Amorphous Intergranular Films in Silicon Nitride Ceramics Quenched from High Temperatures

Abstract

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° and 1550°C. Quenching materials from 1350°C produced no observable changes in the secondary phases at triple‐grain junctions or along grain boundaries. Although quenching from temperatures of ∼ 1450°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 ∼ 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 from 1550°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.