Environmental Barrier Coatings for SiC/SiC-CMCs - Manufacture and Behaviour Under Cyclic Oxidation and Flowing Water Vapour

Abstract

Silicon carbide ceramic matrix composites (SiC/SiC-CMCs) are promising materials for application in the hot section of gas turbines. SiC forms a protective SiO2 scale (TGO) which sufficiently protects the underlying material from oxidation at high temperatures in dry air. However, if rapidly flowing water vapour in a combustion environment is present, the TGO is subjected to severe volatilization by formation of silicon hydroxide. Thus, an environmental barrier coating (EBC) system is required which needs to have thermochemically compatible interfaces and a coefficient of thermal expansion (CTE) matching the SiC/SiC-CMC. In this study, CMCs were coated by magnetron sputtering and EB-PVD with silicon-based bond coats, Y2Si2O7, and Y2SiO5 or combinations thereof. The coating architecture was designed namely to minimize chemical interactions among different layers and with a strain tolerant micro structure. The samples were cyclically tested up to 1250°C in air with and without thermal gradient, respectively. Their degradation behaviour in flowing water vapour was additionally investigated. Emphasis was put on both interfacial chemical reactions, and phase evolution. While the uncoated SiC/SiC-CMC suffered from severe degradation under water vapour, the EBC system proved to be stable and provides good protection of the SiC/SiC-CMC.