| Research Abstract |
Fine and dense ceramic materials with controlled microstructures, including alumina, mullite, forsterite, stabilized zirconia and their composites, were fabricated by using various methods. Small amounts of additives, e.g., MgO or ZrO_2 in alumina, were introduced to control the microstructures, which affected significantly high-temperature mechanical properties of the materials. The hightemperature deformation of fine grained alumina is rate-limited by interface reaction, accompanied with hardening with strain due to dynamic grain growth. However, the addition of an oxide such as zirconia, which segregates at grain boundaries, suppresses appreciably the static and dynamic grain growth, and furthemore affects the yield/flow stress, suggesting that diffusivity or mobility of ions through grain boundaries depend on grain boundary compositions. Fracture and deformation of ceramic composites such as alumina-zirconia (AZ), mullite-zirconia (MZ) and mullite-alumina (MA), were investigated at high temperatures. In AZ materials, high temperature deformation and dynamic grain growth were depressed as compared with pure fine-grained alumina, which may be due to grain boundary segregation of zirconium as in the case of the alumina doped with small amounts of ZrO_2. In MZ and MA materials, experimental data concerning delayd fracture and slow crack growth were obtained and analyzed. Forsterite polycrystals fabricated by sol-gel derived powders were tested to study the high-temperature deformation. Effective diffusion coefficients for rate-determining species were obtained, showing the effects of residual MgO and glassy phases at grain boundaries.
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