Oxynitride glasses and their properties - Implications for high temperature performance of silicon nitride-based ceramics

Oxynitirde glasses are found at triple point junctions and as intergranular films in silicon nitride based ceramics. The glass chemistry, particularly the content of modifyer,usually Y or a rare earth (RE) ion, and the volume fractions of these oxynitride glass phases within the ceramic control the properties of silicon nitride, in particular, creep at high temperature. It is known that, as nitrogen substitutes for oxygen in silicate and alumino-silicate glass networks, increases are observed in glass transition and softening temperatures, viscosities (by two to three orders of magnitude), elastic moduli and microhardness. If changes are made to the RE:Si:Al ratios or different rare earth cation are substituted, properties such as viscosity can be increased by a further two to three orders of magnitude. These effects have implications for the high temperature properties of silicon nitride based ceramics, especially creep resistance. This paper provides an overview of oxynitride glasses and outlines the effect of composition on properties such as glass transition temperature and viscosity and discusses the effects on high temperature behaviour of silicon nitride ceramics.