AMORPHOUS INTERGRANULAR FILMS IN SILICON-NITRIDE CERAMICS QUENCHED FROM HIGH-TEMPERATURES

High-temperature microstructures of an MgO-hot-pressed Si3N4 and a Yb2O3 + Al2O3-sintered/annealed Si3N4 were obtained by quenching thin specimens from temperatures between 1350-degrees and 1550-degrees-C. Quenching materials from 1350-degrees-C produced no observable changes in the secondary phases at triple-grain junctions or along grain boundaries. Although quenching from temperatures of approximately 1450-degrees-C also showed no significant changes in the general microstructure or morphology of the Si3N4 grains, the amorphous intergranular film thickness increased substantially from an initial approximately 1 nm in the slowly cooled material to 1.5-9 nm in the quenched materials. The variability of film thickness in a given material suggests a nonequilibrium state. Specimens quenched froin 1550-degrees-C revealed once again thin (1-nm) intergranular films at all high-angle grain boundaries, indicating an equilibrium condition. The changes observed in intergranular-film thickness by high-resolution electron microscopy can be related to the eutectic temperature of the system and to diffusional and viscous processes occurring in the amorphous intergranular film during the high-temperature anneal prior to quenching.