Oxynitride glasses

Oxynitride glasses are silicates and alumino-silicates in which nitrogen atoms substitute for oxygen in the glass network. They are present as intergranular films and at triple points within the microstructures of silicon nitride-based ceramics as a result of the cooling of liquid phases formed by reaction of sintering additives with the nitride and surface silica. Oxynitride glass chemistry and volume fractions determine the type of microstructure developed and hence the properties of silicon nitride and sialons. In particular, they affect strength and fracture toughness and the control of high temperature creep behaviour. The desire to understand the nature of these grain boundary phases has resulted in a number of investigations on oxynitride glass formation, structure, properties and crystallisation which have shown that they have higher glass transition temperatures, elastic moduli, viscosities and values of hardness and lower thermal expansion coefficients than the corresponding oxide glasses. This paper reviews the development of oxynitride glasses and outlines the effect of glass composition, especially nitrogen content and also cation ratios, on properties and relates this to structural features within the glass. Nucleation and crystallisation studies are also outlined.