MICROSTRUCTURE OF AN EXTRUDED BETA-SILICON NITRIDE WHISKER-REINFORCED SILICON-NITRIDE COMPOSITE

Beta-Si3N4 whisker-reinforced beta'SiAlON composites were fabricated by extrusion and densified, using pressureless sintering. Whisker alignment was observed in both the green and sintered microstructures. SEM analysis of polished, sintered samples showed a microstructure consisting of the original beta-Si3N4 whiskers in a matrix of fine SiAlON grains. SEM of plasma-etched samples and TEM analysis showed that the whiskers, as a result of grain growth, consisted of two phases, a core and a sheath layer. X-ray mapping and EDS analysis revealed that the core material contained no trace of Al, confirming the presence of original beta-Si3N4 whiskers. The composition of the sheath was qualitatively identical to that of the fine beta'SiAlON grains in the matrix. The sheath was thus formed by the precipitation of the beta'SiAlON during liquid-phase sintering and led to substantial growth of the whiskers. Microdiffraction showed that the beta'SiAlON grew epitaxially on the beta-Si3N4 whiskers, resulting in a heavily faulted SiAlON layer.