Abstract: A novel bond coat composed of γ/γ′ phases that are in equilibrium with the superalloy substrate (equilibrium phase (EQ)) is achieved by microstructure regulation on the basis of traditional MCrAlY coating. The EQ-bonded LC-thermal barrier coating (TBC) outperforms the NiCrAlY-bonded HC-TBC regarding the thermally grown oxide (TGO) growth rate, interfacial toughness, as well as interdiffusion with the substrate, remarkably enhancing the interfacial stability of the multi-layer system. For HC-TBC, the high elastic strain energy in TGO due to the large thickness and the inferior toughness at the TGO/top coat interface due to the formation of spinel leads to the premature failure of the coating. By comparison, the improved performance of LC-TBC benefits from the intermixed zone (IMZ) development atop TGO, which effectively decreases the growth rate of TGO while simultaneously strengthening interfacial bonding. Both the spinel in HC-TBC and the IMZ in LC-TBC are intimately associated with the initial oxidation behavior. Preferential segregation of oxygen at the heterogeneous interface of γ′/α-Cr in the bond coat of HC-TBC initiates the in-situ oxidation of α-Cr and further results in the formation of spinel. On account of the remarkably reduced segregation energy of oxygen at coherent γ/γ′ interface in bond coat of LC-TBC, uniform θ-Al₂O₃ forms, facilitating the development of IMZ. The findings in this work challenge conventional wisdom on failure behaviors of TBCs and give new insights into the design of the bond coat with superior interfacial stability.

Key words: Coatings, Superalloy, Oxidation, Interdiffusion, Interfacial stability