EFFECT OF HEAT-TREATMENT OF SI3N4 ON GRAI N-GROWTH BEHAVIOR AND GRAIN-BOUNDARY STRUCTURE

Alpha-Si3N4 containing 8 mass% Y2O3 was hot-pressed at 1750 degrees C (SN1) and then heat-treated at 1900 degrees C for 2h (SN2). The micros tructure of the sintered materials was observed using a scanning elect ron microscope (SEM) and a transmission electron microscope (TEM). Bot h SN1 and SN2 had the microstructure composed of Si3N4 grains and grai n boundary phases located at two-grain junctions and multi-grain junct ions. Heat treatment promoted grain growth of beta-Si3N4; SN2 had a la rger grain size than SN1. The number of multigrain junctions decreased by heat treatment, whereas the size of multi-grain junctions increase d. The average width of the two-grain junction him increased by heat t reatment, suggesting that the composition of films changed. Crystallin e phase was observed at the multi-grain junctions of SN1, whereas thos e of SN2 were amorphous. The bend strength of SN2 at high-temperatures was lower than that of SN1 because of amorphous phase at multi-grain junctions and the composition change of thin films.