ALPHA-]BETA REVERSE-PHASE TRANSFORMATION IN SILICON-CARBIDE IN SILICON-NITRIDE PARTICULATE-REINFORCED SILICON-CARBIDE COMPOSITES

The alpha --> beta reverse transformation in SiC is observed in Si3N4- particulate-reinforced-SiC composites made from as-received alpha-SiC and alpha-Si3N4 powders. However, the transformation does not occur to any great extent in composites made from deoxidized Si3N4-SiC powder compacts, Detailed transmission electron microscopy shows that most in terfaces are covered with an similar to 10 Angstrom thick amorphous in tergranular him in the composites made from as-received powders, where as most interfaces are free of such films in the composites made from deoxidized powder compacts, These observations indicate that the alpha --> beta reverse transformation in SiC is encouraged by a nitrogen-co ntaining liquid phase that occurs at high temperature in the composite s made from the as-received powders, A mechanism is proposed to accoun t for the experimental observations.