Silicon Nitride Ceramics

This chapter reviews one of the important engineering ceramics developed over the last 40 years, silicon nitride and the related SiAlONs, a "family" of structural materials with high flexural strength, good fracture toughness, excellent creep resistance, and wear resistance. Microstructure-property relationships are outlined. Oxide-sintering additives, Y₂O₃ and Al₂O₃, react with silicon nitride and silica present on its particle surfaces to form an oxynitride liquid that promotes liquid-phase sintering and densification by solution-diffusion-precipitation. The liquid cools to form an intergranular glass. The amount and additive ratio in silicon nitride determines glass chemistry, which affects fracture toughness and strength. Thermomechanical properties of bulk oxynitride glasses increase with increasing nitrogen content as a result of substitution of two-coordinated oxygen by three-coordinated nitrogen allowing extra cross-linking of the glass network. Properties are also dependent on Y:Al ratio. Thermal expansion mismatch between the intergranular glass and the silicon nitride causes residual stresses, which can be calculated from bulk Y-Si-Al-O-N glass properties (E, T_g, and α). The tensile residual stresses in the glass-phase increase with increasing Y:Al ratio and this correlates with increasing fracture toughness of the ceramic as a result of easier debonding at the glass/β-Si₃N₄ interface.