Joumal of the American Ceramic Sociery-Kovar et al. Vol. 80. No. 10 Ill. Mechanical Properties of Si3NBN facial sliding resistance. Particular emphas Fibrous Monoliths veloping a methodology to predict the properties, stre materials as Fibrous monoliths are novel materials. therefore. it is a function of architecture. Because fibrous monoliths are in- essary to identify the micromechanical properties that infl tended for use in applications where stresses are primarily gen- ring properties. These include the fracture erated because of bending, we focus on flexural properties tance of cells the interfacial fracture resistance. and the In some respects, fibrous monoliths are similar to ceramic- Panel A. Processing of fibrous monoliths Schematic illustrations of the steps used to fabricate binder, is 83 vol% sinterable Si,N-6 wt%Y203-2 wt% SigNa-BN fibrous monoliths are shown in Fig. Al. We start Al,O3(6Yn2Al-Si3 N4)and 17 wt% BN by mixing conventional ceramic powders in a polyme Sheets of uniaxially aligned green filaments are produced Starck and Co. Newton. MA. or SN-e- by winding the filaments around a mandrel and fixing them into place with a spray adhesive. Fibrous monolith speci New York, NY), consist primarily of equiaxed ax-SiaN4 par- mens are assembled from these sheets. Typically, 25 sheets ticles, nominally 0.5 um in diameter, with a BET specifi are used to produce a specimen. The uniaxially aligned surface area of m/g. The BN powder is a well- chitecture is produced by stacking the sheets without Advanced Ceramics Corp, Cleveland, OH) moplastic; therefore, after stacking, the assembly is molded The thermoplastic extrudable compound is made by into a solid block at temperatures between 1000 and 150C ing ceramic powder with thermoplastic polymers in a heated at a pressure of 2 MPa Shaped objects can be formed using mixer. The solids loading for the cell materials (Sis N conventional compression-molding dies. The filaments, Y2O3, and Al,O3)is 52 vol% ceramic, whereas the cladding which initially have a round cross section, deform during (BN)contains 50 vol% ceramic. After it is mixed, the Siy N4 this warm-pressing operation, filling the interstitial spaces compound is compression molded into a 20 mm diameter etween the filaments and producing flattened hexagon rod. A similar BN compound is compression molded into a shaped cells cylindrical shell, I mm thick, with a 20 mm inner diameter The thermoplastic binder is removed by heating slowly to The bn shell is fitted around the Si3N4 rod to make a 700C in a nitrogen atmosphere Hot pressing at 1750C fo for a piston-style extruder. The feedrod is then 2 h produces a density of 3.05 g/cm3,-98% of the estimated through a heated extrusion die to create 220 um er green filaments with the same Si3 N4 core and BN ng as the feedrod. The flexible green filament is col- onal ZrO,(contamination from the milling media a spool. The ceramic composition, excluding the Extrude feedrod into fine filament feedrod Si NA-filled polymer Form extrusion feedrod heated di spoo上 Hot-press to densify rolysis to remove polymer. binder Laminate sheets of filament to form solid billet Fig. Al. Schematic illustrations showing processing route to fabricate fibrous monolithsIII. Mechanical Properties of Si3N4–BN Fibrous Monoliths Fibrous monoliths are novel materials; therefore, it is nec￾essary to identify the micromechanical properties that influence the engineering properties. These include the fracture resis￾tance of cells, the interfacial fracture resistance, and the inter￾facial sliding resistance. Particular emphasis is placed on de￾veloping a methodology to predict the elastic properties, strength, and energy absorption capability of these materials as a function of architecture. Because fibrous monoliths are in￾tended for use in applications where stresses are primarily gen￾erated because of bending, we focus on flexural properties. In some respects, fibrous monoliths are similar to ceramic￾Panel A. Processing of Fibrous Monoliths Schematic illustrations of the steps used to fabricate Si3N4–BN fibrous monoliths are shown in Fig. A1. We start by mixing conventional ceramic powders in a polymer binder system. The commercial Si3N4 powders (M11, H. C. Starck and Co., Newton, MA, or SN-E-10, Ube Industries, New York, NY), consist primarily of equiaxed a-Si3N4 par￾ticles, nominally 0.5 mm in diameter, with a BET specific surface area of 9–13 m2 /g. The BN powder is a well￾crystallized, hexagonal BN powder consisting of platey par￾ticles 7–10 mm in diameter and 0.1–0.3 mm thick (HCP-BN, Advanced Ceramics Corp., Cleveland, OH). The thermoplastic extrudable compound is made by mix￾ing ceramic powder with thermoplastic polymers in a heated mixer. The solids loading for the cell materials (Si3N4, Y2O3, and Al2O3) is 52 vol% ceramic, whereas the cladding (BN) contains 50 vol% ceramic. After it is mixed, the Si3N4 compound is compression molded into a 20 mm diameter rod. A similar BN compound is compression molded into a cylindrical shell, 1 mm thick, with a 20 mm inner diameter. The BN shell is fitted around the Si3N4 rod to make a feedrod for a piston-style extruder. The feedrod is then forced through a heated extrusion die to create 220 mm diameter green filaments with the same Si3N4 core and BN cladding as the feedrod. The flexible green filament is col￾lected on a spool. The ceramic composition, excluding the binder, is 83 vol% sinterable Si3N4–6 wt% Y2O3–2 wt% Al2O3 (6Y/2Al–Si3N4) and 17 wt% BN. Sheets of uniaxially aligned green filaments are produced by winding the filaments around a mandrel and fixing them into place with a spray adhesive. Fibrous monolith speci￾mens are assembled from these sheets. Typically, 25 sheets are used to produce a specimen. The uniaxially aligned architecture is produced by stacking the sheets without ro￾tation, whereas, for the [0/±45/90] architecture, the filament direction is rotated between layers. The filaments are ther￾moplastic; therefore, after stacking, the assembly is molded into a solid block at temperatures between 100° and 150°C at a pressure of 2 MPa. Shaped objects can be formed using conventional compression-molding dies. The filaments, which initially have a round cross section, deform during this warm-pressing operation, filling the interstitial spaces between the filaments and producing flattened hexagon￾shaped cells. The thermoplastic binder is removed by heating slowly to 700°C in a nitrogen atmosphere. Hot pressing at 1750°C for 2 h produces a density of 3.05 g/cm3 , ∼98% of the estimated theoretical density for this composition. XRD shows the presence of b-Si3N4, hexagonal BN, and a trace of tetrag￾onal ZrO2 (contamination from the milling media). Fig. A1. Schematic illustrations showing processing route to fabricate fibrous monoliths. 2474 Journal of the American Ceramic Society—Kovar et al. Vol. 80, No. 10