B. T. Lee, S. K Sarkar / Scripta Materialia 61(2009)686-689 larger particle size(<15 um), which means there was a difference in flow behavior of the pore-forming core and ceramic bound shell phase of the hot thermoplastic feed role. The irregular interface of the Al,O,m-ZrO2)core and t-ZrO2 shell and that of the rough pore surface are 100m mainly due to these reasons. From Figure 2d, it is seen that the t-zrO grain size was around 400 nm, whereas that of Al,O3 was in the sub-micrometer range. The inclu sion of t-ZrO, phase in the core inhibits substantial grain growth of Al,O3. Figure 2e is a bright-field TEM image of the Al,Ox(m-ZrO2)core, where the white and black con- trasts are Al2O3 and Zro2 particles, respectively. They were homogeneously dispersed and densely packed. Even though Al2O3-m-ZrO2) was initially used in the core twin structures or strain field were not observed. In the Al2O3-(m-ZrO2)system, we previously observed a strong strain field near the m-Zro2 particle due to the tetrago nal-monoclinic phase transformation during the cooling time of the sintering process [16]. This also introduced Figure 2.(a) Cross-sectional SEM image of first-pass porous body. (b) Cross-sectional SEM image of the second-pass porous body. (c) microcracks near the Al O3 and m-ZrO interface However, in this system there were no microcracks or Enlarged image of the pore region. (d) Enlarged image of frame region. twin defects near the Al,O3/zrO2 interface. Figure 2f (e and f) TEM images of the frame region. shows the high-resolution(HR)TEM and electron dif- fraction pattern of the [11 1]zone axis, which was taken t-zro2 matrix. Along the radial direction, there were from Figure 2e. From the electron difraction pattern it around 18 unit cells with Al2O3t-ZrO2) fiber, as de- was confirmed that even though m-ZrO2 was used gned in the green stage during fabrication. Figure 2b initially, t-ZrO2 was obtained after sintering. A few edge shows a cross-sectional SEM image of a sintered sec- dislocations can be observed in the hrtEM image. The ond-pass porous body. The pores are almost circular arrow indicates the low-angle tilt boundary, and a in shape and equidistant. The geometry and distribution periodical interfacial dislocation can be observed in the of the pores are two important factors for the ultimate 1 Table I presents the characteristic dimensions of the mechanical performance of the porous body With irreg- ular shape and a non-uniform distribution, the mechan- sintered porous bodies. In the second-pass porous body ical strength degrades more rapidly with the extent of the pore diameter was around 175 um. The frame thick porosity due to the tortuous and skewed pore frame ness of the porous body was around 110 um. Between where stress concentration zones, flaws or crack initia- two pores there were roughly 36 Al2O3t-ZrO2) fibers tion zones readily arise. In the current processing meth- distributed in the t-ZrO2 matrix. In the second-pass por od, the geometries of the pores and frames were ous body the microstructure in the frame region was significantly improved. The uniform distribution and highly refined, as seen from the AlO3(t-ZrO2)core shape of the pore, and thus uniformity of the frame and t-ZrO2 shell. The microstructure was arbitrarily de- thickness without much irregular curvature (except for signed in the present form. It can be changed very easily the microlevel surface roughness), resulted in improved by changing only the feed role design Changes in com- mechanical strength of the composites. Figure 2c shows position in the Al2O3-t-ZrO2) core and t-ZrO2 shell a closer view of the pores. It can be seen that the pore is phase can be easily achieved by changing their composi- surrounded by a fine fibrous microstructure, as in the tion in the green stage. Fiber dimension and density per first-pass but with significantly reduced dimension. Fig- unit area can also be controlled precisely by controlling ure 2d shows an enlarged image of the frame region the dimension and number of dense Al2O3t-ZrO2)/t The Al,O,t-ZrO2) cores, appearing with dark ZrO, core/shell filaments in the feed role for the first contrast. had a diameter of around 3.5 whereas pass porous body he t-ZrO2 matrix thickness was in the range of 1-2 um ure 3 shows al section SEM images of Figure 3a is an enlarged view in the t-zro2 matrix. No bulk defects, such as shrinkage of the pore region Figure 3b is a SEM image of the pore cavities or large cracks or any other flaws, were observed surface. The pore surface was highly rough with a bimo- in the composites. The Al,m-ZrO2)cores were some- dal distribution of roughness. It was entirely covered what irregular in shape instead of the original circular shape due to the repeated extrusion of the green compos- ites(the core/shell experienced four iterations of the Table 1. Characteristic dimensions of the sintered composite extrusion process). The slight difference in the rheology of the green polymer-bound Al2O3(m-ZrO2)and First pass Second pass(um) t-ZrO2, due to different mixing conditions and particle Pore diameter 144 size of the ceramics contributed to minute differential Frame thickness flow behavior while undergoing a reduction ratio of Al,O3 fiber diameter 71: 1. The polymer-bound carbon has a comparatively t-ZrO, shell thicknesst-ZrO2 matrix. Along the radial direction, there were around 18 unit cells with Al2O3–(t-ZrO2) fiber, as de￾signed in the green stage during fabrication. Figure 2b shows a cross-sectional SEM image of a sintered sec￾ond-pass porous body. The pores are almost circular in shape and equidistant. The geometry and distribution of the pores are two important factors for the ultimate mechanical performance of the porous body. With irreg￾ular shape and a non-uniform distribution, the mechan￾ical strength degrades more rapidly with the extent of porosity due to the tortuous and skewed pore frame where stress concentration zones, flaws or crack initia￾tion zones readily arise. In the current processing meth￾od, the geometries of the pores and frames were significantly improved. The uniform distribution and shape of the pore, and thus uniformity of the frame thickness without much irregular curvature (except for the microlevel surface roughness), resulted in improved mechanical strength of the composites. Figure 2c shows a closer view of the pores. It can be seen that the pore is surrounded by a fine fibrous microstructure, as in the first-pass but with significantly reduced dimension. Fig￾ure 2d shows an enlarged image of the frame region. The Al2O3–(t-ZrO2) cores, appearing with dark contrast, had a diameter of around 3.5 lm, whereas the t-ZrO2 matrix thickness was in the range of 1–2 lm. The Al2O3–(m-ZrO2) cores were uniformly distributed in the t-ZrO2 matrix. No bulk defects, such as shrinkage cavities or large cracks or any other flaws, were observed in the composites. The Al2O3–(m-ZrO2) cores were some￾what irregular in shape instead of the original circular shape due to the repeated extrusion of the green compos￾ites (the core/shell experienced four iterations of the extrusion process). The slight difference in the rheology of the green polymer-bound Al2O3–(m-ZrO2) and t-ZrO2, due to different mixing conditions and particle size of the ceramics, contributed to minute differential flow behavior while undergoing a reduction ratio of 71:1. The polymer-bound carbon has a comparatively larger particle size (615 lm), which means there was a difference in flow behavior of the pore-forming core and ceramic bound shell phase of the hot thermoplastic feed role. The irregular interface of the Al2O3–(m-ZrO2) core and t-ZrO2 shell and that of the rough pore surface are mainly due to these reasons. From Figure 2d, it is seen that the t-ZrO2 grain size was around 400 nm, whereas that of Al2O3 was in the sub-micrometer range. The inclu￾sion of t-ZrO2 phase in the core inhibits substantial grain growth of Al2O3. Figure 2e is a bright-field TEM image of the Al2O3–(m-ZrO2) core, where the white and black con￾trasts are Al2O3 and ZrO2 particles, respectively. They were homogeneously dispersed and densely packed. Even though Al2O3–(m-ZrO2) was initially used in the core, twin structures or strain field were not observed. In the Al2O3–(m-ZrO2) system, we previously observed a strong strain field near the m-ZrO2 particle due to the tetrago￾nal–monoclinic phase transformation during the cooling time of the sintering process [16]. This also introduced microcracks near the Al2O3 and m-ZrO2 interface. However, in this system there were no microcracks or twin defects near the Al2O3/ZrO2 interface. Figure 2f shows the high-resolution (HR) TEM and electron dif￾fraction pattern of the [1 1 1] zone axis, which was taken from Figure 2e. From the electron diffraction pattern it was confirmed that even though m-ZrO2 was used initially, t-ZrO2 was obtained after sintering. A few edge dislocations can be observed in the HRTEM image. The arrow indicates the low-angle tilt boundary, and a periodical interfacial dislocation can be observed in the image. Table 1 presents the characteristic dimensions of the sintered porous bodies. In the second-pass porous body, the pore diameter was around 175 lm. The frame thick￾ness of the porous body was around 110 lm. Between two pores there were roughly 36 Al2O3–(t-ZrO2) fibers distributed in the t-ZrO2 matrix. In the second-pass por￾ous body the microstructure in the frame region was highly refined, as seen from the Al2O3–(t-ZrO2) core and t-ZrO2 shell. The microstructure was arbitrarily de￾signed in the present form. It can be changed very easily by changing only the feed role design. Changes in com￾position in the Al2O3–(t-ZrO2) core and t-ZrO2 shell phase can be easily achieved by changing their composi￾tion in the green stage. Fiber dimension and density per unit area can also be controlled precisely by controlling the dimension and number of dense Al2O3–(t-ZrO2)/t￾ZrO2 core/shell filaments in the feed role for the first￾pass porous body. Figure 3 shows longitudinal section SEM images of the sintered porous body. Figure 3a is an enlarged view of the pore region. Figure 3b is a SEM image of the pore surface. The pore surface was highly rough with a bimo￾dal distribution of roughness. It was entirely covered Table 1. Characteristic dimensions of the sintered composites. Pass no. First pass Second pass (lm) Pore diameter 1.44 mm 175 Frame thickness 600 lm 110 Al2O3 fiber diameter 26 lm 3.5 t-ZrO2 shell thickness 13 lm 1.7 Figure 2. (a) Cross-sectional SEM image of first-pass porous body. (b) Cross-sectional SEM image of the second-pass porous body. (c) Enlarged image of the pore region. (d) Enlarged image of frame region. (e and f) TEM images of the frame region. 688 B. T. Lee, S. K. Sarkar / Scripta Materialia 61 (2009) 686–689