Calcium-magnesium aluminosilicate (CMAS) interactions with ytterbium silicate environmental barrier coating material at elevated temperatures

Thermochemical interactions between a calcium-magnesium aluminosilicate (CMAS) glass and ytterbium disilicate (Yb2Si2O7), a candidate environmental barrier coating (EBC) material, have been investigated. Pellets of Yb2Si2O7 and CMAS glass powder were heat treated at 1200, 1300, 1400 and 1500 degrees C for 1, 10 and 50 h in air. Powder X-ray diffraction was employed to identify the resulting phases. In a second series of experiments, Yb2Si2O7 substrates were prepared by hot pressing, and cylindrical wells were drilled into the material surface. CMAS glass powder was added to the wells to achieve a loading of similar to 35 mg/cm(2) and subsequently heat treated at 1200, 1300, 1400 and 1500 degrees C for durations of 1, 10 and 50 h in air. Sample cross-sections were characterized using scanning electron microscopy, X-ray energy-dispersive spectroscopy, X-ray diffraction, electron microprobe analysis and transmission electron microscopy to evaluate the resulting microstructure, phases and compositions at the CMAS/Yb2Si2O7 interface. Dissolution of ytterbium silicate into molten CMAS followed by precipitation of Yb2Si2O7 coupled with CMAS grain boundary penetration at elevated temperatures were the dominant mechanisms by which CMAS effectively infiltrated and altered the Yb2Si2O7 substrate microstructure.