264 G. Hilmas et al./ Materials Science and Engineering A195(1995)263-268 [6 because of a combination of bridging of tensile consolidated by hot pressing at 1250C for 1 h at 2 cracks by Ni ligaments and shear cracking at the MPa. After machining test bars, the zirconia-Ni speci alumina-Ni interface. For the oxide-metal fibrous mens were annealed at 1100C in an N2+5%H, monoliths,oxide green fibers are dip coated with a atmosphere for 3 h. The density was 6.16 g cm 3, with slurry of Nio, which forms nickel on reduction pore-free zirconia cells and microstructural evidence for porosity in the Ni cell boundaries. The Ni content is 25 vol % by stereological point count analysis on a 2. Experimental procedure polished surface. 2. 1. Fabrication fibrous monoliths 2. 2. Mechanical property characterization The silicon nitride and silicon carbide green fibers Flexural strength was measured at room tempera were produced by melt spinning a blend of 51 vol. ture in four-point bending with a 20 mm inner span (EEA, DPDA NT, Union Carbide Chemicals and driven testimo Malar(Instron model 4483, Instron was compounded with molten polymer, then extruded min Specimens were machined to dimensions of 3 through a 320 um die using a laboratory scale fiber mm x 4 mm x 47 mm and chamfered with a 1000 grit extrusion machine( Bradford University Research Ltd resin-bonded diamond wheel, with the machining Bradford, UK). The silicon carbide green fibers were direction along the axis of the bar. Each hot-pressed made from 78.39 wt %B-SiC powder(B-10 B-SiC, billet produced only five flexural bars, so typically just Hermann C. Starck, Inc, Newton, MA),11.81 wt% one bar was available for each testing temperature aiN powder( Keramont Advanced Ceramic Products, Results are reported as load vS deflection. The Tucson, AZ)and 9.80 wt. Al,O, powder(A-16 SG, flexural stress values are correct until the first load Alcoa, Pittsburgh, PA). The composition of the silicon drop. Beyond that point the bar delaminates, so nitride was 88 wt. Si3 N4 powder(E-10 silicon nitride stresses cannot be calculated from beam theory. We powder, Ube Industries, Tokyo, Japan) with 9 wt% report the peak flexural stress as the apparent flexural Y2O3+3 wt% AL2O3. The green fibers were coated strength of the bar. The specimen continues to support with a 10-20 um layer of boron nitride by dip coating some load to deflections so large that the bar is visibly the fiber in a slurry of bn powder(HCP grade, bent. The test is terminated when the load drops to Advanced Ceramics Corp, Cleveland, OH) 50N. The work of fracture is calculated from the The coated green fibers were arranged into a uni- cross-sectional area of the bar and the area under the xial alignment in a graphite hot pressing die and warm load-deflection curve in the non-elastic region pressed at about 150C. The warm pressing collapses between the first load drop until the load falls to 50 N the green fibers and molds the material into a mono- Young s modulus, determined by the impulse excita- lithic block of green ceramic. The polymer binder was tion technique(Grindo-sonic MK4, J W. Lemmons removed by heating at 30C h-I to 400C in an Inc, St Louis, MO)on bars excited in a flexural mode, actively pumped vacuum Silicon nitride-BN materials was 290#6 GPa for the Si3N4-BN,374+6 GPa for were hot pressed at 1750'C for 1 h at 25 MPa, achiev- the BS 80 SiC-BN, and 21 1 GPa for the zirconia-Ni ing essentially full density at 3. 2 g cm-3. The volume fraction of BN was 12 vol %, as determined by quanti- tative metallography. The bs80 silicon carbide-Bn 3. Results and discussion materials were hot pressed at 2100C for 1 h at 25 MPa, to achieve a density of 3. 1 g cm"3, close to the 3. 1. Structure of the fibrous monoliths rule-of-mixtures theoretical density for this composite Volume fraction of BN was 20 vol % The polytypes in Fig. 1 shows the cell-cell boundary structure of a the BS80 SiC were determined by X-ray diffraction to BS80 SiC-BN fibrous monolith, where the axially be primarily 6H with about 20% 2H and a trace of aligned Sic cells appear grey and the bn cell bound residual 3C aries are in bright contrast. The end-on view(Fig. 1(a) A zirconia-Ni fibrous monolith was prepared by shows that the polycrystalline Sic cells are about 250 extruding green fibers with zirconia-3 moL% yttria um across, with a roughly hexagonal shape created powder(HSY-3.0, Tosoh USA, Atlanta, GA). These during molding of the soft green fibers. The BN cell were prepared as a fibrous monolith with Nio as the boundaries are about 10-20 um wide. Fig. 1(b)is the cell boundary material. After binder burn-out the Nio side view of the same specimen, showing long Sic cells was reduced to nickel metal, and the ceramic was gradually weaving into the plane of polish. The Si,3N4264 G. Hilmas et al. / Materials Science and Engineering A195 (1995) 263-268 [6] because of a combination of bridging of tensile cracks by Ni ligaments and shear cracking at the alumina-Ni interface. For the oxide-metal fibrous monoliths, oxide green fibers are dip coated with a slurry of NiO, which forms nickel on reduction. 2. Experimental procedure consolidated by hot pressing at 1250 °C for 1 h at 25 MPa. After machining test bars, the zirconia-Ni speci￾mens were annealed at 1100 °C in an N2+5% H 2 atmosphere for 3 h. The density was 6.16 g cm -3, with pore-free zirconia cells and microstructural evidence for porosity in the Ni cell boundaries. The Ni content is 25 vol.%, by stereological point count analysis on a polished surface. 2.1. Fabrication of fibrous monoliths 2.2. Mechanical property characterization The silicon nitride and silicon carbide green fibers were produced by melt spinning a blend of 51 vol.% ceramic powder mixture in ethylene ethylacrylate resin (EEA, DPDA 6182 NT, Union Carbide Chemicals and Plastics Company, Inc., Danbury, CT). The powder was compounded with molten polymer, then extruded through a 320 /zm die using a laboratory scale fiber extrusion machine (Bradford University Research Ltd., Bradford, UK). The silicon carbide green fibers were made from 78.39 wt.% r-sic powder (B-10 r-sic, Hermann C. Starck, Inc., Newton, MA), 11.81 wt.% AIN powder (Keramont Advanced Ceramic Products, Tucson, AZ) and 9.80 wt.% A1203 powder (A-16 SG, Alcoa, Pittsburgh, PA). The composition of the silicon nitride was 88 wt.% Si3N 4 powder (E-10 silicon nitride powder, Ube Industries, Tokyo, Japan) with 9 wt.% Y203 + 3 wt.% A1203. The green fibers were coated with a 10-20/zm layer of boron nitride by dip coating the fiber in a slurry of BN powder (HCP grade, Advanced Ceramics Corp., Cleveland, OH). The coated green fibers were arranged into a uni￾axial alignment in a graphite hot pressing die and warm pressed at about 150 °C. The warm pressing collapses the green fibers and molds the material into a mono￾lithic block of green ceramic. The polymer binder was removed by heating at 30 °C h -~ to 400 °C in an actively pumped vacuum. Silicon nitride-BN materials were hot pressed at 1750 °C for 1 h at 25 MPa, achiev￾ing essentially full density at 3.2 g cm-3. The volume fraction of BN was 12 vol.%, as determined by quanti￾tative metallography. The BS80 silicon carbide-BN materials were hot pressed at 2100 °C for 1 h at 25 MPa, to achieve a density of 3.1 g cm -3, close to the rule-of-mixtures theoretical density for this composite. Volume fraction of BN was 20 vol.%. The polytypes in the BS80 SiC were determined by X-ray diffraction to be primarily 6H with about 20% 2H and a trace of residual 3C. A zirconia-Ni fibrous monolith was prepared by extruding green fibers with zirconia-3 mol.% yttria powder (HSY-3.0, Tosoh USA, Atlanta, GA). These were prepared as a fibrous monolith with NiO as the cell boundary material. After binder burn-out the NiO was reduced to nickel metal, and the ceramic was Flexural strength was measured at room tempera￾ture in four-point bending with a 20 mm inner span and 40 mm outer span. Testing was done on a screw￾driven testing machine (Instron model 4483, Instron Corp., Canton, MA) at a cross-head speed of 0.05 mm min-1. Specimens were machined to dimensions of 3 mm x 4 mm x 47 mm and chamfered with a 1000 grit resin-bonded diamond wheel, with the machining direction along the axis of the bar. Each hot-pressed billet produced only five flexural bars, so typically just one bar was available for each testing temperature. Results are reported as load vs. deflection. The flexural stress values are correct until the first load drop. Beyond that point the bar delaminates, so stresses cannot be calculated from beam theory. We report the peak flexural stress as the apparent flexural strength of the bar. The specimen continues to support some load to deflections so large that the bar is visibly bent. The test is terminated when the load drops to 50 N. The work of fracture is calculated from the cross-sectional area of the bar and the area under the load-deflection curve in the non-elastic region between the first load drop until the load fails to 50 N. Young's modulus, determined by the impulse excita￾tion technique (Grindo-sonic MK4, J.W. Lemmons, Inc., St. Louis, MO) on bars excited in a flexural mode, was 290 _+ 6 GPa for the Si3N4-BN, 374_+ 6 GPa for the BS 80 SiC-BN, and 211 GPa for the zirconia-Ni. 3. Results and discussion 3.1. Structure of the fibrous monoliths Fig. 1 shows the cell-cell boundary structure of a BS80 SiC-BN fibrous monolith, where the axially aligned SiC cells appear grey and the BN cell bound￾aries are in bright contrast. The end-on view (Fig. l(a)) shows that the polycrystalline SiC cells are about 250 /~m across, with a roughly hexagonal shape created during molding of the soft green fibers. The BN cell boundaries are about 10-20 pm wide. Fig. l(b) is the side view of the same specimen, showing long SiC cells gradually weaving into the plane of polish. The Si3N 4-