Reversible α⇌ sialon transformation in heat-treated sialon ceramics

Typical β-sialon and mixed α-β-sialon starting compositions have been densified by pressureless sintering using either Ln₂O₃ or equimolar mixtures of Ln₂O₃ + Y₂O₃ where Ln = Sm, Dy, Yb. The resulting materials were heat treated at 1000-1500°C to devitrify the grain boundary glass into crystalline oxynitride phases. The effects of rare earth ionic size and heat-treatment temperature on the stability of devitrified phases, microstructure and mechanical properties have been studied. Microstructural characterisation of the α-sialon phase formed in these systems revealed that α is only stable at high temperatures and transforms to β-sialon plus grain boundary phases at lower temperatures; this provides an excellent mechanism for controlling the mechanical properties of the final material.