Grain-boundary-phase crystallization and strength of silicon nitride sintered with a YSiAlON 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 deg C, only beta -Y sub 2 Si sub 2 O sub 7 was crystallized at the grain boundaries. At a higher temperature of 1450 deg C, primarily YSiO sub 2 N and Y sub 4 Si sub 2 O sub 7 N sub 2 in addition to small amounts of Y sub 2 SiO sub 5 were present. Aluminum existed only in high concentrations in residual amorphous phases, and in solid solution with Si sub 3 N sub 4 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 deg 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. Photomicrographs, Phase diagrams, Graphs. 33 ref.--AA