Three-layered environmental barrier coating System for SiC/SiC CMCs by magnetron sputtering

Abstract

When implemented in an aero-engine, SiC/SiC CMCs require environmental barrier coating (EBC) systems in order to withstand the atmospheric conditions. Physical vapour deposition processes enabled advanced design concept for the EBC systems since its advantages lie within the precise control over microstructure and chemistry. A favourable EBC system consists of an Si-based bond coat followed by a rare earth, Y or Yb, disilicate as intermediate layer and finalised by a rare earth, Y or Yb, monosilicate top layer which protects the component against water vapour attack. The EBC systems presented in this talk were manufactured by using magnetron sputtering for the bond coat and reactive magnetron sputtering for the intermediate and top layer. The X-ray amorphous coating system underwent a subsequent crystallization treatment. The macroscopic homogenous surface as well as local defaults like buckling or spallation of the silicate layers was defined during the crystallization process. Afterwards, the three layered EBC system was tested under cyclic oxidation at 1200 °C up to 1000 cycles where no major macroscopic changes appeared to the EBC system. For comparison, an uncoated SiC substrate and a SiC substrate with the bond coat as single layer were tested as well. The microscopic development in terms of phase stability, layer density and interface reactions were examined by SEM and XRD at different cycle numbers. While the bond coat and the disilicate remained hermetic dense, the monosilicate forms horizontal pores which agglomerated during long term testing. The disilicate and monosilicate remained phase stable while the bond coat started to form a SiO2-based TGO layer on top. The oxidation kinetics of the bond coat within the EBC system was compared to the single layer bond coat during the thermocyclic testing. The behaviour of the EBC system in water vapour was investigated first as short-term experiment under aggressive conditions in 100 % water vapour steam with a gas velocity of about 10-1 m/s and secondly for a longer duration in about 30 % water vapour steam with a gas velocity of about 10-2 m/s at 1200 °C. The degradation behaviour was analysed and correlated to chemistry and morphology of the PVD coatings.