Grain-Boundary Films in A Silicon Nitride Ceramic at High Temperatures

Abstract

The high-temperature microstructure of an MgO-sintered Si3N4 (NC-132) was investigated. Thin samples were heated to temperatures between 1350°C and 1650°C for various times and then quenched to “freeze-in” the high-temperature microstructure. The grain-boundary film thickness was found to depend on temperature and residence time prior to quenching. Rapid heating to temperatures just above the eutectic temperature, followed shortly by quenching, resulted in large increases in intergranular film thickness due to solution of Si3N4 in the glass; the large variation in film widths observed at different grain boundaries indicated a condition of nonequilibrium. For higher temperatures and/or longer times at temperatures, the increased amorphous phase at the grain-boundaries could be redistributed to the multiple-grain junctions by either viscous flow or diffusion of Si3N4 due to a chemical potential gradient in the amorphous phase. Redistribution of glass resulted in film thicknesses slightly greater than those found at room temperature, due to small compositional changes of the glass. Equilibrium film thicknesses were obtained when liquid phase redistribution was not kinetically limited.