The water vapor hot gas corrosion of MGC materials with Al2O3 as a phase constituent in a combustion atmosphere

The hot gas corrosion behavior of two melt grown composite materials (Al2O3/YAG and Al2O3/GAP) in comparison with alumina and sapphire was investigated. The tests were performed in a high temperature burner rig at 1450 degrees C, a total pressure of 1 atm with a water vapor partial pressure of 0.28 atm, a gas flow velocity of 100 m/s and exposure times for up to 450 h. In comparison to sapphire, alumina showed comparable corrosion rates and similar corrosion traces on the corroded surface. The Al2O3/YAG and Al2O3/GAP melt grown composite materials were subject to degradation due to the corrosion attack of the alumina phase. This was the consequence of the formation and evaporation of volatile hydroxides (e.g., Al(OH)(3)). In comparison to polycrystalline alumina the YAG or GAP showed a higher corrosion stability. which causes the formation of a porous YAG or GAP surface layer with increasing corrosion time. For short corrosion times, the porous YAG or GAP surface layer led to a small corrosion protection of the material. After longer corrosion times a switch to linear corrosion kinetics was observed due to spalling of the porous corrosion layer. The corrosion rates for long exposure times were comparable to bulk alumina or sapphire. In principle the regarded polycrystalline and single crystalline materials showed the same corrosion behavior. (C) 2008 Elsevier Ltd. All rights reserved.