FORMATION OF BORON-NITRIDE INCLUSIONS IN HOT ISOSTATICALLY PRESSED SILICON-NITRIDE SILICON-CARBIDE COMPOSITES

Small graphite-like boron nitride inclusions have been detected, using transmission electron microscopy, in silicon nitride particulate-rein forced silicon carbide composites hot isostatically pressed in tantalu m cans. The inclusions are thought to arise indirectly from fine parti cles of boron nitride sprayed onto the internal surface of the tantalu m can to prevent a chemical reaction between the tantalum and the sili con carbide. Boron oxide present on the surface of these particles is able to evaporate and penetrate the powder compacts during hot isostat ic pressing and then react with silicon nitride and silicon carbide to form the observed inclusions.