Ceram. Soc. 88 (2005) 2874–2878. [15] A.K. Gain, B.T. Lee, Mater. Sci. Eng., A 419 (2006) 269–275. [16] K.S. Blanks, A. Kristoffersson, E. Carlstrom, W.J. Clegg, J. Eur. Ceram. Soc. 18 (1998) 1945–1951. [17] B.T. Lee, N.Y. Shin, J.K. Han, H.Y. Song, Mater. Sci. Eng., A (2005) (submitted). Table 1 Material properties of fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites depending on the sintering temperature Samples Properties Shrinkage (%) Relative density (%) Vickers hardness (Hv) Bending strength (MPa) 1200 °C 18.45 83 133 ± 15 72 ± 8 1400 °C 35.65 93 792 ± 24 169 ± 12 408 C. Lee et al. / Materials Letters 61 (2007) 405–408
