Corrosion of silicon nitride materials in acidic and basic solutions and under hydrothermal conditions

Silicon nitride ceramics with different amounts and compositons of the grain boundary phases are produced by gas pressure sintering and the corrosion behaviour was analysed in acids, bases and under hydrothermal conditions. The investigation shows, that the corrosion is strongly affected by the SiO2 content in the grain boundary and the amount of the additives. The results indicate that the grains and the thin films between the grains are very stable in acids and bases but solves partially under hydrothermal conditions. The corrosion behaviour of the investigated materials with a relatively stable grain boundary phase in acids can be described by the same kinetic laws as those used to describe the corrosion of glasses. An interfacial reaction is assumed to be rate-controlling. The less stable material with Y203/Al2O3 additives can be described by the corrosion behaviour of oxide nitride glasses only at the beginning. For higher corrosion depths, the formation of protective barriers consisting of a hydrated SiO2 network is assumed to occur.