This paper explores the polytype distribution and polytypic transformations occurring during the reaction-bonding of a commercial silicon carbide, REFEL. The initially formed polytype is found to be almost exclusively β-SiC, which either deposits epitaxially on the original α-SiC grits or as fine material, in situ in the silicon. The exothermic nature of the reaction causes the epitaxially deposited material to undergo the β→α solid state transformation, usually going almost to completion in a few minutes. Two types of epitaxial material are distinguished ; type I has its stacking sequence directly seeded by the original grain, whilst type II has no direct contact with the original α stack, but can be seeded by 'sideways' nucleation. The fine (type III) material is never found to transform, either because of nucleation problems or because it is formed at a lower temperature. The kinetics β→α transformation indicate a peak local temperature of ~ 2000°C and the rapidity of the transformation is sufficient to rule out any large-scale diffusion. Other microstructural features are noted, including the unexpected presence of some 2H-SiC in certain 'nodules' in the compact, associated with high impurity levels and unusual reaction conditions. The observed behaviour emphasizes the importance of local temperature, impurities and nucleation effects in determining polytype stability. Further, the preferential initial growth of β-SiC confirms it as the high-symmetry, rapidly-forming polytype.
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Ness et al. (1986) studied this question.