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

Abstract

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.