Advanced oxide strengthened silicon bond coats with rare-earth silicate based EBCs for SiC/SiC CMCs

Abstract

The introduction of SiC/SiC CMCs in aero engines demands for well-tailored Environmental Barrier Coatings (EBC). Physical vapour deposition (PVD) coating technology was employed to manufacture these systems since this technology is potentially favourable for the application on near net shape designed CMC components. They have further advantages such as distinct control over microstructure and tailored chemistry. Magnetron sputtered three-layer EBC systems that consist of a silicon-based bond coat (BC), a rare-earth di-silicate intermediate layer and a rare-earth mono-silicate top coat against water vapour attack have been developed and continuously optimised by our group. Thermal sprayed pure silicon is the state-of-the-art choice for the BC. In this talk, not only the performance of the PVD counterpart, but also novel variants with advanced properties will be presented. The longevity and thermomechanical properties of the BC have been enhanced by oxide strengthening with yttria or hafnia dopants. Both dopants evoke significant differences in the morphology of the coatings. Hafnia formed homogeneously distributed particles that form a microscale network within the silicon matrix and was later transformed into hafnon. Contrary, yttria formed larger yttrium-silicates and pores that were closed with time. Four variants were tested and showed improved resistance against spallation and cracking compared to pure silicon after thermal cyclic oxidation testing at 1250°C for 1000hrs. Some variants exhibited faster oxidation kinetics, some were still in the range of the slow silicon oxidation. 3D-reconstruction and TEM analyses gave insight into the microstructural features of the hafnia doped BCs. They revealed the underlying mechanism of oxidation and hafnon formation. The compatibility of rare-earth silicates with the novel bond coats have been validated with both furnace cycle testing and isothermal testing in streaming water vapour at 1250°C. The results proofed their excellent functionality as alternative bond coats for EBCs.