《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_16fibrous molithic-7 Fabrication of pore-gradient Al2O3–ZrO2 sintered bodies by fibrous monolithic process

Availableonlineatwww.sciencedirect.com ScienceDirect E≈RS ELSEVIER Joumal of the European Ceramic Society 26(2006)3525-3530 www.elsevier.comlocate/jeurceramsoc Fabrication of pore-gradient Al2O3-ZrO2 Sintered bodies by fibrous monolithic pr Byong-Taek Lee a, * In Cheol Kang a, Asit Kumar Gain Ki-Ho Kima, Ho-Yeon Song b School of Advanced Materials Engineering, Kongju National University, 182, Shinkwan-dong, Kongju City, Chungnam 314-701, South Korea b Department of Microbiology, School of Medicine, Soonchunhyang University, 3661, Ssangyoung-dong, Cheonan-city Chungnam 330-090, South Korea Received 14 September 2005: received in revised form 12 December 2005: accepted 17 December 2005 Available online 23 February 2006 Functionally pore-gradient Al2O3-ZrO2 composites where the porosity is dependent on the extrusion ratio and number of shell layer were fabricated by a fibrous monolithic process. The size and volume fraction of the pores were controlled by different numbers of shell layers, which contained various sizes and a different volume percentage of the pore-forming agent. In the pore-gradient, Al2O3-tro2 bodies having a dense core part, some defects such as cracks, swelling and delamination occurred during the sintering process due to the low extrusion ratio. However, these defects were completely removed as the extrusion ratio increased, and the shell layers as well as the core part had a continuously porous structure. In the shel part, various sizes of pores from 70 to 250 um in diameter were observed. o 2006 Elsevier Ltd. All rights reserved. Keyword: Pore-gradient; Microstructure; Composites; Al2O3-ZrO2 1. Introduction innovative fibrous monolithic process was adapted to manufac ture highly toughened ceramic bodies as well as to control the Al2O3 and ZrO2 ceramics have received a lot of attention pore size and shape biomaterials from many researchers due to their advantages In this work, this new approach was used to fabricate pore such as excellent biocompatibility, excellent chemical stability gradient Al2O3-ZrO2 ceramic bodies using the fibrous mono- and good corrosion resistance as well as outstanding mechan- lithic process. The various parameters such as extrusion ratio, ical properties -However, the dense Al2O3 and ZrO2 bod- pore-gradient rate, pore size and microstructure were change ies showed low osteoconductivity, which has restricted their to set up optimum conditions for fabrication of pore-gradient application as implant materials. To improve osteoconductivity, Al2O3-ZrO2 bodies. To evaluate their mechanical behavior, porous Al2O3 and ZrO2 bodies are required so that cancellous the microstructure and the tendency to phase transformation bone can be easily grown on the artificial implants. Thus, many depending on the fabrication procedure, three-point bending researchers have focused on the formation of porous materials tester, BSE-SEM, optical microscopy and XRD techniques were and made a rough pore surface. In general, the suitable pore size used or implant materials is about 100-150 um, 140-160 um'and 200-1000μm 2. Experimental procedure Several fabrication processes such as ceramic/carbon mixture,tape casting and centrifugal molding technique have Al2O3(about 0.3 um, AKP-50, Sumitomo, Japan), ZrO2 been used to make pore-gradient materials, but, these processes (about 80 nm, m-ZrO2, Tosho, Japan)and carbon(50-200 um, do not control the pore size and pore-gradient rate as well as the SMC, Korea)powders were used as starting materials. Ethy modification of the uniform porous core structure. Recently, an lene vinyl acetate(EVA)(ELVAX 210 and 250, Dupont, USA)and stearic acid(CH3(CH2)16COOH, Daejung Chemi cals &Metals Co., Korea) were added as binder and lubricant Corresponding author. Tel. +82 41 8508677: fax: +82 41 858 2939 respectively. Different volume ratio(25: 75, 40: 60, 60: 40 and E-mail address: Ibt@kongju ac kr(B.-T. Lee) 80: 20 voL %)of (75 vol. Al2O3-25 vol ZrO2) and car- 0955-2219/S-see front matter o 2006 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc 2005.12 01

Journal of the European Ceramic Society 26 (2006) 3525–3530 Fabrication of pore-gradient Al2O3–ZrO2 sintered bodies by fibrous monolithic process Byong-Taek Lee a,∗, In Cheol Kang a, Asit Kumar Gain a, Ki-Ho Kima, Ho-Yeon Song b a School of Advanced Materials Engineering, Kongju National University, 182, Shinkwan-dong, Kongju City, Chungnam 314-701, South Korea b Department of Microbiology, School of Medicine, Soonchunhyang University, 366-1, Ssangyoung-dong, Cheonan-city, Chungnam 330-090, South Korea Received 14 September 2005; received in revised form 12 December 2005; accepted 17 December 2005 Available online 23 February 2006 Abstract Functionally pore-gradient Al2O3–ZrO2 composites where the porosity is dependent on the extrusion ratio and number of shell layer were fabricated by a fibrous monolithic process. The size and volume fraction of the pores were controlled by different numbers of shell layers, which contained various sizes and a different volume percentage of the pore-forming agent. In the pore-gradient, Al2O3–ZrO2 bodies having a dense core part, some defects such as cracks, swelling and delamination occurred during the sintering process due to the low extrusion ratio. However, these defects were completely removed as the extrusion ratio increased, and the shell layers as well as the core part had a continuously porous structure. In the shell part, various sizes of pores from 70 to 250
m in diameter were observed. © 2006 Elsevier Ltd. All rights reserved. Keyword: Pore-gradient; Microstructure; Composites; Al2O3–ZrO2 1. Introduction Al2O3 and ZrO2 ceramics have received a lot of attention as biomaterials from many researchers due to their advantages such as excellent biocompatibility, excellent chemical stability and good corrosion resistance as well as outstanding mechan￾ical properties.1–3 However, the dense Al2O3 and ZrO2 bod￾ies showed low osteoconductivity, which has restricted their application as implant materials. To improve osteoconductivity, porous Al2O3 and ZrO2 bodies are required so that cancellous bone can be easily grown on the artificial implants. Thus, many researchers have focused on the formation of porous materials and made a rough pore surface. In general, the suitable pore size for implant materials is about 100–150
m,4 140–160
m5 and 200–1000
m.6 Several fabrication processes such as ceramic/carbon mixture,7 tape casting8 and centrifugal molding technique9 have been used to make pore-gradient materials, but, these processes do not control the pore size and pore-gradient rate as well as the modification of the uniform porous core structure. Recently, an ∗ Corresponding author. Tel.: +82 41 850 8677; fax: +82 41 858 2939. E-mail address: lbt@kongju.ac.kr (B.-T. Lee). innovative fibrous monolithic process was adapted to manufac￾ture highly toughened ceramic bodies as well as to control the pore size and shape.10–12 In this work, this new approach was used to fabricate pore￾gradient Al2O3–ZrO2 ceramic bodies using the fibrous mono￾lithic process. The various parameters such as extrusion ratio, pore-gradient rate, pore size and microstructure were changed to set up optimum conditions for fabrication of pore-gradient Al2O3–ZrO2 bodies. To evaluate their mechanical behavior, the microstructure and the tendency to phase transformation depending on the fabrication procedure, three-point bending tester, BSE-SEM, optical microscopy and XRD techniques were used. 2. Experimental procedure Al2O3 (about 0.3
m, AKP-50, Sumitomo, Japan), ZrO2 (about 80 nm, m-ZrO2, Tosho, Japan) and carbon (50–200
m, SMC, Korea) powders were used as starting materials. Ethy￾lene vinyl acetate (EVA) (ELVAX 210 and 250, Dupont, USA) and stearic acid (CH3(CH2)16COOH, Daejung Chemi￾cals &Metals Co., Korea) were added as binder and lubricant, respectively. Different volume ratio (25:75, 40:60, 60:40 and 80:20 vol.%) of (75 vol.% Al2O3–25 vol.% ZrO2) and car- 0955-2219/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2005.12.017

B -T Lee et al. Joumal of the European Ceramic Society 26 (2006)3525-3530 Core (AL,O /ZrO,: Polymer (ALO/ZrO, Mixture H- (AL,O,/ZrO,& powders: polymer) mixture ( Carbon: Pol Ist Core Stacking Ist pass filaments Extruded body Extruded bod Fig. 1. Schematic diagram of fabrication process of pore-gradient Al2O3-ZrO2 sintered body: (a)dense and (b) continuously porous core. bon powders were homogeneously mixed in ethanol by a ball tures were examined using optical photography, a back-scattered mill process using AlO3 balls as milling media. After mix- electron scanning electron microscope(BSE-SEM, JEOL-JSM ing, each composition was dried on a hotplate while stirring. 5410)and transmission electron microscopy (TEM, JEM-2010, The different volume ratio of ball milled mixture of pow- JEOL)techniques. The crystal phases were analyzed by X-ray ders(Al2O3-Zro2/carbon)/EVA/stearic acid(volume fraction diffraction (XRD, D/MAX-250, Rigaku, Japan)using Cu Ko 60: 30: 10) was separately mixed using a shear mixer(Shina of 0. 1542 nm. To measure the fracture strength, a three-point Platec Co. Korea)at 120C for I h. Using these four kinds bending strength test was carried out using UTM(R&B Co of shear-mixed materials, 2 mm thick shell layer of each com- Korea). position was made by warm pressing. To make the dense core pore-gradient bodies, the rod type core, 21 mm in diameter, was 3. Results and discussion made by extrusion process using the Al2O3-Zro2/EVA mixture These four-shell layers and core were assembled together and Fig. 2 shows photographs of sintered Al2O3-ZrO2 bodies extruded into filament at 120C with different extrusion ratios depending on the number of shell layers and the extrusion ratio (2.5: 1, 11.9: 1 and 19.3: 1) as shown in Fig. 1(a) In Fig. 2(a), the sample consisted of two-shell layers and was On the other hand, to make the continuously porous core extruded with a low extrusion ratio(2.5: 1 ). After sintering, shell tructure, at first, Al2O3-Zro2/EVA mixture was used to make delamination occurred as indicated by an arrowhead, due to 4 mm thick shell layer by warm pressing and a rod type core the low extrusion ratio. In Fig. 2(b), the sample consisted of was made by the extrusion process using only pore forming four shell layers and was extruded with a ratio of 11.9: 1. Some agent(carbon yEVAmixture This shell and core were assembled delamination phenomenon, as indicated with an arrowhead, was together and extruded in a heated die to make the lst passed observed, also due to the low extrusion ratio On the other hand filaments having 3.5 mm in diameter. The Ist passed filaments using the same number of shell layers and increasing the extru were cut and reloaded in a steel die and extruded to make a sion ratio(19.3: 1 ), bulk defects, which appeared in Fig. 2(a)and 21 mm diameter rod type core. Then, the before making four (b), were not observed. However, in Fig. 2(d), the core structure ell layers and this core were assembled together and extruded was modified with the continuously porous structure. The pore into filament at 120C with extrusion ratio(19.3: 1)as shown in size of the continuously porous core region was about 255 um Fig. 1(b) in diameter To remove the eVa binder and pore-forming agent(car Fig. 3 shows the XRD patterns of(a) raw Al2O3 and(b) bon)in both samples, the lst and 2nd burning-out processes m-zro2 powder, (c)after the Ist burning-out, (d)after the 2nd rere carried out at 700 and 1000C for 2 h under N2 atmo- burning-out and(e) pore-gradient Al2O3-Zro2 sintered bodies sphere and air, respectively. Finally, the pressureless sintering at 1550.C. After the lst burn-out, carbon(pore-forming agent) was carried out at 1550C for 2 h in air atmosphere. Microstruc- peaks were detected as well as Al2O3 and m-ZrO2 phases. How

3526 B.-T. Lee et al. / Journal of the European Ceramic Society 26 (2006) 3525–3530 Fig. 1. Schematic diagram of fabrication process of pore-gradient Al2O3–ZrO2 sintered body: (a) dense and (b) continuously porous core. bon powders were homogeneously mixed in ethanol by a ball mill process using Al2O3 balls as milling media. After mix￾ing, each composition was dried on a hotplate while stirring. The different volume ratio of ball milled mixture of pow￾ders (Al2O3–ZrO2/carbon)/EVA/stearic acid (volume fraction 60:30:10) was separately mixed using a shear mixer (Shina Platec Co., Korea) at 120 ◦C for 1 h. Using these four kinds of shear-mixed materials, 2 mm thick shell layer of each com￾position was made by warm pressing. To make the dense core pore-gradient bodies, the rod type core, 21 mm in diameter, was made by extrusion process using the Al2O3–ZrO2/EVA mixture. These four-shell layers and core were assembled together and extruded into filament at 120 ◦C with different extrusion ratios (2.5:1, 11.9:1 and 19.3:1) as shown in Fig. 1(a). On the other hand, to make the continuously porous core structure, at first, Al2O3–ZrO2/EVA mixture was used to make a 4 mm thick shell layer by warm pressing and a rod type core was made by the extrusion process using only pore forming agent (carbon)/EVA mixture. This shell and core were assembled together and extruded in a heated die to make the 1st passed filaments having 3.5 mm in diameter. The 1st passed filaments were cut and reloaded in a steel die and extruded to make a 21 mm diameter rod type core. Then, the before making four shell layers and this core were assembled together and extruded into filament at 120 ◦C with extrusion ratio (19.3:1) as shown in Fig. 1(b). To remove the EVA binder and pore-forming agent (car￾bon) in both samples, the 1st and 2nd burning-out processes were carried out at 700 and 1000 ◦C for 2 h under N2 atmo￾sphere and air, respectively. Finally, the pressureless sintering was carried out at 1550 ◦C for 2 h in air atmosphere. Microstruc￾tures were examined using optical photography, a back-scattered electron scanning electron microscope (BSE-SEM, JEOL-JSM 5410) and transmission electron microscopy (TEM, JEM-2010, JEOL) techniques. The crystal phases were analyzed by X-ray diffraction (XRD, D/MAX-250, Rigaku, Japan) using Cu K of 0.1542 nm. To measure the fracture strength, a three-point bending strength test was carried out using UTM (R&B Co., Korea). 3. Results and discussion Fig. 2 shows photographs of sintered Al2O3–ZrO2 bodies depending on the number of shell layers and the extrusion ratio. In Fig. 2(a), the sample consisted of two-shell layers and was extruded with a low extrusion ratio (2.5:1). After sintering, shell delamination occurred as indicated by an arrowhead, due to the low extrusion ratio. In Fig. 2(b), the sample consisted of four shell layers and was extruded with a ratio of 11.9:1. Some delamination phenomenon, as indicated with an arrowhead, was observed, also due to the low extrusion ratio. On the other hand, using the same number of shell layers and increasing the extru￾sion ratio (19.3:1), bulk defects, which appeared in Fig. 2(a) and (b), were not observed. However, in Fig. 2(d), the core structure was modified with the continuously porous structure. The pore size of the continuously porous core region was about 255
m in diameter. Fig. 3 shows the XRD patterns of (a) raw Al2O3 and (b) m-ZrO2 powder, (c) after the 1st burning-out, (d) after the 2nd burning-out and (e) pore-gradient Al2O3–ZrO2 sintered bodies at 1550 ◦C. After the 1st burn-out, carbon (pore-forming agent) peaks were detected as well as Al2O3 and m-ZrO2 phases. How-

B.-T. Lee et al. / Joumal of the European Ceramic Society 26 (2006)3525-3530 35 Imm Imm (d) Imm I mm Fig. 2. Photographs of pore-gradient Al2O3-ZrO2 sintered bodies depending on the number of layers and extrusion ratio: (a)two layers: 2.5: 1, (b)four layers: 11. 9: 1 (c)four layers: 19.3: 1 and (d)four layers: 19.3: 1 ever, after the 2nd burn-out, the carbon was not detected as shown body contained 40 vol. carbon while the outer shell con- in Fig 3(d). After sintering, the pore-gradient bodies were com- tained 70 vol %o carbon. After sintering, different sizes of pores posed of Al2O3 and m-ZrO2 phases as shown in Fig 3(e) about 70-100 um in diameter were produced in the inner shell Fig 4 shows SEM micrographs of (a)before and (b)after sin- while the pore size of the outer shell was about 150-250 um in ring of pore-gradient Al2O3-ZrO2 bodies having a dense core diameter(b). Fig. 4(c)and(d)are partially enlarged images of structure and two shell layers. The inner shell of the extruded outer shell and shell boundary regions of Fig 4(b). As shown in Fig. 4(c), the outer shell has various sizes of pore sizes, :m1,,,,0dsremscNamstensetonisEMm ★ Carbon graph at the interface between the layers in Fig 4(d), no defects such as swelling or delamination in the boundary were found. Moreover, each layer shows a uniform pore size, respectively. i However, some cracks were found as shown in Fig 4(b)as indi- cated with arrowheads between the dense core and porous shell 欢r Fig. 5 shows BSE-SEM images of (a and b) longitudi nal and (c and d)cross-sectional directions of pore-gradient Al2O3-zro bodies(a and c) before and(b and d) after sinter ing, respectively. As shown in Fig. 5(a)and(c), the pore-forming agent which appeared with a dark contrast, as indicated with 70 arrowheads, was homogeneously and gradient dispersed in the AlO3-ZrO matrix. However, after the burning-out and sinter- Fig 3. XRD profiles of (a)raw Al2 O3 powder, (b)raw m-Z10z powder, (c) ing process, the pore-forming agent was removed and formed Ist burn-out, (d) after 2nd burn-out and(e)sintered at 1550.C of extruded many pores as indicated with arrowheads in Fig. 5(b and d).Aft

B.-T. Lee et al. / Journal of the European Ceramic Society 26 (2006) 3525–3530 3527 Fig. 2. Photographs of pore-gradient Al2O3–ZrO2 sintered bodies depending on the number of layers and extrusion ratio: (a) two layers: 2.5:1, (b) four layers: 11.9:1, (c) four layers: 19.3:1 and (d) four layers: 19.3:1. ever, after the 2nd burn-out, the carbon was not detected as shown in Fig. 3(d). After sintering, the pore-gradient bodies were com￾posed of Al2O3 and m-ZrO2 phases as shown in Fig. 3(e). Fig. 4 shows SEM micrographs of (a) before and (b) after sin￾tering of pore-gradient Al2O3–ZrO2 bodies having a dense core structure and two shell layers. The inner shell of the extruded Fig. 3. XRD profiles of (a) raw Al2O3 powder, (b) raw m-ZrO2 powder, (c) after 1st burn-out, (d) after 2nd burn-out and (e) sintered at 1550 ◦C of extruded Al2O3–ZrO2 bodies. body contained 40 vol.% carbon while the outer shell con￾tained 70 vol.% carbon. After sintering, different sizes of pores about 70–100
m in diameter were produced in the inner shell while the pore size of the outer shell was about 150–250
m in diameter (b). Fig. 4(c) and (d) are partially enlarged images of outer shell and shell boundary regions of Fig. 4(b). As shown in Fig. 4(c), the outer shell has various sizes of pore sizes, which offers the advantages of good adherence and growth of cells. From observation of the cross-sectional SEM micro￾graph at the interface between the layers in Fig. 4(d), no defects such as swelling or delamination in the boundary were found. Moreover, each layer shows a uniform pore size, respectively. However, some cracks were found as shown in Fig. 4(b) as indi￾cated with arrowheads between the dense core and porous shell region. Fig. 5 shows BSE–SEM images of (a and b) longitudi￾nal and (c and d) cross-sectional directions of pore-gradient Al2O3–ZrO2 bodies (a and c) before and (b and d) after sinter￾ing, respectively. As shown in Fig. 5(a) and (c), the pore-forming agent which appeared with a dark contrast, as indicated with arrowheads, was homogeneously and gradient dispersed in the Al2O3–ZrO2 matrix. However, after the burning-out and sinter￾ing process, the pore-forming agent was removed and formed many pores as indicated with arrowheads in Fig. 5(b and d). After sintering, bulk defects such as cracks, delamination or swelling

B.-T Lee et al. Joumal of the European Ceramic Sociery 26 (2006)3525-3530 Yol-AL.O/ZrO I m Fig 4. SEM micrographs of pore-gradient Al2O3-ZrO2 bodies(different pore size):(a) before and( b)after sintering.(c)Outer shell and(d) interface of shell layers. were not found between the pore-gradient regions, but some core structure. Fig. 6(a) and (c) show BSE-SEM images cracks were observed in the dense core region. It was confirmed of longitudinal and cross-sectional direction of pore-gradient that the reason cracks were created was derived from thermal Al2O3-ZrO2 extruded bodies, respectively. Before sintering. strain due to the different shrinkage between the dense core and the core part, the pore forming agent(carbon) and EVA having porous shell region. 255 um in diameter were homogeneously distributed as ind Fig. 6 shows the other pore-gradient microstructure of cated with an arrowhead (c). After the burning-out and sintering A12O3-ZrOz bodies, which consist of a continuously porous process, the eVa binder and pore-forming agent(carbon) were 00ma0001o B日g1 Fig. 5. BSE-SEM micrographs of pore-gradient Al2O3-ZrO2 bodies(same pore size and dense core structure): (a and b)longitudinal and (c and d) cross-sectional direction and(a and c)before and (b and d) after sintering

3528 B.-T. Lee et al. / Journal of the European Ceramic Society 26 (2006) 3525–3530 Fig. 4. SEM micrographs of pore-gradient Al2O3–ZrO2 bodies (different pore size): (a) before and (b) after sintering. (c) Outer shell and (d) interface of shell layers. were not found between the pore-gradient regions, but some cracks were observed in the dense core region. It was confirmed that the reason cracks were created was derived from thermal strain due to the different shrinkage between the dense core and porous shell region. Fig. 6 shows the other pore-gradient microstructure of Al2O3–ZrO2 bodies, which consist of a continuously porous core structure. Fig. 6(a) and (c) show BSE–SEM images of longitudinal and cross-sectional direction of pore-gradient Al2O3–ZrO2 extruded bodies, respectively. Before sintering, in the core part, the pore forming agent (carbon) and EVA having 255
m in diameter were homogeneously distributed as indi￾cated with an arrowhead (c). After the burning-out and sintering process, the EVA binder and pore-forming agent (carbon) were Fig. 5. BSE–SEM micrographs of pore-gradient Al2O3–ZrO2 bodies (same pore size and dense core structure): (a and b) longitudinal and (c and d) cross-sectional direction and (a and c) before and (b and d) after sintering

B.-T. Lee et al. / Joumal of the European Ceramic Society 26 (2006)3525-3530 3529 25kU35 500um 000010 Fig. 6. BSE-SEM micrographs of Al2O3-ZrOz pore-gradient bodies(same pore size and porous core structure):(a and b) longitudinal and(c and d)cross-sectional direction and (a and c)before and(b and d) after sintering. perfectly removed and formed continuous pores in the core part, by an arrowhead (a). An HRTEM micrograph(b) was taken but in the shell part, pores were present gradiently as shown in from an m-ZrO2 grain, which is marked b in Fig. 7(a). In the Fig. 6(d). However, in this case, bulk defects were not found m-ZrO2 grain, many twin boundaries were clearly observed as between the shell layers or in the continuously porous core indicated with arrowheads(b). In general, these types of twins were formed to dissipate a shear stress, which was generated by Fig. 7(a) shows a TEM image (a)of the pore-gradient a phase transformation during the sintering process Al2O3-ZrO2 bodies. In the TEM micrograph(a), the al2o Fig. 8 shows the load-distance curves of pore-gradient grains showed a bright contrast about 700 nm in diameter while Al2O3-ZrOz bodies having(a)dense core and(b) porous core m-ZrO2 grains were seen with a dark contrast about 250 nm in structures. At the initial stage, about 0. 2 mm in distance, the load diameter. Most of the m-ZrOz phases were located at the grain was increased about 7 kg and dramatically dropped in both sam boundaries. However, in the Al2O3 grains, a few t-ZrO2 particles ples, due to the fracture of the porous shell. But, in the case of the less than 150 nm in diameter were clearly observed as indicated pore-gradient Al2O3-ZrO2 bodies having a dense core, the load (a) 00n I Onm Fig. 7. TEM (a) and HRTEM(b)micrographs of pore frame region of pore-gradient Al2O3-ZrO2 bodies

B.-T. Lee et al. / Journal of the European Ceramic Society 26 (2006) 3525–3530 3529 Fig. 6. BSE–SEM micrographs of Al2O3–ZrO2 pore-gradient bodies (same pore size and porous core structure): (a and b) longitudinal and (c and d) cross-sectional direction and (a and c) before and (b and d) after sintering. perfectly removed and formed continuous pores in the core part, but in the shell part, pores were present gradiently as shown in Fig. 6(d). However, in this case, bulk defects were not found between the shell layers or in the continuously porous core part. Fig. 7(a) shows a TEM image (a) of the pore-gradient Al2O3–ZrO2 bodies. In the TEM micrograph (a), the Al2O3 grains showed a bright contrast about 700 nm in diameter while m-ZrO2 grains were seen with a dark contrast about 250 nm in diameter. Most of the m-ZrO2 phases were located at the grain boundaries. However, in the Al2O3 grains, a few t-ZrO2 particles less than 150 nm in diameter were clearly observed as indicated by an arrowhead (a). An HRTEM micrograph (b) was taken from an m-ZrO2 grain, which is marked b in Fig. 7(a). In the m-ZrO2 grain, many twin boundaries were clearly observed as indicated with arrowheads (b). In general, these types of twins were formed to dissipate a shear stress, which was generated by a phase transformation during the sintering process. Fig. 8 shows the load–distance curves of pore-gradient Al2O3–ZrO2 bodies having (a) dense core and (b) porous core structures. At the initial stage, about 0.2 mm in distance, the load was increased about 7 kg and dramatically dropped in both sam￾ples, due to the fracture of the porous shell. But, in the case of the pore-gradient Al2O3–ZrO2 bodies having a dense core, the load Fig. 7. TEM (a) and HRTEM (b) micrographs of pore frame region of pore-gradient Al2O3–ZrO2 bodies

B.-T Lee et al. Joumal of the European Ceramic Sociery 26 (2006)3525-3530 Distance(mm) Distance(mm) Fig. 8. Load-distance curves of AlzO3-ZrO2 pore-gradient bodies: (a) dense core and(b) porous core. was increased up to ll kg as the distance increased from 0. 25 Acknowledgement to 0.55 mm and dramatically dropped due to the fracture of the dense core. The existence of cracks obtained in Fig. 5(d)may be This work was supported by NRl research program of the promoted as the typical catastrophic failure. However, the pore- Korean Ministry of Science and Technology progran gradient AlzO3-ZrOz bodies having a porous core showed a dif- ferent behavior: ie. the load fluctuated before the main fracture References due to the continuously porous structure as shown in Fig. 6(d) In general, the common ceramics showed a brittle fracture; thus, Li, Wand Gao, L, Fabrication of HAp-ZrO2(3Y)nano-composite by SPS the load-distance curve increased linearly and dropped dramati Biomaterials. 2003. 24. 937-940. 2. Jordan, D. R Online clinical commun. for ophthalmologists (TM) cally after the fracture. However, the pore-gradient Al2O3-ZrO2 http://www.occojournal.com/.Forn,2001,31 bodies showed semi-brittle fracture behavior, which appears in 3. Willmann, G, Fruh, H J and Pfaff, H G, Wear characteristics of slid fibrous monolithic composites. The maximum values of bending ing pairs of ZrO2(Y-TZP)for hip endoprostheses. Biomaterials, 1996, 17, strength of pore-gradient Al2O3-ZrO2 bodies having a dense 2157-2162. core and porous core microstructure were about 219.7 and 4. Hing, K. A, Best, S M. and Bonfield, w, Characteristic of porous hyrox 195.5 MPa, respectively. From our previous work, the values of 5. Hench. LL. Bioceramics.. Am. Ceram Soc. 1998.81.1705-1728 bending strength strongly depended on porosity and pore size. 6. Chiroff, R.T., White, E.W. Webber,JN and Roy, DM,Tissue ingrowth In the continuously porous Al2O3 bodies having 63%0 relative of replamineform implants J. Biomed. Mater. Res Symp., 1975. 6. 29 density and 150 um in diameter pore size, the bending strength value was about 90 MPa.3 7. Maca, K, Dobsak, P. and Boccaccini, A. R, Fabrication of graded porous ceramics using alumina-carbon powder mixtures. Cera. Int, 2001, 27, 577-584 4. Conclusions 8. Werner, J, Krcmar, B. L, Friess, W. and Greil, P, Mechanical properties and in vitro cell compatibility of hydroxyapatite ceramics with graded pore Pore-gradient Al203-ZrO2 sintered bodies were success- 9. Chen, C H. Takita, K, Honda, S. and Awaji, J. fracture behavior of cylin- fully fabricated using the fibrous monolithic process. The pore- drical porous alumina with pore gradient. J. Eur Ceram. Soc., 2005, 25, gradient microstructure was easily controlled depending on the 385-391. pore size and porosity, using the various sizes of carbon powder 10. Kaya, C, Butler, E.G. and Lewis, M H, Co-extrusion of Al2O3/ZrO2 bi- and changing the volume fraction of pore-agent In pore-gradient phase high temperature ceramics with fine scale aligned microstructures. J bodies having a porous core structure, no bulk defects such a cts such as 11. Lienard. S. Y. Kovar. D. Moon. R I. Bowman. K J. and Halloran. I. w cracks and shrinkage cavities were found due to the reduction of Texture development in Sig N4/BN fibrous monolithic ceramics. J Mater the thermal strain by the formation of continuously porous core Sci,2000,35,3365-3371 structure 12. Kim, T. S. Kang. I. C, Goto, T. and Lee, B. T, Fabrication of continuously The load-distance curves of pore-gradient Al2O3-ZrO2 bod porous alumina body by fibrous monolithic and sintering process. Mater es showed some different behavior, which appeared with that 13. Lee. B. T. Kang. I C. Cho. S.H. and Song. H. Y. Fabrication of contin- of common due to microstructure control, especially uously oriented porous AlO3 body and its in-vitro study. J. A. Cera pore-gradient and porous core structure. Soc.2005,88,2262-2266

3530 B.-T. Lee et al. / Journal of the European Ceramic Society 26 (2006) 3525–3530 Fig. 8. Load–distance curves of Al2O3–ZrO2 pore-gradient bodies: (a) dense core and (b) porous core. was increased up to 11 kg as the distance increased from 0.25 to 0.55 mm and dramatically dropped due to the fracture of the dense core. The existence of cracks obtained in Fig. 5(d) may be promoted as the typical catastrophic failure. However, the pore￾gradient Al2O3–ZrO2 bodies having a porous core showed a dif￾ferent behavior; i.e., the load fluctuated before the main fracture due to the continuously porous structure as shown in Fig. 6(d). In general, the common ceramics showed a brittle fracture; thus, the load–distance curve increased linearly and dropped dramati￾cally after the fracture. However, the pore-gradient Al2O3–ZrO2 bodies showed semi-brittle fracture behavior, which appears in fibrous monolithic composites. The maximum values of bending strength of pore-gradient Al2O3–ZrO2 bodies having a dense core and porous core microstructure were about 219.7 and 195.5 MPa, respectively. From our previous work, the values of bending strength strongly depended on porosity and pore size. In the continuously porous Al2O3 bodies having 63% relative density and 150
m in diameter pore size, the bending strength value was about 90 MPa.13 4. Conclusions Pore-gradient Al2O3–ZrO2 sintered bodies were success￾fully fabricated using the fibrous monolithic process. The pore￾gradient microstructure was easily controlled depending on the pore size and porosity, using the various sizes of carbon powder and changing the volume fraction of pore-agent. In pore-gradient bodies having a porous core structure, no bulk defects such as cracks and shrinkage cavities were found due to the reduction of the thermal strain by the formation of continuously porous core structure. The load–distance curves of pore-gradient Al2O3–ZrO2 bod￾ies showed some different behavior, which appeared with that of common ceramics due to microstructure control, especially pore-gradient and porous core structure. Acknowledgement This work was supported by NRL research program of the Korean Ministry of Science and Technology. References 1. Li, W. and Gao, L., Fabrication of HAp–ZrO2 (3Y) nano-composite by SPS. Biomaterials, 2003, 24, 937–940. 2. Jordan, D. R., Online clinical commun. for ophthalmologists (TM) – http://www.occojournal.com/. Forum, 2001, 31. 3. Willmann, G., Fruh, H. J. and Pfaff, H. G., Wear characteristics of slid￾ing pairs of ZrO2(Y-TZP) for hip endoprostheses. Biomaterials, 1996, 17, 2157–2162. 4. Hing, K. A., Best, S. M. and Bonfield, W., Characteristic of porous hyrox￾yapaptite. J. Mater. Sci. Mater. Med., 1999, 10, 135–145. 5. Hench, L. L., Bioceramics. J. Am. Ceram. Soc., 1998, 81, 1705–1728. 6. Chiroff, R. T., White, E. W., Webber, J. N. and Roy, D. M., Tissue ingrowth of replamineform implants. J. Biomed. Mater. Res. Symp., 1975, 6, 29– 45. 7. Maca, K., Dobsak, P. and Boccaccini, A. R., Fabrication of graded porous ceramics using alumina–carbon powder mixtures. Ceram. Int., 2001, 27, 577–584. 8. Werner, J., Krcmar, B. L., Friess, W. and Greil, P., Mechanical properties and in vitro cell compatibility of hydroxyapatite ceramics with graded pore structure. Biomaterials, 2002, 23, 4285–4294. 9. Chen, C. H., Takita, K., Honda, S. and Awaji, J., Fracture behavior of cylin￾drical porous alumina with pore gradient. J. Eur. Ceram. Soc., 2005, 25, 385–391. 10. Kaya, C., Butler, E. G. and Lewis, M. H., Co-extrusion of Al2O3/ZrO2 bi￾phase high temperature ceramics with fine scale aligned microstructures. J. Eur. Ceram. Soc., 2003, 23, 935–942. 11. Lienard, S. Y., Kovar, D., Moon, R. J., Bowman, K. J. and Halloran, J. W., Texture development in Si3N4/BN fibrous monolithic ceramics. J. Mater. Sci., 2000, 35, 3365–3371. 12. Kim, T. S., Kang, I. C., Goto, T. and Lee, B. T., Fabrication of continuously porous alumina body by fibrous monolithic and sintering process. Mater. Trans., 2003, 44, 1851–1856. 13. Lee, B. T., Kang, I. C., Cho, S. H. and Song, H. Y., Fabrication of contin￾uously oriented porous Al2O3 body and its in-vitro study. J. Am. Ceram. Soc., 2005, 88, 2262–2266