Microstructural Evolution in Liquid‐Phase‐Sintered SiC: Part II, Effects of Planar Defects and Seeds in the Starting Powder

Abstract

The effects of planar‐defect density in a β‐SiC starting powder and the addition of α‐SiC seeds to that powder on microstructural evolution in liquid‐phase‐sintered (LPS) SiC have been studied separately. Planar‐defect density is altered by appropriate heat treatment of an as‐received β‐SiC starting powder. It was found that a decrease in the planar‐defect density in the powder retards the β→α phase transformation rate. It is proposed that, because nucleation of α‐SiC occurs on the planar defects present in the β‐SiC starting powders, the nucleation rate and the attendant rate of transformation decrease with a reduction in planar‐defect density. Consequently, this reduces the frequency of formation of elongated β/α composite grains, resulting in lower average aspect ratios, as the initial untransformed β‐SiC grains coarsen in an equiaxed manner. In contrast, addition of external α‐SiC seeds has no effect on the β→α phase transformation rate, although a significant reduction in the average aspect ratio occurs. It is proposed that preferential equiaxed coarsening of the α‐SiC seeds over elongated coarsening of β/α composite grains occurs, resulting in a reduction of overall coarsening anisotropy.