Grain-Boundary-Phase Crystallization and Strength of Silicon Nitride Sintered with a YSlAlON Glass

Densifying silicon nitride with a YSiAlON glass additive produced 99% dense materials by pressureless sintering. Subsequent heat-treating led to nearly complete crystallization of the amorphous intergranular phase. Transmission electron microscopy revealed that for heat treatments at 1350°C, only β-Y₂Si₂O₇ was crystallized at the grain boundaries. At a higher temperature of 1450°C, primarily YSiO₂N and Y₄Si₂O₇N₂ in addition to small amounts of Y₂SiO₅ were present. Al existed only in high concentrations in residual amorphous phases, and in solid solution with Si₃N₄ and some crystalline grain-boundary phases. In four-point flexure tests materials retained up to 73% of their strengths, with strengths of up to 426 MPa, at 1300°C. High-strength retention was due to nearly complete crystallization of the intergranular phase, as well as to the high refractoriness of residual amorphous phases.