HIGH-TEMPERATURE REACTIONS WITHIN SIC-AL2O3 COMPOSITES

Composites Of SiC-Al2O3 and SiC-mullite are unstable at high temperatures. The reactions occurring within the composites between 1700 and 1850-degrees-C in stagnant inert atmospheres were characterized. Gaseous products cause excessive weight losses which cannot be attributed to passive oxidation. These losses can be successfully retarded by processing under high pressures. Compatible phases were determined by x-ray analysis for mixtures lying in the section SiC-Al4C3-Al2O3-SiO2. The reactions produced condensed phases of Al2OC and Al4O4C as well as gaseous SiO and CO. The condensed phases have high vapor pressures above 1700-degrees-C. The effect of these reactions on densification of composites by firing at different temperatures for various periods under different pressures was studied. Dense materials prepared under high pressures at 1825-degrees-C were tested at 1700-degrees-C under normal pressure in argon, where active oxidation is expected, and weight losses were insignificant.