Effect of heat treatment of Si₃N₄ on grain growth behavior and grain boundary structure

Alpha-Si₃N₄ containing 8 mass% Y²O₃ was hot-pressed at 1750°C (SN1) and then heat-treated at 1900°C for 2h (SN2). The microstructure of the sintered materials was observed using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Both SN1 and SN2 had the microstructure composed of Si₃N₄ grains and grain boundary phases located at two-grain junctions and multi-grain junctions. Heat treatment promoted grain growth of β-NSi₃N₄; SN2 had a larger grain size than SN1. The number of multigrain junctions decreased by heat treatment, whereas the size of multi-grain junctions increased. The average width of the two-grain junction film increased by heat treatment, suggesting that the composition of films changed. Crystalline phase was observed at the multi-grain junctions of SN1, whereas those 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.