B. T. Lee, S. K Sarkar/ Scripta Materialia 61(2009)686-689 Table 2. Material properties of the composite Sintering Relative temperature(°C) strength(MP 1450 63.77±0.0 479±24 1500 64.27±0.16 588±32 Table 2 shows the material properties of the po ody sintered at different temperatures. The bending strength of the porous ceramics was remarkably im- proved in the porous body and sintered at 1500C,it was 588 MPa. The bending strength value was also remarkably higher compared to that of monolithic por- Figure 3.(a)Longitudinal section SEM image of second-pass porou ous ceramics of Al]O3(90 MPa) and porous t-ZrO body. (b) SEM image of the pore surface. (c) SEM image of the fibro (270 MPa)made by a similar extrusion process and hav pore region.(d) Enlarged SEM image of frame region. ing the same value of porosity [13, 14]. In conclusion, fibrous frame porous Al,O3(t-ZrO2)/ t-ZrO2 composites were fabricated by a multipass extru- with t-zrO2 phase, as evident from the white contrast of sion process. The composite frame of the porous body the image. The thin layer of t-ZrO2 on the pore surface has a very fine and homogeneous distribution of was strongly attached to the matrix phase. This extreme AlO3 t-ZrO2) fiber in a t-ZrO2 matrix. The Al2O t roughness along with preferential pore size is favorable ZrO2)fibers were around 3.5 um and the t-zrO2 matrix for a range of applications. For biomaterials applica- had a dimension of 1-2 um between the Al,Ox(t-ZrO2) tions, the pores allow proliferation of bone cells and fibers. The pores were around 175 um in diameter. The the rough pore surface allows a microlevel mechanical pore was in circular channel form and the surface was terlocking that can enhance attachment of the bone rough. The relative density was around 64%, whereas cells. It should be much easier to integrate a functional the actual density was less then 3.5 g cm(see Table coating or incorporate another microsystem inside these 2). The bending strength of the porous bodies was im- ores. Figure 3c is a longitudinal section SEM image of proved significantly and the highest value was observed the frame region. Unidirectionally aligned continuous fi- to be 588.62 MPa when sintered at 1500C. The porous bers of Al, O,(t-ZrO2) can be clearly observed. The dis- body did not show any sign of bulk defects continuities of a few fibers seen in the image are simply overlap of the matrix phase, and immediately beneath []B.T. Lee, A Nishiyama, K. Hiraga, Mater. Trans. JIM 34 them continuous fibers can be found. Figure 3d is an en larged image from Figure 3c 2A. H D. Aza, J. Chevalier, G. Fantozzi, M. Schehl, R The interface between the Al,O3t-ZrO2)cores and t- Torrecillas, Biomaterials 23(3)(2002)937 ZrO, matrix in the pore frame was distinct except in some 3]H. Chen, J. Gu, J. Shi. Adv. Mater. 17(2005)2010 instances where the t-ZrO2 phase of the matrix and that of 4V. Biasini, M. Parasporo, A. Bellosi, Thin Solid Film 297 997)207 the Al,Oxt-ZrOo,) core were continuous due to signifi 5]K. Maca, P. Dobsak, A.R. Boccaccini, Ceram. Int. 27 cant grain growth during sintering. The ZrO2 phase was (2001)577 homogeneously dispersed in the Al_O3 phase. This not 6 M.H. Bocanegra-Bernal, D D.L. Torre, J. Mater. Sci. 37 only inhibited the grain growth of both phases but also 2002)4947 introduced microcracking in the grain boundary of [7J. Chevalier, Biomaterials 27(2006)535 AlO3 and t-ZrO2 phase, which can lead to improved frac [8]V. Gonzalez- Pena, C. Marquez-Alvarez, I. Diaz, M ture toughness [9]. This is because of the differences in the Grande, T. Blasco, J. Perez-Pariente, Micropor Mesop coefficient of thermal expansion and elasticity of these two Mater.80(2005)17 materials. During the furnace-cooling period after sinter- 9A.K. Gain, H.Y. Song. B.T. Lee, Scr. Mater. 54(12) ing, this mismatch creates a strain field near the grain [10]H. Sieber, Mater. Sci. Eng. A 412(1-2)(2005)43 boundary and microcracks appear. The mismatch also [l1]CR. Rambo, H. Sieber, Adv. Mater. 17(8)(2005)1088 imposes residual stress on the core and matrix phases, [12]H. Masuda, K. Fukuda, Science 268(1995)1466. the core being in compression and the matrix in tensio [3]BT. Lee, I.C. Kang, S.H. Cho, H.Y. Song, J. Am. Ceram.Soc.88(8)(2005)2262 crack tip zones and reducing the crack propagation en- [14]AK. Gain, B.T. Lee, Mater. Sci Eng. A 419(1-2)(2006) ergy [16]. It was reported that the difference in the degree of residual stress increases the crack deflection and hence [5]BT. Lee, K H. Kim, H.C. Youn, H.Y. Song, J. Am. the fracture toughness [17]. Miyazaki et al. proposed that Ceram.Soc.90(2)(2007)62 [16 B T Lee, K H Lee, K. Hiraga, Scr. Mater. 38(1998)1101 the efect of a crack deflection on the toughness is almost [17]T. Adachi, T. Sekino, T. Kusunose, T. Nakayama, A proportional to the variation in residual stress across the Hikasa, Y.H. Choa, K. Niihara, J. Ceram Soc. Jpn. Ill interface[18]. From these two findings it can be inferred (2003)4 that the fracture toughness can be effectively increased [18]H Miyazaki, Y. Yoshizawa, K. Hirao, J. Eur. Ceram in this system Soc.26(16)(2006)3539with t-ZrO2 phase, as evident from the white contrast of the image. The thin layer of t-ZrO2 on the pore surface was strongly attached to the matrix phase. This extreme roughness along with preferential pore size is favorable for a range of applications. For biomaterials applica￾tions, the pores allow proliferation of bone cells and the rough pore surface allows a microlevel mechanical interlocking that can enhance attachment of the bone cells. It should be much easier to integrate a functional coating or incorporate another microsystem inside these pores. Figure 3c is a longitudinal section SEM image of the frame region. Unidirectionally aligned continuous fi- bers of Al2O3–(t-ZrO2) can be clearly observed. The dis￾continuities of a few fibers seen in the image are simply overlap of the matrix phase, and immediately beneath them continuous fibers can be found. Figure 3d is an en￾larged image from Figure 3c. The interface between the Al2O3–(t-ZrO2) cores and t￾ZrO2 matrix in the pore frame was distinct except in some instances where the t-ZrO2 phase of the matrix and that of the Al2O3–(t-ZrO2) core were continuous due to signifi- cant grain growth during sintering. The ZrO2 phase was homogeneously dispersed in the Al2O3 phase. This not only inhibited the grain growth of both phases but also introduced microcracking in the grain boundary of Al2O3 and t-ZrO2 phase, which can lead to improved frac￾ture toughness [9]. This is because of the differences in the coefficient of thermal expansion and elasticity of these two materials. During the furnace-cooling period after sinter￾ing, this mismatch creates a strain field near the grain boundary and microcracks appear. The mismatch also imposes residual stress on the core and matrix phases, the core being in compression and the matrix in tension. The microcracks play an important role in deflecting the crack tip zones and reducing the crack propagation en￾ergy [16]. It was reported that the difference in the degree of residual stress increases the crack deflection and hence the fracture toughness [17]. Miyazaki et al. proposed that the effect of a crack deflection on the toughness is almost proportional to the variation in residual stress across the interface [18]. From these two findings it can be inferred that the fracture toughness can be effectively increased in this system. Table 2 shows the material properties of the porous body sintered at different temperatures. The bending strength of the porous ceramics was remarkably im￾proved in the porous body and sintered at 1500 C, it was 588 MPa. The bending strength value was also remarkably higher compared to that of monolithic por￾ous ceramics of Al2O3 (90 MPa) and porous t-ZrO2 (270 MPa) made by a similar extrusion process and hav￾ing the same value of porosity [13,14]. In conclusion, fibrous frame porous Al2O3–(t-ZrO2)/ t-ZrO2 composites were fabricated by a multipass extru￾sion process. The composite frame of the porous body has a very fine and homogeneous distribution of Al2O3–(t-ZrO2) fiber in a t-ZrO2 matrix. The Al2O3–(t￾ZrO2) fibers were around 3.5 lm and the t-ZrO2 matrix had a dimension of 1–2 lm between the Al2O3–(t-ZrO2) fibers. The pores were around 175 lm in diameter. The pore was in circular channel form and the surface was rough. The relative density was around 64%, whereas the actual density was less then 3.5 g cm1 (see Table 2). The bending strength of the porous bodies was im￾proved significantly and the highest value was observed to be 588.62 MPa when sintered at 1500 C. The porous body did not show any sign of bulk defects. [1] B.T. Lee, A. Nishiyama, K. Hiraga, Mater. Trans. JIM 34 (88) (1993) 682. [2] A.H.D. Aza, J. Chevalier, G. Fantozzi, M. Schehl, R. Torrecillas, Biomaterials 23 (3) (2002) 937. [3] H. Chen, J. Gu, J. Shi. Adv. Mater. 17 (2005) 2010. [4] V. Biasini, M. Parasporo, A. Bellosi, Thin Solid Film 297 (1997) 207. [5] K. Maca, P. Dobsak, A.R. Boccaccini, Ceram. Int. 27 (2001) 577. [6] M.H. Bocanegra-Bernal, D.D.L. Torre, J. Mater. Sci. 37 (2002) 4947. [7] J. Chevalier, Biomaterials 27 (2006) 535. [8] V. Gonza´lez-Pen˜a, C. Ma´rquez-Alvarez, I. Dı´az, M. Grande, T. Blasco, J. Pe´rez-Pariente, Micropor. Mesopor. Mater. 80 (2005) 173. [9] A.K. Gain, H.Y. Song, B.T. Lee, Scr. Mater. 54 (12) (2006) 2081. [10] H. Sieber, Mater. Sci. Eng. A 412 (1–2) (2005) 43. [11] C.R. Rambo, H. Sieber, Adv. Mater. 17 (8) (2005) 1088. [12] H. Masuda, K. Fukuda, Science 268 (1995) 1466. [13] B.T. Lee, I.C. Kang, S.H. Cho, H.Y. Song, J. Am. Ceram. Soc. 88 (8) (2005) 2262. [14] A.K. Gain, B.T. Lee, Mater. Sci. Eng. A 419 (1–2) (2006) 269. [15] B.T. Lee, K.H. Kim, H.C. Youn, H.Y. Song, J. Am. Ceram. Soc. 90 (2) (2007) 629. [16] B.T. Lee, K.H. Lee, K. Hiraga, Scr. Mater. 38 (1998) 1101. [17] T. Adachi, T. Sekino, T. Kusunose, T. Nakayama, A. Hikasa, Y.H. Choa, K. Niihara, J. Ceram. Soc. Jpn. 111 (2003) 4. [18] H. Miyazaki, Y. Yoshizawa, K. Hirao, J. Eur. Ceram. Soc. 26 (16) (2006) 3539. Table 2. Material properties of the composite. Sintering temperature (C) Relative density (%) Bending strength (MPa) 1450 63.77 ± 0.05 479 ± 24 1500 64.27 ± 0.16 588 ± 32 Figure 3. (a) Longitudinal section SEM image of second-pass porous body. (b) SEM image of the pore surface. (c) SEM image of the fibrous pore region. (d) Enlarged SEM image of frame region. B. T. Lee, S. K. Sarkar / Scripta Materialia 61 (2009) 686–689 689