Grain boundary glasses in silicon nitride: A review of chemistry, properties and crystallisation

Silicon nitride for engineering applications is densified by liquid phase sintering using oxide additives such as yttria and alumina. The oxynitride liquid remains as an intergranular glass. This paper provides a review of microstructural development in silicon nitride, grain boundary oxynitride glasses and effects of chemistry on properties. Nitrogen increases T _g, viscosities, elastic moduli and microhardness. These property changes are compared with known effects of grain boundary glass chemistry in silicon nitride ceramics where significant improvements in fracture resistance of silicon nitride can be achieved by tailoring the intergranular glass chemistry. Crystallisation of the grain boundary Y-Si-Al-O-N glass phase can improve properties. Nucleation and crystallisation of a Y-Si-Al-O-N glass, similar to that found in grain boundaries of silicon nitride densified with yttria and alumina, can be optimised to form different Y-disilicate polymorphs at different temperatures. One solution to provide a single disilicate phase over a range of temperatures is discussed.