Environmental barrier coatings for SiC-SiC CMC components by Magnetron Sputtering and EB-PVD

Abstract

SiC-SiC ceramic matrix composites (CMCs) operating in combustion atmospheres require sufficient protection against water vapor volatilization of silicon hydroxide, which leads to severe surface recession, and steam enhanced oxidation. Therefore, environmental barrier coatings (EBCs) are mandatory that protect the underly-ing CMC. In this presentation, multilayer EBCs manufactured by PVD (physical vapor deposition) methods are introduced. Two different coating deposition processes were used, namely magnetron sputtering and electron beam physical vapor deposition (EB-PVD). Both SiC-based CMCs and model SiC-alloys were used as sub-strate materials in this study. A first layer consists of a silicon-based bond coat that was partially modified by rare earth oxides with a gradient in content. Oxidation of those Si-based bond coats at high temperatures and the corresponding SiO2 oxide scale growth rate is a life limiting factor for the EBCs. The upper layers consist of an intermediate rare-earth di-silicate, and a mono-silicate layer for protection against water vapor recession on top. The coating architecture was designed to minimize chemical interactions between different layers and to have a strain tolerant microstructure. In another approach, multi-component oxides were designed to provide a low thermal conductivity and good protection against water vapor and CMAS in one single layer. Samples were tested up to 1300°C in air in a furnace cycle test and under flowing water vapor at isothermal conditions. The EBC system showed no spallation up to 1000hrs of cyclic testing and retains stable interfaces between the layers. The oxide scale growth rates for both sputtered and EB-PVD Si-bond coats were measured and its influence on the life time of EBC system will be presented. Special emphasis is put on the phase for-mation during a mandatory heat treatment of the initially amorphous EBC top layers, the chemistry of the lay-ers, and the consequences of the inherent CMC surface roughness for PVD layers. While the uncoated CMC suffered from severe degradation under flowing water vapor and showed rapid loss of the matrix material after only 1h of testing, the EBC systems considerably lowered the mass loss and provide a good protection of the CMC. Finally, the perfect coverage and good adhesion of PVD-based EBCs on sharp edges and on a real CMC vane is addressed.