Thermal Spray Deposited Hafnia-Silicon Bond Coats for Protection of SiC/SiC CMCs

Environmental barrier coatings (EBCs) are used to shield silicon carbide (SiC) based ceramic matrix composites (CMCs) from the water vapor atmosphere in the gas turbine engine environment. A baseline EBC structure consists of a Si bond coat layer that protects the SiC from oxidation and a topcoat layer that acts as a volatilization barrier. Research on EBC degradation revealed that delamination failure occurs at bond coat –topcoat interface during thermal cycling due to reversible α-β transition in thermally grown silica which is accompanied by ~5% volume change.

In this work, Si-HfO₂ feedstock was plasma sprayed as an alternative to Si-bond coats. The concept was proposed by NASA and aims to consume formed silica by HfSiO₄ formation. Phase composition, thermal expansion behavior, and performance of the plasma sprayed Si-HfO₂ coating will be discussed and compared to Si in detail based on high-temperature XRD, dilatometry, and furnace cycling investigations.