MICROSTRUCTURAL ANALYSIS OF SECONDARY PHASES IN SILICON CARBIDE FABRICATED WITH SIC NANO-POWDER AND SINTERING ADDITIVES

Chemical composition, structure and distribution of secondary phases in two kinds of monolithic SiC ceramics fabricated by liquid phase sintering with SiC nano-powder and 6 or 9 wt.% sintering additives of Al2O3 and Y2O3 were investigated. The major secondary phase in both samples was identified as crystalline yttrium aluminum garnet (YAG) by XRD pattern. EPMA-WDS analysis revealed that the sample containing 9 wt.% additives had the relatively distinct Y-rich and Al-rich regions, by contrast with relatively-homogeneous distribution of the additives elements in the sample containing 6 wt.% additives. The dominating secondary phases at triple junctions of YAG for Y-rich regions and Al2O3 for Al-rich regions in SiC (9 wt.% additives) were identified by TEM-EELS analysis. In SiC (6 wt.% additives), the dominating YAG was observed at triple junctions. High-resolution TEM revealed that amorphous films with content Y, Al and O existed at both the SiC-SiC and SiC-YAG grain boundaries.