Stability of interfaces in hybrid EBC/TBC coatings for Si-based ceramics in corrosive environments

Abstract

SiC/SiC ceramic matrix composites (CMCs) are being used increasingly in the hot-sections of gas turbines, especially for aerospace applications. These CMCs are subject to recession of their surface if exposed to a flow of high-velocity water vapor, and to hot-corrosion when exposed to molten alkali salts. This research involves developing a hybrid system containing an environmental barrier coating (EBC) for protection of the CMC from chemical attack and a thermal barrier coating (TBC) that allows a steep temperature gradient across it to lower the temperature of the CMC for increased lifetimes. The EBC coating is a functionally graded mullite (3Al2O3•2SiO2) deposited by chemical vapor deposition (CVD), and the TBC layer is yttria-stabilized zirconia (YSZ) deposited by air plasma spray (APS). The stability of this system is investigated, which includes the adhesion between the two coating layers and the substrate, the physical and chemical stability of each layer at high temperature, and the performance under severe thermal shock and during exposure to hot corrosion. The effect of vertical cracks in the TBC on the EBC layer below it is also examined.