Oxynitride glasses: Effects of composition on glass formation and properties with implications for high temperature behaviour of silicon nitride ceramics

Oxynitride glasses are silicates or alumino-silicates, containing Mg, Ca, Y or rare earth (RE) ions as modifiers, in which nitrogen atoms substitute for oxygen atoms in the glass network. These glasses are found as intergranular films and at triple point junctions in silicon nitride ceramics and these grain boundary phases affect their fracture behaviour and creep resistance. This paper provides an overview of the preparation of M-SiA1ON glasses and outlines the effects of composition on properties. As nitrogen substitutes for oxygen in oxynitride glasses, increases are observed in glass transition and softening temperatures, viscosities, elastic moduli and microhardness. These property changes are related to structure of the glasses and have also been compared with known effects of grain boundary glass chemistry in silicon nitride. The implication for behaviour of silicon nitride ceramics, particularly at high temperatures, is discussed. The paper also outlines new research on oxynitride glasses containing other anions, such as fluorine. The effects of glass chemistry on glass formation and on physical and mechanical properties is presented.