Oxynitride glasses as grain boundary phases in silicon nitride: Correlations of chemistry and properties

Silicon nitride is recognized as a high performance material for both wear resistant and high temperature structural applications. Oxide sintering additives, such as yttrium or rare earth oxides plus alumina or magnesia, are used in processing the ceramic to provide conditions for liquid phase sintering. The oxynitride liquid promotes densification and on cooling remains as an oxynitride glass at triple point junctions and also as intergranular films between the elongated hexagonal β-Si₃N₄ grains. The properties of silicon nitride, especially fracture behavior and creep resistance at high temperatures are influenced by the glass chemistry, particularly the concentration of modifier, and the volume fraction within the ceramic. This paper provides an overview of liquid phase sintering of silicon nitride ceramics, grain boundary oxynitride glasses and the effects of chemistry and structure on properties. As nitrogen substitutes for oxygen in bulk oxynitride glasses, increases are observed in glass transition and softening temperatures, viscosities, elastic moduli and thermal expansion coefficient. These property changes are compared with known effects of grain boundary glass chemistry on properties of silicon nitride ceramics.