Microstructure and mechanical properties of nanoscale SiC/Ca alpha-SiAlON composites

Ceramic composites comprising nanoscale (less than 200 nm) silicon carbide particles distributed in a matrix of calcium alpha-SiAION (alpha') have been prepared by uniaxial hot pressing, and the reaction sequence, during densification of a sample containing 20 wt% SiC, has been investigated in the temperature range 1400-1800 degrees C. Samples containing up to 20 wt % SiC were produced to near full density by pressure sintering at temperatures as low as 1600 degrees C for 1 h. For samples with higher SiC contents subjected to a similar treatment, there was residual porosity which was detrimental to the mechanical properties. The SiC was preferentially distributed intergranularly within the alpha-SiAION and was effective in controlling grain growth of the alpha-SiAION during processing. There was an increase in the uniformity of the grain structure and a significant refinement of the grain size of the composite microstructures with increasing SiC content. The hardness and the three-point bending strength of the composite samples increased markedly with increasing SiC content up to a level of 20 wt %. For a sample containing 20 wt % SiC, the bending strength was about similar to 1.5 times that of single-phase alpha-SiAION. For samples with higher SiC contents, the rate of increase in hardness was diminished and the bending strength decreased because of incomplete densification. The initial improvement in fracture strength with increasing SiC content is plausibly attributable to the uniform refined grain structure of the composite materials, assuming that the maximum flaw size scales with the grain size.