Pressureless-sintered ceramics based on the compound Si 2N 2O

Abstract

Si 2N 2O is a tetrahedral structure of SiN 3O units which has many of the required intrinsic properties of a major phase in high temperature ceramics, similar to the more intensively studied Si 3N 4. The rapid decomposition of Si 2N 2O above 1700°C dictates the use of ternary eutectics to effect liquid phase sintering below this temperature. In this work the eutectic has been used to sinter Si 2N 2O ceramics to nearly the theoretical density without the application of pressure. Sintered microstructures are composed of submicron faceted Si 2N 2O crystals within a semicontinuous matrix of YSiAlON eutectic glass of about 10 vol.%. The crystal morphology is that of elongated pseudo-hexagonal prisms, similar to the β′ phase in Syalon ceramics. The prism axis is [001] with facet planes parallel to (100), (110) and (1 1 0) in the orthorhombic unit cell. A post-sintering heat treatment at 1250–1350°C induces a crystallization of the matrix glass to Y 2Si 2O 7 with a redistribution of aluminium and oxygen by substitution in Si 2N 2O. A brief property survey shows that hardness, fracture toughness and oxidation resistance are comparable with the corresponding values for the related ceramics based on β-Si 3N 4.