Pull Adhesion Testing of Hafnium Oxide Coatings

The introduction of silicon carbide/silicon carbide (SiC/SiC) ceramic matrix composites (CMCs) into aviation turbine engine hot section enabled increased inlet temperature and reduced cooling requirements. Environmental barrier coatings (EBCs) are required to achieve the durability needs of CMC aerospace turbine components. Future EBC oxide topcoats will need to provide sufficient thermal capability to support turbine inlet temperatures in excess of 3000°F (1650°C), thermal barrier function and acceptable mechanical properties such as adhesion strength. Hafnium oxide (HfO2) has been evaluated as a prospective oxide topcoat material and can be deposited in a pseudocolumnar structure for increased in-plane compliance to better manage the thermal expansion mismatch stresses with Si-based ceramics, thus, to help adhesion to Si-based ceramics. Coatings were applied using two different processing methods on bulk α-SiC substrates to compare the properties of the two deposition methods. Samples were exposed to aging heat treatment from 2500-2800°F (1371-1538°C) in air and mechanical pull testing was conducted at room temperature to track the adherence with aging. The pull adhesion test results revealed sensitivities to the strength of the monolithic SiC substrate and potential confounding effects associated with bonding adhesive layer and preparation conditions.