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

Al2O3-SiC particulate composites were fabricated by hot-pressing mixtu res 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 particul ate composites. Samples starting with alpha-alumina resulted in primar ily 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 S iC by grain boundary breakaway. The alpha-alumina transformation in th e samples starting with gamma-alumina resulted in the entrapment of ap proximately 48% of the 5 vol% of SiC added whereas 79% of the SiC was entrapped in the alpha-alumina grains in samples starting with boehmit e. Only SiC particles less-than-or-equal-to 0.2 mum were entrapped in the alpha-alumina grains during the phase transformation. With increas ing SiC content, the relative volume of intragranular SiC decreased, b ut the amount of intragranular SiC was constant and independent of the amount of SiC added before transformation. The formation of intragran ular composites from gamma-alumina and boehmite samples was explained with a model that attributes particle entrapment to the vermicular gro wth of alpha-alumina into the transition alumina matrix during the alp ha-alumina phase transformation. Seeding the boehmite-based samples di d not affect the concentration of entrapped SiC, but did lower the hot -pressing densification temperature by as much as 150-degrees-C.