High-Temperature Corrosion Mechanisms of La2Si2O7 Environmental Barrier Coatings Exposed to Molten Calcium–Magnesium–Aluminosilicate (CMAS) and Water Vapor/Oxygen

La2Si2O7 is a candidate environmental barrier coating (EBC) material for silicon carbide ceramic matrix composites in next-generation gas turbine engines, yet its degradation behavior under the simultaneous attack of molten CMAS and water vapor remains insufficiently understood. This study systematically examines atmospheric plasma-sprayed La2Si2O7 coatings exposed to four environments (air, CMAS alone, H2O/O2 alone, and combined CMAS + H2O/O2) at 1400 °C for 8 h. Under dry air, the coating recrystallizes to La2Si2O7 with negligible corrosion. CMAS attack triggers a dissolution–reprecipitation mechanism forming needle-like CaLa4(SiO4)3O apatite within a denser reaction zone, which partially impedes further infiltration. Water vapor accelerated the growth of the thermally grown oxide at the bond coat interface. The combined CMAS + H2O/O2 environment produced a pronounced synergistic acceleration: water vapor reduced CMAS melt viscosity, enabling deeper CMAS penetration, while concurrent silica volatilization disrupted the protective apatite barrier, generating extensive porosity and through-coating cracking. These findings reveal that the inherent CMAS resistance of La2Si2O7 via apatite formation is critically compromised by simultaneous water vapor, highlighting a key challenge for its application in realistic engine environments.