May 1984 Communications of the American Ceramic Society C-85 Effect of Crystallization of the Grain-Boundary Phase on the Thermal Diffusivity of a Sialon

Crystallization of the glassy grain-boundary phase in a series of sialon ceramics fabricated using a range of hot-pressing schedules increased the thermal diffusivity at room temperature by an average of =lo%. For samples made by a given hot-pressing schedule, the relative difference in thermal diffusivity between composites containing a glassy grain-boundary phase and those in which this phase had crystallized decreased with increasing temperature. This behavior is attributed to enhanced phonon scattering in the crystalline grain-boundary phase. T H E DENSIFTCATION during sintering of polycrystalline ceramics for high-performance applications can be enhanced significantly by the addition of a sintering aid. This is particularly so for Si,N4-based ceramics, which do not densify readily without the assistance of additives. For Si3N4, additives commonly consist of the oxides of magnesium, yttrium, or alumi-num. At the fabrication temperatures, the presence of these additives results in the formation of a liquid phase at the grain boundaries. This liquid phase can act either as a lubricant to promote particle transport or as a medium for material transport by solution and reprecipitation of the matrix phase. On cooling, the liquid at the grain boundaries and triple points be-comes a glass. The presence of this grain-boundar