TEM ANALYSIS OF CORE RIM STRUCTURE IN BET A-SILICON NITRIDE CERAMICS/

Fine beta-Si3N4 powders containing 5 mass% of large beta-Si3N4 particl es were gas-pressure sintered at 1900-degrees-C for 4h using Y2O3 and Al2O3 as sintering aid. A core/rim structure was observed by scanning and transmission electron microscopy. The core was pure beta-Si3N4 and the rim was beta-sialon. The electron diffraction patterns indicated that the core and rim had the same orientation, showing that the rim w as epitaxially grown on the core. Lattice mismatch was observed at the core/rim boundary by HREM. These results show that the relatively lar ge beta-Si3N4 particles in raw material powder acted as seeds for grai n growth and that the rim was formed by epitaxial precipitation from l iquid containing Al. When beta-Si3N4 is used as the raw material powde r, the particle size distribution is especially important for the deve lopment of in-situ composite microstructure.