Transformation of β-SiC in Sintering Process

It has been reported that a unique polytype transformation (6H→4H) in the sintering process of SiC ceramics with BeO addition was observed by means of analytical electron microscopy, in which α-SiC powder was used as a starting material. In this report, the sintering process of β-SiC was investigated. It was expected that the transformation would occur more remarkably in β-SiC powder than in α-SiC because there exists α β to α transformation in the sintering process. SiC powders with and without addition of BeO, AlN and B+C were prepared to study the effects of additives on the transformation. Sintered SiC bodies were hot-pressed at 1850°, 1950°, 2050° and 2200°C, and the structural changes were examined by means of X-ray diffraction and analytical electron microscopy. The results are summarized as follows. Without additives, grain growth was observed. However, the β to α transformation did not occur up to 2050°C. With additives, the transformation occurred at a temperatures dependent on the additives. With addition of BeO or AlN, α rapid β to α (4H) transformation occurred between 1850° and 1950°C. The transformation with B+C addition occurred above 2050°C and 6H polytype dominated. The different transformation temperatures might be responsible for the difference in polytypes after the transformation. The 4H structure formed in the presence of BeO or AlN was stable even at 2200°C.