Oxidation performance of Si-HfO2 environmental barrier coating bond coats deposited via plasma spray-physical vapor deposition

Current state of the art (SOA) environmental barrier coating (EBCs) systems necessary for SiC/SiC ceramic matrix composites (CMCs) rely upon a metallic silicon bond coat. While this layer provides durability and adhesion, the upper use temperature of these systems is limited by the melting point of silicon (1414 °C). Turbine engine temperatures already exceed this melting point and in order to reduce or eliminate cooling for advanced components, new bond coat materials are required for EBC systems. One potential EBC bond coat that has been proposed is a composite layer of silicon and hafnium oxide (Si-HfO2). This coating concept intends to provide a well-bonded interface between silicon and the substrate, similar to the ‘pure’ metallic silicon bond coat, but with a higher temperature capability owed to the addition of HfO2. Two-layer systems of Si-HfO2 bond coat and a Yb2Si2O7 EBC top coat were deposited using Plasma Spray- Physical Vapor Deposition (PS-PVD), which is a hybrid coating method technique capable of vapor or liquid deposition. Coatings were deposited on bulk α-SiC and tested for oxidation performance in both laboratory air and in 90%/10% H2O/O2. A thermally grown oxide (TGO) of SiO2 was formed at the substrate/bond coat interface and the growth of this layer was measured and compared to literature values to determine TGO growth rates. •Two-layer EBC systems were heat treated in air or thermally cycled in 90% H2O/O2.•Oxidation rates under the coatings were compared to literature values for coated and uncoated SiC.•Si-HfO2 bond coats exhibited limited oxidation protection at 1426 °C in steam environments.