Delamination analysis of a ductile phase-toughened environmental barrier coating for oxygen-rich turbopumps

Abstract

The turbine in an oxygen-rich turbopump subjects materials to high-pressure gaseous oxygen environments, where the risk of metal fires presents a potentially catastrophic failure mode. A candidate approach for mitigating particle impact ignition, one key metal ignition mechanism, is to coat metallic components with an inert ceramic environmental barrier coating; however, such coatings are susceptible to delamination during the rapid thermal transients upon engine startup and shutdown. Here, we investigate the delamination risk of a ductile phase-toughened composite environmental barrier coating under a nominal flight cycle of a reusable boost-stage rocket engine. The coating of present interest comprises a borate-based glass-ceramic matrix reinforced with Ni and is applied as an aqueous slurry that bonds to the substrate upon firing. We first varied the Ni content in the coating and characterized its structure, finding that the Ni reinforcement sinters to the substrate and percolates through the brittle glass-ceramic when the Ni volume fraction exceeds 0.3, in excellent agreement with percolation theory. We also experimentally assessed the thermomechanical properties of the coating and used these measurements to compute its energy release rate under the thermal transients expected in service. Our analysis shows that thermal stresses and energy release rates are greatest during engine startup and shutdown, with energy release rates on the order of 100 J/m2 for a 100-μm thick coating on an IN718 substrate. Our analysis then highlights strategies for reducing the energy release rate, which include tailoring the thermal expansion coefficient to balance transient vs. steady-state thermal stresses and using active heating to promote film boiling upon exposure to cryogenic liquid oxygen, thus mitigating the severity of cold shocks. Most importantly with respect to materials design, we predict that interpenetrating composite coatings with Ni concentrations greater than 0.3 exhibit enhanced toughness via crack bridging, allowing them to resist delamination under even the most aggressive operating conditions. These results highlight the potential of high-toughness metal/ceramic interpenetrating composite coatings in thermal and environmental protection applications where extreme thermal transients delaminate conventional monolithic ceramic coatings.