HIGH-TEMPERATURE ENVIRONMENTAL STABILITY OF THE COMPOUNDS IN THE AL2O3-Y2O3 SYSTEM

Heat treatments in several environments were performed on a series of compounds in the Al2O3 and Y2O3 system: Al2O3-Y3Al5O12, eutectic, Y3Al 5O12, YAlO3, Y4Al2O9, and Y2O3, The yttrium aluminates were found to b e stable at high temperatures under vacuum and in air. However, when t hey were heat-treated under vacuum in proximity to SiC, degradation wa s observed, This was found to be primarily a result of carbothermal re duction, In a similarly reducing environment without Si, the yttrium a luminates, acid Al2O3 and Y2O3, all exhibited degradation by carbother mal reduction, Based upon the experimental results, a degradation mech anism for yttrium aluminates was proposed.