PVD-based Environmental Barrier Coatings for SiC/SiC CMC components: current status, challenges and future directions.

Abstract

Ceramic matrix composites (CMCs) are promising materials for components in the hot section of gas turbines and are already applied in the latest generation of aero-engines. They can withstand higher service temperatures than the currently used Ni-based superalloys. However, SiC-SiC CMCs possess an insufficient oxidation resistance under the presence of rapidly flowing water vapor as it is present in combustion atmospheres. They suffer from both volatilization of silicon hydroxide, which leads to severe surface recession, and steam enhanced oxidation. Therefore, environmental barrier coatings (EBCs) are mandatory that protect the underlying CMC. In this presentation, multilayer EBCs manufactured by PVD (physical vapor deposition) methods are introduced. Both SiC-based CMCs and model SiC-alloys were coated by magnetron sputtering and by high rate EB-PVD processes. A first layer consisted of a silicon-based bond coat. 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. In this work, two different coating deposition methods were used, namely magnetron sputtering and electron beam physical vapor deposition (EB-PVD) to deposit Si-based bond coats on SiC-SiC substrates. The upper layers consisted 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. Samples were tested up to 1250°C in air in a furnace cycle test and under flowing water vapor at isothermal conditions. The EBC system showed no spallation after 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 formation during a mandatory heat treatment of the initially amorphous EBC top layers and the consequences of 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 in this test. Finally, the perfect coverage and good adhesion of PVD-based EBCs on sharp edges and on a real CMC vane structure is addressed. In addition, as CaO-MgO-Al2O3- SiO2 (CMAS) is a well-known threat to the EBC functionality, an additional CMAS resistant top layer is necessary which adds more complexity to the multi-layer system. A multi-phase CMAS resistant coating containing Y-Fe-Si oxides has been developed and its applicability on a state-of-the art EBC system will be demonstrated and the CMAS infiltration results at high temperature will be presented.