Journal of the American Ceramic Society-Lee and Kriven Vol 84. No 4 of 1C/min. The laminated green bodies were cold isostatically pressed(CIPed) at a pressure of 270 MPa for 5 min and then POROUS INTERPHASE pressureless-sintered at 1600C for 10 h(for the 3Al2O3. 2SiO matrix)or 2 h(for the Al,O, matrix)to densify the samples After the densified laminates were co-fired, they were cut int ALUMINA PLATELETS the form of bend bars The cutt on was along the longitudinal axis of the specimens in the plane of the lamination The bend bars, which were 30 mm long, 4.0 mm thick, and 3.0 mm wide, were tested in three-point flexure, without any surface polishing of the bend-bar specimens (2) Fabrication of Fibrous Monoliths The conventional fibrous monolithic forming method was adopted" but with some modification: a specific burnout process, RACK PROPAGATION as well as a post-burnout CIP step, was included. The first step in the fabrication of the fibrous composites was to batch the respec. tive polymer and ceramic powder in a high-shear, twin-roll mixer (C. w. Brabender Instruments, Inc, South Hackensack, NJ). In the Fig. 1. preparation of the fibrous ceramics, a paste(which consisted of matic illustration of proposed crack-deflection mechanism in 30-40 vol% polymer and 60-70 vol% ceramic powder)was posite containing porous alumina platelets and weak interphases Synergistic, energy-dissipating mechanisms of crack deflection, crack prepared from ethylene vinyl acetate(Elvax 470TM, which is a blunting, and grain bridging within pores are in operation. co-polymer thermoplastic that is manufactured by DuPont(Or- ange, TX) with a softening temperature of 120C and a density of -1 The mechanism will be demonstrated in the two matrices- powders(Al2O, for the matrix and Al2O, platelets for the Al,O3 and stoichiometric mullite(3A1,03 2SiO,hto compare interphase). The ceramic powders and the polymers were loaded their strengths. Conventional tape casting and co-extrusion tech into the shearing chamber, which was electrically heated to 150C niques will be used to engineer a series of composites in laminated When the high-shear mixing began, the viscosity of the resulting and fibrous monolithic+-configurations, respectively. The effects paste was monitored as a function of time of innovative modifications in design on the mechanical properties The AlzO3-platelet paste was warm-pressed into relatively thi of the composites also will be investigated. These properties weak layers, for use as cell boundaries in the fibrous composite include a bimodal variation of matrix interphase thickness ratio in AlO3-matrix sheets also were warm-pressed( Carver Press, Fred the laminated composites. In the fibrous monoliths, a triple- layer S. Carver, Inc., Menomonee Falls, wi) to a predetermined thick- repeating unit that has a"core/interphase/matrix"construction will be fabricated; this assembly ensures that no continuous weak path A2 O3 matrix and the Al2Or-platelet interphase. The pastes wer through the composite has been designed into the microstructure pressed at a temperature of 140.C under a pressure of 130 MPa for min. Hardened paste pieces of Al2O3 also were loaded into a cylinder assembly that had an inner diameter of 16 mm. Formation IL. Experimental Procedure of the Al,O, feeder rod via compression molding in a universal (I Fabrication of laminates testing machine (Model 4502, Instron Corp, Canton, MA)was Laminates of mullite(3AI03 2SiO2)and alumina(Al203)were conducted in the pressure range of 1-3 MPa; the ram spee bricated using the tape-casting process. These experiments used onitored at 20 mm/s, and the extrusion temperature was set at 3AL2032SiO, powder(KM Mullite-101, Kyoritsu, Inc, Nagoya, Japan), Al,O, powder(Al6SG, Alcoa, New Milford, CT), and After the specimens were formed, the Al2O3-platelet cell Al2O3 platelets(Atochem, Pierre-Benite, France). The slurries for boundary and Al,,-matrix layers were incorporated into the tape casting had the following composition: - 30 vol% oxide mIcrostructure. These layers were applied by wrapping the warm pressed sheet around the Al,, feeder rod; first the Al,O3-platelet butyral)(PVB)(0.5 wt%)(Monsanto, Inc. St. Louis, Mo) was layer, then the AlzO -matrix layer. Atter the green body was added to the slurries as a dispersant. The solvent was composed of tightly wrapped, it was warm-extruded through an orifice(2 mm in mixtures of toluene, n-butyl alcohol, and ethanol(all manufactured diameter) at a ram rate of 5 mm/s, with the extrusion temperature by Aldrich Chemical Co., Milwaukee, WI). The approximate set at 140C. As the green, layered filament came through the mixing ratio of toluene, n-butyl alcohol, and ethanol was 20: 20: 60 orifice of the spinneret, it was cut and then tightly packed into the (by vol%). The organics included PVB, which was used as a extrusion block. Then, a second warm-extrusion pass binder, and polyethylene glycol(PEG 2000) and dioctyl phthalate formed through the same spinneret. The exiting (Aldrich Chemical Co. ) each was used as a plasticizer. The collected and cut into segments 51 mm long, to be organics ratio was 50 wt% binder 50 wt% plasticizer. After th rectangular mold, and then warm-pressed at 140C slurry was pulverized, the binder and plasticizers were added and pressure of 27 MPa into a billet. ball-milled for 24 h. In the case of the Al,O3-platelet slurry, the The organic additives were removed by heating to 700C for 5h mixing was performed by just in an air atmosphere, with a slow heating schedule(0.1C/min) the working vIscosity. a acuI Of he& for 24 h, instead of ball platelet morphology. The within the range of 120%-200oC. After the burnout process, the slurries were stirred in a va to remove any bubbles and adjust samples wer of 270 MPa for 5 min and fter the slurries were aged for I d, they pressureless-sintered at 1600.C for 2 h. The sintered samples were ere tape-cast, using a doctor-blade opening of 130-300 um, to cut and tested in flexure in the same way as that mentioned for the obtain green sheets 50-150 um thick. The cast tapes were dried lamination process under a saturated solvent atmosphere for l d The green laminate composites had mm X 51 mm after the green sheets were exude Bend-bar samples were used to per- arrangement. Thermocompression was per three-point testing at room temperature in the 10 MPa for 10 min at a temperature of 80C, which was the previously mentioned universal testing machine, using a span oftening point of the organics. Subsequently, the organic addi- ength of 20 mm and a crosshead speed of 0.01 mm/min. The tives were removed by heating to 600%C in air, using a heating rate apparent work of fracture (WOF) was obtained by dividing theThe mechanism will be demonstrated in the two matrices— Al2O3 and stoichiometric mullite (3Al2O3z2SiO2)—to compare their strengths. Conventional tape casting and co-extrusion tech￾niques will be used to engineer a series of composites in laminated and fibrous monolithic42 configurations, respectively. The effects of innovative modifications in design on the mechanical properties of the composites also will be investigated. These properties include a bimodal variation of matrix:interphase thickness ratio in the laminated composites. In the fibrous monoliths, a triple-layer repeating unit that has a “core/interphase/matrix” construction will be fabricated; this assembly ensures that no continuous weak path through the composite has been designed into the microstructure. II. Experimental Procedure (1) Fabrication of Laminates Laminates of mullite (3Al2O3z2SiO2) and alumina (Al2O3) were fabricated using the tape-casting process. These experiments used 3Al2O3z2SiO2 powder (KM Mullite-101, Kyoritsu, Inc., Nagoya, Japan), Al2O3 powder (A16SG, Alcoa, New Milford, CT), and Al2O3 platelets (Atochem, Pierre-Be´nite´, France). The slurries for tape casting had the following composition: ;30 vol% oxide powders, ;55 vol% solvent, and ;15 vol% organics. Poly(vinyl butyral) (PVB) (0.5 wt%) (Monsanto, Inc., St. Louis, MO) was added to the slurries as a dispersant. The solvent was composed of mixtures of toluene, n-butyl alcohol, and ethanol (all manufactured by Aldrich Chemical Co., Milwaukee, WI). The approximate mixing ratio of toluene, n-butyl alcohol, and ethanol was 20:20:60 (by vol%). The organics included PVB, which was used as a binder, and polyethylene glycol (PEG 2000) and dioctyl phthalate (Aldrich Chemical Co.); each was used as a plasticizer. The organics ratio was 50 wt% binder:50 wt% plasticizer. After the slurry was pulverized, the binder and plasticizers were added and ball-milled for 24 h. In the case of the Al2O3-platelet slurry, the mixing was performed by just stirring for 24 h, instead of ball milling, to prevent breakage of the platelet morphology. The slurries were stirred in a vacuum to remove any bubbles and adjust the working viscosity. After the slurries were aged for 1 d, they were tape-cast, using a doctor-blade opening of 130–300 mm, to obtain green sheets 50–150 mm thick. The cast tapes were dried under a saturated solvent atmosphere for 1 d. The green laminate composites had area dimensions of 25 mm 3 51 mm after the green sheets were stacked in an alternating arrangement. Thermocompression was performed under a load of 10 MPa for 10 min at a temperature of 80°C, which was the softening point of the organics. Subsequently, the organic addi￾tives were removed by heating to 600°C in air, using a heating rate of 1°C/min. The laminated green bodies were cold isostatically pressed (CIPed) at a pressure of 270 MPa for 5 min and then pressureless-sintered at 1600°C for 10 h (for the 3Al2O3z2SiO2 matrix) or 2 h (for the Al2O3 matrix) to densify the samples. After the densified laminates were co-fired, they were cut into the form of bend bars. The cutting direction was along the longitudinal axis of the specimens in the plane of the lamination. The bend bars, which were 30 mm long, 4.0 mm thick, and 3.0 mm wide, were tested in three-point flexure, without any surface polishing of the bend-bar specimens. (2) Fabrication of Fibrous Monoliths The conventional fibrous monolithic forming method was adopted40 but with some modification: a specific burnout process, as well as a post-burnout CIP step, was included. The first step in the fabrication of the fibrous composites was to batch the respec￾tive polymer and ceramic powder in a high-shear, twin-roll mixer (C. W. Brabender Instruments, Inc., South Hackensack, NJ). In the preparation of the fibrous ceramics, a paste (which consisted of 30–40 vol% polymer and 60–70 vol% ceramic powder) was prepared from ethylene vinyl acetate (Elvax 470™, which is a co-polymer thermoplastic that is manufactured by DuPont (Or￾ange, TX) with a softening temperature of 120°C and a density of 0.94 g/cm3 ), drops of PEG 2000 (as a plasticizer), and the ceramic powders (Al2O3 for the matrix and Al2O3 platelets for the interphase). The ceramic powders and the polymers were loaded into the shearing chamber, which was electrically heated to 150°C. When the high-shear mixing began, the viscosity of the resulting paste was monitored as a function of time. The Al2O3-platelet paste was warm-pressed into relatively thin, weak layers, for use as cell boundaries in the fibrous composite. Al2O3-matrix sheets also were warm-pressed (Carver Press, Fred S. Carver, Inc., Menomonee Falls, WI) to a predetermined thick￾ness that was based on the desired thickness ratio between the Al2O3 matrix and the Al2O3-platelet interphase. The pastes were pressed at a temperature of 140°C under a pressure of 130 MPa for 1 min. Hardened paste pieces of Al2O3 also were loaded into a cylinder assembly that had an inner diameter of 16 mm. Formation of the Al2O3 feeder rod via compression molding in a universal testing machine (Model 4502, Instron Corp., Canton, MA) was conducted in the pressure range of 1–3 MPa; the ram speed was monitored at 20 mm/s, and the extrusion temperature was set at 145°C. After the specimens were formed, the Al2O3-platelet cell boundary and Al2O3-matrix layers were incorporated into the microstructure. These layers were applied by wrapping the warm￾pressed sheet around the Al2O3 feeder rod: first the Al2O3-platelet layer, then the Al2O3-matrix layer. After the green body was tightly wrapped, it was warm-extruded through an orifice (2 mm in diameter) at a ram rate of 5 mm/s, with the extrusion temperature set at 140°C. As the green, layered filament came through the orifice of the spinneret, it was cut and then tightly packed into the extrusion block. Then, a second warm-extrusion pass was per￾formed through the same spinneret. The exiting filament was collected and cut into segments 51 mm long, to be laid up in a rectangular mold, and then warm-pressed at 140°C under a pressure of 27 MPa into a billet. The organic additives were removed by heating to 700°C for 5 h in an air atmosphere, with a slow heating schedule (0.1°C/min) within the range of 120°–200°C. After the burnout process, the samples were CIPed at a pressure of 270 MPa for 5 min and then pressureless-sintered at 1600°C for 2 h. The sintered samples were cut and tested in flexure in the same way as that mentioned for the lamination process. (3) Characterization (A) Flexural Testing: Bend-bar samples were used to per￾form three-point flexural testing at room temperature in the previously mentioned universal testing machine, using a span length of 20 mm and a crosshead speed of 0.01 mm/min. The apparent work of fracture (WOF) was obtained by dividing the Fig. 1. Schematic illustration of proposed crack-deflection mechanism in a composite containing porous alumina platelets and weak interphases. Synergistic, energy-dissipating mechanisms of crack deflection, crack blunting, and grain bridging within pores are in operation. 768 Journal of the American Ceramic Society—Lee and Kriven Vol. 84, No. 4