PROCESSING AND MICROSTRUCTURE DEVELOPMENT IN ALUMINA-SILICON CARBIDE INTRAGRANULAR PARTICULATE COMPOSITES

Al2O3-SiC particulate composites were fabricated by hot-pressing mixtures or 5-30 vol% SiC with either alpha-Al2O3, gamma-Al2O3, or boehmite (gamma-AlOOH) to determine whether grain growth or the alpha-alumina phase transformation could be used to fabricate intragranular particulate composites. Samples starting with alpha-alumina resulted in primarily intergranular SiC of 0.3 mum and an alumina grain size of 1.5-4.1 mum. Heat treatments resulted in SiC coarsening but no entrapment of SiC by grain boundary breakaway. The alpha-alumina transformation in the samples starting with gamma-alumina resulted in the entrapment of approximately 48% of the 5 vol% of SiC added whereas 79% of the SiC was entrapped in the alpha-alumina grains in samples starting with boehmite. Only SiC particles less-than-or-equal-to 0.2 mum were entrapped in the alpha-alumina grains during the phase transformation. With increasing SiC content, the relative volume of intragranular SiC decreased, but the amount of intragranular SiC was constant and independent of the amount of SiC added before transformation. The formation of intragranular composites from gamma-alumina and boehmite samples was explained with a model that attributes particle entrapment to the vermicular growth of alpha-alumina into the transition alumina matrix during the alpha-alumina phase transformation. Seeding the boehmite-based samples did not affect the concentration of entrapped SiC, but did lower the hot-pressing densification temperature by as much as 150-degrees-C.