Wetting, Spreading and Interfacial Reactions of CaO–MgO–Al2O3–SiO2 Melts on Yttria-Stabilized Zirconia

Abstract

Yttria-stabilized zirconia (YSZ) is often used as a thermal barrier coating (TBC) material for gas turbines. When calcium-magnesium–aluminum-silicate (CMAS) deposits derived from sand, dust and volcanic ash accumulates on the TBC during operation, the molten CMAS wets and spreads, and reacts with the TBC leading to the formation of Y-depleted ZrO₂, as a result, compromising the thermal protection of the TBC. In this study, the contact angles of CMAS melts on YSZ and Al₂O₃ substrates were measured using the sessile drop method to observe the effects of interfacial reactions on the wetting and spreading behaviors of CMAS melts. To elucidate the thermodynamics of the formation of Y-depleted ZrO₂ in CMAS melts, the saturation solubility of ZrO₂ in four different CMAS melts was measured. Sessile drop measurements and electron probe microanalyzer observations revealed that the contact angles of CMAS melts on the YSZ substrates continuously decreased owing to the progress of interfacial reaction, leading to the formation of Y-depleted ZrO₂. On the other hand, the contact angles of CMAS melts on the Al₂O₃ substrates remained constant because anorthite covered the entire interface and the local equilibrium at the interface was reached. The ZrO₂ solubility measurements revealed that the reprecipitation of Y-depleted ZrO₂ is attributed to the reduction in the saturation solubility of ZrO₂ in the CMAS melts near the interface, resulting from a decrease in the CaO and MgO concentrations and an increase in the Y₂O₃ concentration in the CMAS melt owing to the CMAS/YSZ interfacial reaction.