ADDITIVE-ASSISTED PRESSURELESS SINTERING OF CARBOTHERMAL BETA-SIALON - AN X-RAY AND SOLID-STATE MAS NMR-STUDY

beta-Sialon pellets (Si6-zAlzOzN8-z; z = 2.45) prepared from carbother mally synthesised sialon powder were sintered in N-2 at 1400-1700 degr ees C with additions of 15 mol% each of Al2O3, MgO and Y2O3, and 10 mo l% each of various combinations of these oxides. The most effective de nsifying agent is Y2O3, both alone and in combination; followed by MgO and Al2O3. XRD and multinuclear MAS NMR (Al-27, Si-29, Mg-25 and Y-89 ) studies revealed traces of X-phase sialon, SiC and an X-ray amorphou s glass-like phase in the starting material, and were used to monitor phase changes during Sintering. Y2O3 by itself and With other additive s forms Y3Al5O12 (YAG) with beta'-sialon; at higher temperatures, an a luminium-rich polytypoid sialon and yttrium-containing glass are forme d, to the detriment of the strength of the sintered body, MgO initiall y forms MgAl2O4, lowering the sialon z-value. At higher temperatures, this further reacts to form a glass containing magnesium in both tetra hedral and octahedral sites. Al2O3 reacts with beta'-sialon; increasin g its z-value. The NMR spectra confirm the XRD phase analyses and prov ide information about phases not detectable by XRD, and about the poss ible constitution of the glasses-formed.