High-temperature interactions of desert sand CMAS glass with yttrium disilicate environmental barrier coating material

A calcium-magnesium aluminosilicate (CMAS) glass was prepared by melting a sample of desert sand to evaluate the high-temperature interactions between molten CMAS and yttrium disilicate (Y2Si2O7), an environmental barrier coating (EBC) candidate material. Cold-pressed pellets of 80 wt% Y2Si2O7 powder and 20 wt% CMAS glass powder were heat treated at 1200 degrees C, 1300 degrees C, 1400 degrees C and 1500 degrees C for 20 h in air. The resulting phases were evaluated using powder X-ray diffraction. In the second set of experiments, free standing hot-pressed Y2Si2O7 substrates with cylindrical wells were filled with CMAS powder to a loading of similar to 35 mg/cm(2) and heat treated in air at 1200 degrees C, 1300 degrees C, 1400 degrees C and 1500 degrees C for 20 h. Scanning electron microscopy, energy-dispersive spectroscopy and electron microprobe analysis were used to evaluate the microstructure and phase compositions of specimens after heat treatment. An oxyapatite silicate (Ca2Y8(SiO4)(6)O-2) phase was identified in all specimens after CMAS exposure regardless of heat treatment temperature. Apatite appeared to form by dissolution of Y2Si2O7 into molten CMAS, reacting with CaO in the melt according to the reaction 4Y(2)Si(2)O(7) + 2CaO -> Ca2Y8(SiO4)(6)O-2 + 2SiO(2), and followed by precipitation of the apatite phase.