STRESS-ENHANCED OXIDATION OF SILICON-NITRIDE CERAMICS

Silicon nitride ceramics show an accelerated oxidation rate under load in air. This phenomenon was observed for porous and dense ceramics with and without additives in a wide temperature range (700-degrees-1450-degrees-C) and can be interpreted as stress corrosion in oxygen-containing environments. Stresses cause an alteration of the amount and composition of oxidation products, formation of pits and cracks on stressed parts of specimens, and changes of the surface coloration and oxide scale morphology. Both tensile and compressive stresses can affect the oxidation process. An exponential dependence of mass gain on stress was found. On the other hand, oxidation of silicon nitride-based ceramics can affect the material response to mechanical stresses as, for example, deformation, cavitation and cracking. Stress-assisted chemical reaction at lower temperatures and stress-affected diffusion at higher temperatures seem to be the main reasons for the susceptibility of Si3N4 ceramics to stress corrosion. The effect of stress corrosion on mechanical properties is discussed.