B -T Lee et al. /Journal of the European Ceramic Society 28(2008)229-233 SUum 500nm 500nm Fig. 2. Longitudinal SEM micrographs of (a and b)4th passed double-network type fibrous Al2O3-m-zrO2Mt-ZrO2 composite and enlarged SEM image from(c) AlO3-(m-ZrO2) core and(d)t-ZrO outer network regions composites sintered at 1500C. Fig. 1(a) was taken from the ZrO2)cores and t-ZrO2 shells during the sintering process 3rd passed filament and shows the unit microstructure of the Using one pass further, a modified nanostructured composite tailored composites. The dark contrasts in the image which can be made by this top down fabrication process. However, were the two-phase(Al2O3-m-ZrO2)cores were encircled by using the present technology it can be easily conceived that t-ZrO2 cylinder, observed as white contrast in the SEM image. N extrusion passes might be expected to lead to an(N-2) The t-zrO2 enclosures of the cores were adjoined in an inter- network type microstructure, provided that the particle size of connected formation and made a network-like microstructural the constituent materials are significantly lower compared to arrangement. This entire arrangement is confined within the the lowest dimension achievable for the inner network thick boundary of another t-ZrO2 enclosure, evident in the SEM ness image by the thicker white contrast. This outer t-ZrO2 phase Fig. 2 shows the longitudinal SEM micrographs of (a) 4th is adjoined with the bordering of the same kind and made a passed double-network type composites sintered at 1500C. network type formation. This outer network of t-ZrO2 is more A homogeneous, fibrous microstructure with a highly unidi learly evident in the 4th passed SEM image in Fig. 1(b). The rectional orientation was observed in the low magnification microstructure has just been scaled down in dimension in the 4th image(a). From this image the hierarchical sub-micrometer the same. Ultimately it was observed that a t-ZrO2 network was Al2O3-(m-ZrO2)core was also observed in unidirectional align- enclosing a two-phase system of Al2O3-(m-ZrO2)which itself ment where the thickness was about 2.5 um. The inserted image was enclosed within a networked t-ZrO2 phase. This type of a in Fig. 2(a) clearly depicted this point. In the thermal etched network-like microstructure inside another network led to think enlarged image(b), the t-ZrO2 shells and Al2O3-(m-Zro2)cores this microstructure as a double-network type microstructure comprised a dense microstructure without any deleterious phe and this group of words was used through the manuscript to nomena like cracking, delimitation, etc. This result indicated that describe this hierarchically oriented microstructure In Fig. 1(c), the submicron-sized continuous fibrous microstructures were the enlarged SEM image of the 4th passed filament showed well controlled using the multi-pass extrusion process. Enlarged clearly the double network of the t-ZrO2 phase SEM images(c and d)were taken from the Al,O3-(m-zrO2) The thicknesses of the outer and inner t-ZrO2 network in core and t-ZrO2 shell regions, respectively. In the enlargedimage the 3rd passed composite were about 60 and 3 um, respectively, of Al2O3-(m-ZrO2)core(c), the bright and dark contrasts were and the Al2O3-(m-ZrO2) core was about 25 um in diameter. m-ZrO2 and Al2O3 phases, respectively. The average AlO3 In the 4th passed composite the Al2O3-(m-zrO2)core was grain size was about 0.7 um in diameter while the m-ZrO2 grain about 2.5 um in diameter and the outer and inner t-zro2 shell was about 0.3 um in diameter. However, most of the fine m- thicknesses were about 15 and 0.8 um, respectively. The outer ZrO2 phases were located in the Al2O3 grain boundary. The network appeared almost hexagonal in shape. This shape is presence of m-zrO2 phase in the core region decreased the grain attributed to the arrangement of the lst passed filaments in the growth of Al2O3, which undergoes excessive grain coarsening die which follow a near hexagonal assembly. The inner micro- during high temperature sintering. The average grain size of the groups also exhibit the same geometric feature. The hexagonal t-ZrO2 outer network was about 0. 4 um in diameter as shown shape is also retained in the 4th passed composites as shown in in Fig. 2(d). Although the particle size of m-ZrO2 and t-ZrO2 Fig. 1(c). Moreover, the hexagonal cells containing the typical was same initially, after sintering the grain size was different louble-network microstructures were seen as indicated by the due to the pinning effect. In the Al2O3-(m-zrO2) core the m- dotted lines in Fig. 1(b). These were the individual 3rd passed ZrO2 was surrounded by Al2O3 grains and grain growth was filaments. However, as the number of extrusion passes increased, hindered the microstructure became finer and ultimately the inner network Fig. 3 shows the relative density and bending strength of 4th thickness became sub-micrometer as shown in Fig. I(c). This is a passed double-network type fibrous(Al2O3-m-zrO2 )/t-ZrO2 unique way to fabricate a hierarchically arranged microstructure composites depending on the sintering temperature. The bend where the lowest microstructural dimension can reach even up to ing test was performed on the round bar with smooth surface ab-micrometer level The microstructure did not show any bulk without any additional surface preparation. The following equa defects such as cracks and delamination between AlzO3-(m- tion was used to calculate the bending strength of the samplesB.-T. Lee et al. / Journal of the European Ceramic Society 28 (2008) 229–233 231 Fig. 2. Longitudinal SEM micrographs of (a and b) 4th passed double-network type fibrous Al2O3–(m-ZrO2)/t-ZrO2 composite and enlarged SEM image from (c) Al2O3–(m-ZrO2) core and (d) t-ZrO2 outer network regions. composites sintered at 1500 ◦C. Fig. 1(a) was taken from the 3rd passed filament and shows the unit microstructure of the tailored composites. The dark contrasts in the image which were the two-phase (Al2O3–m-ZrO2) cores were encircled by t-ZrO2 cylinder, observed as white contrast in the SEM image. The t-ZrO2 enclosures of the cores were adjoined in an inter￾connected formation and made a network-like microstructural arrangement. This entire arrangement is confined within the boundary of another t-ZrO2 enclosure, evident in the SEM image by the thicker white contrast. This outer t-ZrO2 phase is adjoined with the bordering of the same kind and made a network type formation. This outer network of t-ZrO2 is more clearly evident in the 4th passed SEM image in Fig. 1(b). The microstructure has just been scaled down in dimension in the 4th passed filament from the 3rd passed filament keeping its design the same. Ultimately it was observed that a t-ZrO2 network was enclosing a two-phase system of Al2O3–(m-ZrO2) which itself was enclosed within a networked t-ZrO2 phase. This type of a network-like microstructure inside another network led to think this microstructure as a double-network type microstructure and this group of words was used through the manuscript to describe this hierarchically oriented microstructure. In Fig. 1(c), the enlarged SEM image of the 4th passed filament showed clearly the double network of the t-ZrO2 phase. The thicknesses of the outer and inner t-ZrO2 network in the 3rd passed composite were about 60 and 3m, respectively, and the Al2O3–(m-ZrO2) core was about 25 m in diameter. In the 4th passed composite the Al2O3–(m-ZrO2) core was about 2.5 m in diameter and the outer and inner t-ZrO2 shell thicknesses were about 15 and 0.8m, respectively. The outer network appeared almost hexagonal in shape. This shape is attributed to the arrangement of the 1st passed filaments in the die which follow a near hexagonal assembly. The inner micro￾groups also exhibit the same geometric feature. The hexagonal shape is also retained in the 4th passed composites as shown in Fig. 1(c). Moreover, the hexagonal cells containing the typical double-network microstructures were seen as indicated by the dotted lines in Fig. 1(b). These were the individual 3rd passed filaments. However, as the number of extrusion passes increased, the microstructure became finer and ultimately the inner network thickness became sub-micrometer as shown in Fig. 1(c). This is a unique way to fabricate a hierarchically arranged microstructure where the lowest microstructural dimension can reach even up to sub-micrometer level. The microstructure did not show any bulk defects such as cracks and delamination between Al2O3–(m￾ZrO2) cores and t-ZrO2 shells during the sintering process. Using one pass further, a modified nanostructured composite can be made by this top down fabrication process. However, using the present technology it can be easily conceived that, N extrusion passes might be expected to lead to an (N − 2) network type microstructure, provided that the particle size of the constituent materials are significantly lower compared to the lowest dimension achievable for the inner network thick￾ness. Fig. 2 shows the longitudinal SEM micrographs of (a) 4th passed double-network type composites sintered at 1500 ◦C. A homogeneous, fibrous microstructure with a highly unidi￾rectional orientation was observed in the low magnification image (a). From this image the hierarchical sub-micrometer and micro-level orientation of t-ZrO2 phase was confirmed. The Al2O3–(m-ZrO2) core was also observed in unidirectional align￾ment where the thickness was about 2.5 m. The inserted image in Fig. 2(a) clearly depicted this point. In the thermal etched enlarged image (b), the t-ZrO2 shells and Al2O3–(m-ZrO2) cores comprised a dense microstructure without any deleterious phe￾nomena like cracking, delimitation, etc. This result indicated that the submicron-sized continuous fibrous microstructures were well controlled using the multi-pass extrusion process. Enlarged SEM images (c and d) were taken from the Al2O3–(m-ZrO2) core and t-ZrO2 shell regions, respectively. In the enlarged image of Al2O3–(m-ZrO2) core (c), the bright and dark contrasts were m-ZrO2 and Al2O3 phases, respectively. The average Al2O3 grain size was about 0.7 m in diameter while the m-ZrO2 grain was about 0.3 m in diameter. However, most of the fine m￾ZrO2 phases were located in the Al2O3 grain boundary. The presence of m-ZrO2 phase in the core region decreased the grain growth of Al2O3, which undergoes excessive grain coarsening during high temperature sintering. The average grain size of the t-ZrO2 outer network was about 0.4m in diameter as shown in Fig. 2(d). Although the particle size of m-ZrO2 and t-ZrO2 was same initially, after sintering the grain size was different due to the pinning effect. In the Al2O3–(m-ZrO2) core the m￾ZrO2 was surrounded by Al2O3 grains and grain growth was hindered. Fig. 3 shows the relative density and bending strength of 4th passed double-network type fibrous (Al2O3–m-ZrO2)/t-ZrO2 composites depending on the sintering temperature. The bend￾ing test was performed on the round bar with smooth surface without any additional surface preparation. The following equa￾tion was used to calculate the bending strength of the samples