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

High-temperature microstructures of an MgO-hot-pressed Si3N4 and a Yb2 O3 + Al2O3-sintered/annealed Si3N4 were obtained by quenching thin spe cimens from temperatures between 1350-degrees and 1550-degrees-C. Quen ching materials from 1350-degrees-C produced no observable changes in the secondary phases at triple-grain junctions or along grain boundari es. 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 thic kness increased substantially from an initial approximately 1 nm in th e slowly cooled material to 1.5-9 nm in the quenched materials. The va riability of film thickness in a given material suggests a nonequilibr ium state. Specimens quenched froin 1550-degrees-C revealed once again thin (1-nm) intergranular films at all high-angle grain boundaries, i ndicating an equilibrium condition. The changes observed in intergranu lar-film thickness by high-resolution electron microscopy can be relat ed to the eutectic temperature of the system and to diffusional and vi scous processes occurring in the amorphous intergranular film during t he high-temperature anneal prior to quenching.