Fine SiC inclusions and grain boundary phases in pressureless sintered SiAlON

Abstract

Abstract The starting powder, synthesized by carbothermal reduction, was pressureless sintered at temperatures over 1670°C to produce a dense SiAlON (Si 6−z Al z . O z N 8−z ) of z = 0·5. The dense material was characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and analytical electron microscopy (AEM), showing a microstructure featuring 1 μm, equiaxed, β′-grains bonded with a glassy phase. Fine SiC grains, 10–50 nm in size, were identified as inclusions in β′-SiAlON grains or at the grain boundaries. α′-SiAlON grains or whiskers which alloyed with Y and Ca were occasionally found in sizes less than 0·5 μm located in glassy pockets at grain boundaries or on the oversintered surface. Several other phases, normally non-detectable from XRD spectra, were found at grain boundaries by TEM, including yttrium-aluminum garnet (YAG), (Fe,Zr) Si x′ amorphous SiO x and elemental Si. The formation of SiC and other grain boundary phases is primarily due to the reducing atmosphere, chemistry of the matrix and neighboring glassy phase.