Properties of rare earth oxynitride glasses and the implications for high temperature behaviour of silicon nitride ceramics

Rare earth oxynitride glasses occur as intergranular phases in silicon nitride and sialon ceramics. The volume fractions and compositions of these glass phases control the high temperature creep behaviour of the material. Investigations on oxynitride glass formation and properties have shown that the substitution of nitrogen for oxygen increases the glass transition and softening temperatures, viscosity, elastic modulus and hardness. By changing the cation ratios or the type of rare earth cation, properties such as viscosity can be increased further. This paper outlines the effects of composition on properties such as glass transition temperature and viscosity which have important implications for creep resistance of silicon nitride based ceramics containing intergranular oxynitride films.