Corrosion and recession of mullite in water vapor environment

In the development of environmental barrier coating system for silicon nitride ceramic, a corrosion behavior of mullite was examined under static state water vapor environment at elevated temperatures. Results showed that a small amount of sodium, which was present in mullite bulk as an impurity, accelerated the water vapor corrosion of silica component and decomposition of mullite into alumina phase. To improve the selective corrosion of the boundary phase in silicate compounds, a mullite/Lu2Si2O7 eutectic system without boundary glassy phase was developed. The eutectic composition was Al2O3:SiO2:Lu2O3 = 27.3:54.6:18.1 in molar ratio and the eutectic temperature was estimated to be ∼1500–1520 °C. No weight loss was measured after the static state water vapor corrosion test at 1300 °C for 100 h. However, the recession of mullite phase in the eutectic oxide occurred after exposure to high speed steam jet environment at 1200 °C.