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: Al₂O₃Y₃Al₅O₁₂ eutectic, Y₃Al₅O₁₂, YAlO₃, Y₄Al₂O₉, and Y₂O₃. The yttrium aluminates were found to be stable at high temperatures under vacuum and in air. However, when they were heat-treated under vacuum in proximity to SiC, degradation was observed. This was found to be primarily a result of carbothermal reduction. In a similarly reducing environment without Si, the yttrium aluminates, and Al₂O₃ and Y₂O₃, all exhibited degradation by carbothermal reduction. Based upon the experimental results, a degradation mechanism for yttrium aluminates was proposed.