Fine SiC inclusions and grain boundary phases in pressureless sintered SiAlON

Abstract

The starting powder, synthesized by carbothermal reduction, was pressureless sintered at temperatures over 1670[deg]C to produce a dense SiAlON (Si6-zAlz. OzN8-z) of z = 0[middle dot]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 [mu]m, equiaxed, [beta]'-grains bonded with a glassy phase. Fine SiC grains, 10-50 nm in size, were identified as inclusions in [beta]'-SiAlON grains or at the grain boundaries. [alpha]'-SiAlON grains or whiskers which alloyed with Y and Ca were occasionally found in sizes less than 0[middle dot]5 [mu]m 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) Six' amorphous SiOx 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.