MICROSTRUCTURE CHARACTERIZATION OF GAS-PRESSURE-SINTERED BETA-SILICON NITRIDE CONTAINING LARGE BETA-SILICON NITRIDE SEEDS

Fine beta-Si3N4 powders with or without the addition of 5 wt% of large beta-Si3N4 particles (seeds) were gas-pressure sintered at 1900-degrees-C for 4 h using Y2O3 and Al2O3 as sintering aids. The microstructures were examined on polished and plasma-etched surfaces. These materials had a microstructure of in situ composites with similar small matrix grains and different elongated grains. The elongated grains in the materials with seeds had a larger diameter and a smaller aspect ratio than those in the materials without seeds. A core/rim structure was observed in the elongated grains; the core was pure beta-Si3N4 and the rim was beta-SiAlON. These results show that the large beta-Si3N4 particles acted as seeds for abnormal grain growth and the rim was formed by precipitation from the liquid containing aluminum.