B -T Lee et al. / Joumal of the European Ceramic Society 28(2008)229-233 the microstructure can be tailored with many desired features Al2O3 balls(Al2O3/m-ZrO2 )EVA/stearic acid(volume ratio, ke fibrous microstructure, fibrous microstructure with soft 50: 40: 10)and t-ZrO2/EVA/stearic acid (volume ratio, 47: 40: 13) interface,etc, which can impart some unique characteristics. were separately mixed using a shear mixture(C w. Braben- In the fibrous monolithic process, the ceramic powder is mixed der Instruments, Shina Platech Co, Hwaseong Gyeong-Gi-Do with polymer to make an extrudable material and extrusion is Korea. The polymer bound(Al2O3/m-zrO2) and t-zrO2 mix carried out Selecting combination of materials and changing the tures were used to make rod-like cores(22 mm diameter) and arrangements of filaments during loading for extrusion, various tube -like shells(4 mm thick) by warm press, respectively. This kinds of microstructures can be fabricated In our previous work, t-ZrO2 shell makes the inner network of the final composites. we levites 2.14 A coating of HAp inside the contnuous pores roll which consisted of 60/40 volume fraction of the core and ed continuously porous Al2O3, ZrO2 and their These core and shell were assembled together to prepare the feed of ZrO2and functionally gradient HAp(t-ZrO2)/Al2O3-(m- shell. The feed roll was then extruded at 120C with 8 mm/min ZrO2)composites 6 were fabricated by the same extrusion velocity to make the Ist passed filaments, which were method 3.5 mm in diameter. The lst passed filaments were cut 80mm In this work, as a new approach, to improve the fracture length and reloaded in a steel die and again rod-like cores were strength and toughness, a novel double-network type microstruc- prepared (22 mm diameter). Then, these core and previously pre ture with fibrous(Al2O3-m-ZrO2)t-ZrO2 composites were pared t-ZrO2 shell were assembled again and extruded to make fabricated using the multi-pass extrusion process. To utilize the the 2nd passed filaments, 3.5 mm in diameter. The t-ZrO2 shell phase transformation toughening mechanism of t-ZrO2, a net- in this stage will make the outer network of the final compos work boundary was fabricated surrounding the fine core/shell ites. The 2nd passed filaments were cut and reloaded to make microstructure of(Al2O3-m-ZrO2)/t-ZrO2 where the adjoin- the 3rd passed filaments. Subsequently, the 4th passed filaments the two-phase core 25 vol. m-ZrO2 was dispersed in AlO3 passed filaments. To obtain the sintered body from nee e 3rd ing t-ZrO2 shell phase forms a continuous inner network. In were made in the same way by assembling and extruding th matrix. This was done decrease the grain coarsening and to composites, first a binder burning-out process was carried out at introduce microcracking near the phase boundary of Al2O3 and 700C for 2 h in a N2 atmosphere with a very slow heating rate m-zrO2, which is reported to improve the fracture toughness of and then again at 1000C for 2 h in an air atmosphere. Finally, the system as stated earlier. A thicker outer cylinder of t-ZrO2 the pressureless sintering process was carried out at different enclosed this assembly and all the cylinders made a formation of temperatures ranging from 1300 to 1500C for I h in an air macro-scale network. The microstructure of the fabricated mate- atmosphere rial contained sub-micrometer level dimension which renders Microstructures and fracture surfaces were observed using the use of nanopowders obvious. It also improves the mechani scanning electron microscope (SEM, JSM-6335F, JEOL, cally property of the composite compared to the coarse powder. Japan). The relative density was measured by the Archimede In the work the microstructural details were characterized with method. The average bending strength was measured by a four- the SEM technique. Hardness, relative density, bending strength point bending test method with a universal testing machine and fracture toughness was ated for different sintering (Unitech TM, R&B, Korea) using eight specimens(2.75 mm temperatures in diameter x 30 mm in length) with a crosshead speed of 0.1 mm/min. To measure the vickers hardness and fracture 2. Experimental procedure toughness, 2.75 mm diameter cylindrical-shaped samples were cut about 4 mm in length and polished, using up to 1 um dia Alumina(a-Al2O3, about 300 nm, AKP-50, Sumitomo, mond paste. The average Vickers hardness was measured by apan), monoclinic zirconia(m-ZrO2, about 80 nm, Tosoh Cor- indenting with a load of 5 kg(10 points/sample). The fracture poration, Nanyo Manufacturing Complex, Japan), tetragonal toughness was calculated by the indentation method using an zirconia(t-zrO2, about 80 nm, Tosoh Corporation, Nanyo Man- indentation load of 10kg facturing Complex), ethylene vinyl acetate(EVA)(ELVAX210 and 250, Dupont, USA) and stearic acid(CH3(CH2)16 COoH, 3. Results and discussion Daejung Chemicals Metals Co., Korea) were used as start ing materials. 75 vol. Al2O3 and 25 vol %o m-zro2 powders ig. 1 shows the cross-sectional SEM micrographs of were homogeneously mixed in ethanol by ball milling using the double-network type fibrous (Al2O3-m-zro2)/t-ZrO2 20 Fig 1 SEM micrographs of(a) 3rd and(b and c)4th passed double-network type fibrous Al2O3-(m-ZrO2 M-ZrO2 composite sintered at 1500C230 B.-T. Lee et al. / Journal of the European Ceramic Society 28 (2008) 229–233 the microstructure can be tailored with many desired features like fibrous microstructure,11 fibrous microstructure with soft interface,12 etc., which can impart some unique characteristics. In the fibrous monolithic process, the ceramic powder is mixed with polymer to make an extrudable material and extrusion is carried out. Selecting combination of materials and changing the arrangements of filaments during loading for extrusion, various kinds of microstructures can be fabricated. In our previous work, we fabricated continuously porous Al2O3, 4 ZrO2 13 and their composites.2,14 A coating of HAp inside the continuous pores of ZrO2 15 and functionally gradient HAp–(t-ZrO2)/Al2O3–(m￾ZrO2) composites16 were fabricated by the same method. In this work, as a new approach, to improve the fracture strength and toughness, a novel double-network type microstruc￾ture with fibrous (Al2O3–m-ZrO2)/t-ZrO2 composites were fabricated using the multi-pass extrusion process. To utilize the phase transformation toughening mechanism of t-ZrO2, a net￾work boundary was fabricated surrounding the fine core/shell microstructure of (Al2O3–m-ZrO2)/t-ZrO2 where the adjoin￾ing t-ZrO2 shell phase forms a continuous inner network. In the two-phase core 25 vol.% m-ZrO2 was dispersed in Al2O3 matrix. This was done decrease the grain coarsening and to introduce microcracking near the phase boundary of Al2O3 and m-ZrO2, which is reported to improve the fracture toughness of the system as stated earlier. A thicker outer cylinder of t-ZrO2 enclosed this assembly and all the cylinders made a formation of macro-scale network. The microstructure of the fabricated mate￾rial contained sub-micrometer level dimension which renders the use of nanopowders obvious. It also improves the mechani￾cally property of the composite compared to the coarse powder. In the work the microstructural details were characterized with the SEM technique. Hardness, relative density, bending strength and fracture toughness was investigated for different sintering temperatures. 2. Experimental procedure Alumina (-Al2O3, about 300 nm, AKP-50, Sumitomo, Japan), monoclinic zirconia (m-ZrO2, about 80 nm, Tosoh Cor￾poration, Nanyo Manufacturing Complex, Japan), tetragonal zirconia (t-ZrO2, about 80 nm, Tosoh Corporation, Nanyo Man￾ufacturing Complex), ethylene vinyl acetate (EVA) (ELVAX 210 and 250, Dupont, USA) and stearic acid (CH3(CH2)16COOH, Daejung Chemicals & Metals Co., Korea) were used as start￾ing materials. 75 vol.% Al2O3 and 25 vol.% m-ZrO2 powders were homogeneously mixed in ethanol by ball milling using Al2O3 balls. (Al2O3/m-ZrO2)/EVA/stearic acid (volume ratio, 50:40:10) and t-ZrO2/EVA/stearic acid (volume ratio, 47:40:13) were separately mixed using a shear mixture (C.W. Braben￾der Instruments, Shina Platech Co., Hwaseong Gyeong-Gi-Do, Korea). The polymer bound (Al2O3/m-ZrO2) and t-ZrO2 mix￾tures were used to make rod-like cores (22 mm diameter) and tube-like shells (4 mm thick) by warm press, respectively. This t-ZrO2 shell makes the inner network of the final composites. These core and shell were assembled together to prepare the feed roll which consisted of 60/40 volume fraction of the core and shell. The feed roll was then extruded at 120 ◦C with 8 mm/min extrusion velocity to make the 1st passed filaments, which were 3.5 mm in diameter. The 1st passed filaments were cut 80 mm length and reloaded in a steel die and again rod-like cores were prepared (22 mm diameter). Then, these core and previously pre￾pared t-ZrO2 shell were assembled again and extruded to make the 2nd passed filaments, 3.5 mm in diameter. The t-ZrO2 shell in this stage will make the outer network of the final compos￾ites. The 2nd passed filaments were cut and reloaded to make the 3rd passed filaments. Subsequently, the 4th passed filaments were made in the same way by assembling and extruding the 3rd passed filaments. To obtain the sintered body from these green composites, first a binder burning-out process was carried out at 700 ◦C for 2 h in a N2 atmosphere with a very slow heating rate and then again at 1000 ◦C for 2 h in an air atmosphere. Finally, the pressureless sintering process was carried out at different temperatures ranging from 1300 to 1500 ◦C for 1 h in an air atmosphere. Microstructures and fracture surfaces were observed using a scanning electron microscope (SEM, JSM-6335F, JEOL, Japan). The relative density was measured by the Archimedes method. The average bending strength was measured by a four￾point bending test method with a universal testing machine (Unitech TM, R&B, Korea) using eight specimens (2.75 mm in diameter × 30 mm in length) with a crosshead speed of 0.1 mm/min. To measure the Vickers hardness and fracture toughness, 2.75 mm diameter cylindrical-shaped samples were cut about 4 mm in length and polished, using up to 1 m dia￾mond paste. The average Vickers hardness was measured by indenting with a load of 5 kg (10 points/sample). The fracture toughness was calculated by the indentation method using an indentation load of 10 kg. 3. Results and discussion Fig. 1 shows the cross-sectional SEM micrographs of the double-network type fibrous (Al2O3–m-ZrO2)/t-ZrO2 Fig. 1. SEM micrographs of (a) 3rd and (b and c) 4th passed double-network type fibrous Al2O3–(m-ZrO2)/t-ZrO2 composite sintered at 1500 ◦C.