High toughness, high strength sintered silicon nitride

Abstract

A silicon nitride sintered body has a composition consisting essentially of 85 to 94% by weight .beta. silicon nitride, 6 to 15% by weight grain boundary phase consisting essentially of (i) at least two rare earth elements, wherein yttrium is considered a rare earth, and, optionally, strontium which, calculated as SrO, is 0-2 percent by weight of the total body, and (ii) at least two of Si, N, O and C, and an additive consisting essentially of a metal-carbon compound present in the amount of about 0.2 to 3.5% by volume. The additive is substantially homogeneously dispersed within the sintered body. The sintered body has a density at least 95% of theoretical and the ceramic has high strength and high toughness and is especially suited for industrial applications such as components for gas turbine and automotive engines. The sintered body has a microstructure wherein (i) the .beta. silicon nitride grains are acicular and have an average grain width ranging from 0.5 to 1.5 .mu.m, (ii) at least 25% of the grains have width greater than 0.7 .mu.m, and at least 10% of the grains have width greater than 1 .mu.m, and (iii) no more than 5% of grains have width greater than 3.5 .mu.m and apparent aspect ratio greater than 5, with the proviso that the average aspect ratio is at least 1.8.

Dependencies are inferred from the claim text.