High-temperature stability of alumina in argon and argon/water-vapor environments

The high-temperature stability of alumina (Al2O3) in argon and argon/water-vapor (Ar/H2O) environments has been investigated. Samples were exposed at temperatures of 1300 degrees C-1700 degrees C for 10 h, The microstructure, flexural strength, and volume all showed significant changes in the Ar/H2O environment at 1700 degrees C. Samples also became whiter, because of the oxidation of graphite impurities that had diffused from the hot-processing dies, In the Ar/H2O environment at 1700 degrees C, grain-boundary etching occurred and was much more severe than in the pure-argon environment, which was very likely caused by the enhanced formation of gaseous Al(OH)(3) and Al(OH)(2) along grain boundaries. In addition, in the Ar/H2O environment, substantial grain growth occurred in the surface vicinity. This grain growth, together with grain-boundary etching, led to a decrease in flexural strength.