Oxynitride glasses: Preparation and properties - Implications for high temperature behaviour of silicon nitride ceramics

Oxynitride 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 influence the properties of silicon nitride, especially fracture behavior and also creep at high temperatures. As nitrogen substitutes for oxygen in silicate and alumino-silicate glass networks, increases are observed in glass transition and softening temperatures, viscosities, elastic moduli and microhardness. If changes are made to the cation ratios or different rare earth elements are substituted, properties such as viscosity can be increased further. These effects have implications for the mechanical properties of silicon nitride based ceramics, especially high temperature creep resistance. This paper provides an overview of the preparation and properties of oxynitride glasses and outlines the effect of composition on glass transition temperature and viscosity and discusses the effects on high temperature behavior of silicon nitride ceramics.