Microstructural developments in pressureless-sintered US -SiC materials with Al, B, and C additions

Abstract

Fine-grained US -SiC powder with various combinations of Al, B, and C additions was pressureless-sintered to near-theoretical density and investigated using analytical electron microscopy. The additions were found to enhance the US -to- phase transformation and subsequent basal growth of -SiC, as confirmed by quantitative X-ray diffraction analysis. Formation of graphite was reduced when Al was present in the carbon-rich regions because fine Al4C3 particles formed. For low sintering temperatures, Al was distributed in amorphous carbon as Al4C3 particles, whereas at high sintering temperature (>1800C) Al was concentrated in pores and along grain boundaries as an Al-rich second phase. This phase was identified as mostly Al8B4C7 by energy-dispersive X-ray analysis, electron energy-loss spectroscopy, and selectedarea diffraction analysis. Formation of homogeneous microstructures and rapid polytypic transformation in this material may be explained by the homogeneous Al distribution at relatively low temperature, the local crystallographic coherency of the and US -phases, diffusion of the Al-rich second phase on the basal plane of -SiC, and formation of an Al4C3 layer grown epitaxially.