DFT-assisting experimental insights into the thermally grown oxide of thermal barrier coatings with Cr₂AlC as the bond coat

Because of the high oxidation resistance and appropriate coefficient of thermal expansion (CTE), Cr₂AlC has emerged as a promising bond coat (BC) for thermal barrier coatings (TBCs), but the insufficient understanding for its thermally grown oxide (TGO) and underlying mechanisms potentially hinders its practical applications. Herein, the isothermal oxidation behavior and associated TGO of TBCs with Cr₂AlC as the BC was experimentally examined at 900-1150°C with the mechanism revealed by density functional theory (DFT). Interestingly, facilitated by enhanced Al diffusivity and reactivity, a dense, continuous and nanocrystalline α-Al₂O₃-dominated TGO layer formed on top of the Cr₂AlC BC, accompanied by its decomposition into Cr₃C₂. Besides, the favorable CTE match among Cr₂AlC, Cr₃C₂, Al₂O₃ and 8YSZ ensured the TGO integrity, while it exhibited cubic oxidation kinetics, with consistently reduced thickness compared to MCrAlY. DFT simulations corroborated these experimental trends and revealed the intrinsic mechanisms: lower oxygen adsorption energy (Δ E _ads) on Cr₂AlC (-5.22 to -4.33 eV) versus MCrAlY (-0.94 eV). This was attributed to high oxygen affinity of Al in Cr₂AlC and inherent Al-O reactivity, promoting oxygen capture and initial nucleation of protective oxides. Moreover, cramped Cr-C interlayers were found to effectively inhibit the oxygen penetration through the (0001) surface, providing an additional mechanism for enhanced oxidation resistance of Cr₂AlC. These findings provide fundamental insights into oxidation mechanisms of the Cr₂AlC BC and offer a guidance to optimize its oxidation resistance for high-temperature applications.