MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF NANOSCALE SIC CA ALPHA-SIALON COMPOSITES/

Ceramic composites comprising nanoscale (less than 200 nm) silicon car bide particles distributed in a matrix of calcium alpha-SiAION (alpha' ) have been prepared by uniaxial hot pressing, and the reaction sequen ce, during densification of a sample containing 20 wt% SiC, has been i nvestigated in the temperature range 1400-1800 degrees C. Samples cont aining up to 20 wt % SiC were produced to near full density by pressur e sintering at temperatures as low as 1600 degrees C for 1 h. For samp les with higher SiC contents subjected to a similar treatment, there w as residual porosity which was detrimental to the mechanical propertie s. The SiC was preferentially distributed intergranularly within the a lpha-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 hardnes s and the three-point bending strength of the composite samples increa sed markedly with increasing SiC content up to a level of 20 wt %. For a sample containing 20 wt % SiC, the bending strength was about simil ar to 1.5 times that of single-phase alpha-SiAION. For samples with hi gher SiC contents, the rate of increase in hardness was diminished and the bending strength decreased because of incomplete densification. T he initial improvement in fracture strength with increasing SiC conten t is plausibly attributable to the uniform refined grain structure of the composite materials, assuming that the maximum flaw size scales wi th the grain size.