Lawrence Berkeley National Laboratory

Free sintering of SiC with Al, B, and C additions in two successive stages, first under nitrogen and then under argon, produced a near full-density ceramic with equiaxed grain structure. The beta to alpha transformation proceeded to completion; however, the grain shape remained equiaxed due to the action of nitrogen present during the first stage of sintering. It is found that the beta to alpha transformation is necessary but not sufficient for producing the microstructure of interlocking plates found in high-toughness SiC.