B.-T. Lee et al. Materials Science and Engineering A 458(2007)11-16 0 Fig. 1. SEM micrographs of(a),(b) third passed cross-sectional HAp-(I-ZrO2)Al2O3-(m-ZrO2)bodies sintered at 1200C and(c). (d), enlarged images of core and Fig 5 shows the relative density and vickers hardness of the Fig. 6 shows the bending strength and fracture toughness third passed fibrous HAp-(t-ZrO2)Al2O3-(m-ZrO2)composites of third passed HAp composites depending on the sintering depending on the the sintering temperature. In the sample sintered temperature. In the sample sintered at 1200oC, their value at 1200C, the values of the relative density and hardness werewere low about 75 MPa and 1.8 MPam, respectively comparatively low about 83% and 170 MPa, respectively, due to However, as the sintering temperature increased, the values the low densification as mentioned in Fig. 1. However, as increas- of bending strength and fracture toughness increased due to ing the sintering temperature, the values of relative density and the enhancing of densification. Thus, at 1500C, their values hardness increased due to the enhancing of the densification. were about 280 MPa and 4. 1 MPam", respectively. In general, Thus, the maximum relative density and hardness values were using the conventional processes, the fracture toughness of the obtained at 1500C, and the values were 96% and 890 Hv, HAp-(50 vol %o ZrO2) composites did not exceed the value of respectively. The hardness value also remarkably increased as 2.5 MPa m[16]. However, in this work, using the multi-pass the sintering temperature increased due to the high densification. extrusion process, the fracture toughness of the third passed Shell Fig. 2. SEM ohs of (a)third passed cross-sectional HAp-(t-ZrO2MAlO3-(m-ZrO2) bodies sintered at 1500C and(b), enlarged images of core and shellB.-T. Lee et al. / Materials Science and Engineering A 458 (2007) 11–16 13 Fig. 1. SEM micrographs of (a), (b) third passed cross-sectional HAp-(t-ZrO2)/Al2O3-(m-ZrO2) bodies sintered at 1200 ◦C and (c), (d), enlarged images of core and shell regions. Fig. 5 shows the relative density and Vickers hardness of the third passed fibrous HAp-(t-ZrO2)/Al2O3-(m-ZrO2) composites depending on the sintering temperature. In the sample sintered at 1200 ◦C, the values of the relative density and hardness were comparatively low about 83% and 170 MPa, respectively, due to the low densification as mentioned in Fig. 1. However, as increas￾ing the sintering temperature, the values of relative density and hardness increased due to the enhancing of the densification. Thus, the maximum relative density and hardness values were obtained at 1500 ◦C, and the values were 96% and 890 Hv, respectively. The hardness value also remarkably increased as the sintering temperature increased due to the high densification. Fig. 6 shows the bending strength and fracture toughness of third passed HAp composites depending on the sintering temperature. In the sample sintered at 1200 ◦C, their values were low about 75 MPa and 1.8 MPa m1/2, respectively. However, as the sintering temperature increased, the values of bending strength and fracture toughness increased due to the enhancing of densification. Thus, at 1500 ◦C, their values were about 280 MPa and 4.1 MPa m1/2, respectively. In general, using the conventional processes, the fracture toughness of the HAp-(50 vol.% ZrO2) composites did not exceed the value of 2.5 MPa m1/2 [16]. However, in this work, using the multi-pass extrusion process, the fracture toughness of the third passed Fig. 2. SEM micrographs of (a) third passed cross-sectional HAp-(t-ZrO2)/Al2O3-(m-ZrO2) bodies sintered at 1500 ◦C and (b), enlarged images of core and shell regions