T-S. Kim et al. Scripta Materialia 52(2005)725-729 with a binder in a shear mixer( C w. Brabender Instru- using Archimedes principle. Hardness of the sintered ments, Inc, PL2000 Plasti-Corder, USA), in which body was measured using a Vickers hardness tester Al2O3 25 vol %(m-ZrO2) was prepared using a ball mill-(HV-112, Akashi, Japan)under a pressure of 1.96N ing process; m-ZrO2 and t-ZrO2 indicate monoclinic and for 10 s The fracture toughness, KIc was obtained using tetragonal ZrO2, respectively. The m-ZrO2 was added to a combination of the indentation method and the evans suppress the grain growth of Al_O3 during sintering. The equation at a load of 10 kg as follows, than 300 nm for A10,(AKP-50, Sumimoto, Japan) Kic=(E/)(/ce' and 80 nm for ZrO2 powder( Toso, Japan). The binder where A is a material-independent constant (=0.016), E material was composed of ethylene vinyl acetate is the elastic modulus of the materials being indented, H (EVA)in granules(Elvax 250, Dupont). For lubrication the hardness, P the applied load and c the crack half during blending, stearic acid( CH3(CH2)16COOH, Dae- length jung Chemicals Metals Co, Ltd)was added. The vol to measure the bending strength, four-po ume per cent of the mixture was 50/50 for the Al2O3 bending tests were carried out using a universal testing powder/polymer and 40/60 for the ZrO2 powder/poly- machine (UnitechM, R&B, Korea), in which the mer. In order to blend each powder with polymer, the round-type of filament as extruded was used for the test mixing head and chamber of the blender were first specimen. The distance between the upper rolls was heated to 130C using an oil-type heating source, fol- 30 mm and between the bottom rolls 10 mm lowed by addition of the polymer. After rotating the head for about 120 s each ceramic powder was added slowly 3. Results and discussion Mixtures of Al2O3-ZrO2/polymer and ZrO/polymer vere extruded into filaments of 3. 5 mm in diameter with Fig. 1(a)(d) shows BSE-SEM micrographs of a sin an area reduction ratio of 70: 1. The first passed Al2O3- ered Al2O3(m-ZrO2)/ZrO2 composite body fabricated te ZrO2/polymer filament and ZrO/polymer filament were by the multi-extrusion process, in which Fig. 1(aH(d) reloaded into the extrusion die with an identical volume. show the second-, third-, fourth- and fifth- pass fila The extrusion was repeated until the fifth-pass filament ments, respectively. The composite presents a homoge- was obtained. The extrusion temperature was 120C neous distribution of AlO3(m-ZrO2) and ZrO2 and the rate was 15 mm/min. Binder burning-out of phases. The Zro2 phase appears to be brighter than the filaments was carried out at 700C under flowing Al2O3-(m-ZrO2) phases. In order to arrange both the fil- nitrogen atmosphere, and sintered at 1450C aments in a certain pattern as shown in Fig. 1, the fila Microstructure and fracture surface of the filaments ments of composition Al2O3(m-ZrO2)and ZrO2 were vere examined using backscattered electrons(BSE)in initially prepared using the extrusion process. The size a scanning electron microscope (SEM) (JSM 6331 F, of both the filaments was 3. 5 mm in diameter. The same Japan). The density of the sintered body was measured number of each extruded filament were put in the con 含 Fig 1. BSE-SEM micrographs of sintered AlO]-(m-ZrO2)Zro2 composite body fabricated by fibrous monolithic process:(akd) were obtained by the second, third, fourth and fifth extrusions, respectivelywith a binder in a shear mixer (C.W. Brabender Instru￾ments, Inc., PL2000 Plasti-Corder, USA), in which Al2O3 25 vol.% (m-ZrO2) was prepared using a ball mill￾ing process; m-ZrO2 and t-ZrO2 indicate monoclinic and tetragonal ZrO2, respectively. The m-ZrO2 was added to suppress the grain growth of Al2O3 during sintering. The average particle size of the starting materials was less than 300 nm for Al2O3 (AKP-50, Sumimoto, Japan) and 80 nm for ZrO2 powder (Toso, Japan). The binder material was composed of ethylene vinyl acetate (EVA) in granules (Elvax 250, Dupont). For lubrication during blending, stearic acid (CH3(CH2)16COOH, Dae￾jung Chemicals & Metals Co., Ltd) was added. The vol￾ume per cent of the mixture was 50/50 for the Al2O3 powder/polymer and 40/60 for the ZrO2 powder/poly￾mer. In order to blend each powder with polymer, the mixing head and chamber of the blender were first heated to 130 C using an oil-type heating source, fol￾lowed by addition of the polymer. After rotating the head for about 120 s, each ceramic powder was added slowly. Mixtures of Al2O3–ZrO2/polymer and ZrO2/polymer were extruded into filaments of 3.5 mm in diameter with an area reduction ratio of 70:1. The first passed Al2O3– ZrO2/polymer filament and ZrO2/polymer filament were reloaded into the extrusion die with an identical volume. The extrusion was repeated until the fifth-pass filament was obtained. The extrusion temperature was 120 C and the rate was 15 mm/min. Binder burning-out of the filaments was carried out at 700 C under flowing nitrogen atmosphere, and sintered at 1450 C. Microstructure and fracture surface of the filaments were examined using backscattered electrons (BSE) in a scanning electron microscope (SEM) (JSM 6331 F, Japan). The density of the sintered body was measured using Archimedes principle. Hardness of the sintered body was measured using a Vickers hardness tester (HV-112, Akashi, Japan) under a pressure of 1.96 N for 10 s. The fracture toughness, K1C was obtained using a combination of the indentation method and the Evans equation at a load of 10 kg as follows, K1C ¼ AðE=HÞ 1=2 ðP=c3=2 Þ where A is a material-independent constant (=0.016), E is the elastic modulus of the materials being indented, H the hardness, P the applied load and c the crack half￾length. In order to measure the bending strength, four-point bending tests were carried out using a universal testing machine (UnitechTM, R&B, Korea), in which the round-type of filament as extruded was used for the test specimen. The distance between the upper rolls was 30 mm and between the bottom rolls 10 mm. 3. Results and discussion Fig. 1(a)–(d) shows BSE-SEM micrographs of a sin￾tered Al2O3–(m-ZrO2)/ZrO2 composite body fabricated by the multi-extrusion process, in which Fig. 1(a)–(d) show the second-, third-, fourth- and fifth- pass fila￾ments, respectively. The composite presents a homoge￾neous distribution of Al2O3–(m-ZrO2) and ZrO2 phases. The ZrO2 phase appears to be brighter than Al2O3–(m-ZrO2) phases. In order to arrange both the fil￾aments in a certain pattern as shown in Fig. 1, the fila￾ments of composition Al2O3–(m-ZrO2) and ZrO2 were initially prepared using the extrusion process. The size of both the filaments was 3.5 mm in diameter. The same number of each extruded filament were put in the con￾Fig. 1. BSE-SEM micrographs of sintered Al2O3–(m-ZrO2)/ZrO2 composite body fabricated by fibrous monolithic process: (a)–(d) were obtained by the second, third, fourth and fifth extrusions, respectively. 726 T.-S. Kim et al. / Scripta Materialia 52 (2005) 725–729