Thermochemical interaction of biofuel impurities with yttria-stabilized zirconia thermal barrier coatings

Abstract Thermal barrier coatings (TBCs) protect superalloy components from the extreme heat in gas turbines. Airborne contaminants that interact with TBCs during flight, such as CMAS particulate where C is calcia, M is magnesia, A is alumina, and S is silica, degrade these ceramic coatings. Biofuels can directly introduce some or all these same impurities into the gas turbine based on their starting biomass, how they were harvested, and the synthesizing process. Because the impurities are unreacted until combustion, new combinations of these impurities can form harmful deposits with compositions such as S, C–S, C-A, C-A-S, and C-M-S. In this study, we applied combinations of precursors for C, M, A, and S impurities to the surface of a TBC with subsequent ablation to ~1400 °C. Observations revealed that the TBCs delaminated by two failure mechanisms dependent on the impurities applied. In the first case, impurity combination(s) of S, C-A, and C–S interacted thermochemically with the topcoat, ultimately causing the spallation of the TBC. The second failure mechanism was characterized by molten impurities infiltrating the topcoat and inducing the propagation of cracks in the proximity of the bondcoat through a thermomechanical interaction with the TBC. This occurred for impurity combinations of C-A-S, C-M-S, and C-M-A-S. Regardless of the interaction type, different combinations of impurities common to biofuels can accelerate failure and reduce lifetimes of TBCs.