HIGH-TEMPERATURE THERMAL AND ENVIRONMENTAL STABILITIES OF BORON-NITRIDE, ALUMINUM NITRIDE AND SILICON-NITRIDE CERAMICS

The thermal and environmental stabilities of boron nitride (BN), aluminium nitride (AIN), and silicon nitride (Si3N4) ceramics have been evaluated using thermochemical modelling. The volatility diagrams of these materials have been established at 2000 K. Master volatility diagrams with equilibrium pressure of dominant vapour species at 1400, 1600. 1800 and 2000 K have also been constructed. The phase stability diagrams of BN and Si3N4 have been established to indicate the stability of various oxide and oxynitride phases in equilibrium with these nitrides at different oxygen and nitrogen pressures. These data show that in order to prevent dissociation and volatilization, nitrogen over-pressures are required during processing and/or operation at higher temperatures. Vaporization rate data indicate that under congruently vaporizing conditions, the mass losses of silicon nitride are greatest, followed by those of aluminium nitride and then boron nitride.