Techniques for processing composites of SiC-Al2O3 and Si3N4-Al2O3

Vita.Composites of SiC - AI2O3 and SiC - mullite are unstable at high temperatures. The reactions occurring within the composites above 1700°C in stagnant inert atmospheres were characterized. Gaseous products cause excessive w eight losses which cannot be attributed to active oxidation. These losses can be successfully retarded by processing under high pressures. Compatible phases were determined by X -ray analysis and used to construct the tie tetrahedra existing at high temperatures in the Si - C - Al - O system. The reactions produced condensed phases of Al2OC and AI4O4C as well as gaseous SiO and CO. The condensed phases have high vapor pressures above 1700°C. The effect of these reactions on densification of composites by firing at different temperatures for various periods under different pressures was studied. Formed composites prepared under high pressures a t 1825°C were fired at 1700°C under normal pressure in argon, where active oxidation is expected, and in spite of the low loss in weight, deterioration in microstructure was observed. In Si3N4 - Al2O3 composites, α - Si3N4 transforms to β - Si3N4 over the investigated temperatures (above 1800°C), and β' - SiAlON and X -SiAlON were formed. Wet mixing different grain sizes, precipitating AI2O3 using different methods on dispersed SiC or Si3N4 particles, and precipitating AI2O3 on these particles and filling the voids were among the techniques studied for processing. The optimum conditions and techniques for preparing dense composites rich in alumina were established and the best microstructure and fracture toughness were obtained by coating Si3N4 particles filling the voids with AI2O3