Formation and growth of silica layer beneath environmental barrier coatings under water-vapor environment

Environmental barrier coatings (EBCs) have widely been employed as protective coatings for SiC-based ceramic matrix composites (CMC-SiCs) against combustion corrosion in advanced gas turbines. During service, a layer of oxides forms beneath EBCs, which plays key role in lifetime of coated CMC-SiC parts. In this study, the kinetics of thermally grown silica oxide (silica-TGO) were examined through rare earth disilicates coated CMC-SiC composites subjected to corrosion under 50%H₂O-50%O₂ gas flow environment. Results revealed that growth of silica-TGO followed parabolic behavior, indicating diffusion controlled process. Calculated growth rates and activation energies of silica-TGO suggested that oxygen ion was the main active oxidant involved at relatively low partial pressure of water vapor. Corrosion environments containing different H₂O/O₂ ratios were also investigated and the data indicated that water vapor was also involved in growth of silica-TGO. The role of EBCs in corrosion process was also analyzed and discussed.