grained and very porous, consisting mainly of the unconverted 3C phase. As the a polytypes developed at higher temperatures, elongated, interlocking crystals formed, increasing in size with increasing temperature.
DISCUSSIONThe properties and structure of an S i c body are controlled by several factors that influence sintering behavior. The characteristics of the initial powder, fabricating methods, and sintering conditions control the phase transformations and the resultant crystal morphology produced by firing. In p-Sic, the chunky cubic-phase crystals ot the starting material progressively transform to elongated a polytype crystals at 2000°C. This morphology is less conducive to additional sintering than a more equiaxed structure since the crystal surface areas and intergranular contact areas are reduced. The entrapment of pores and C in these long grains also retards further densification. The major portion of the shrinkage occurs below 2150°C when substantial quantities (-40%) of untransformed p phase remain and the newly formed a phases are relatively fine-grained. Above this temperature the grain growth of the increasingly dominant 6H crystals does not contribute to further shrinkage.
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