Comparative study of temperature-dependent oxidation and interdiffusion behavior on NiCoCrAlYHf-coated nickel-based single-crystal superalloys

The oxidation of bond coat (BC) and interdiffusion between BC layer and superalloys were two mechanisms that contribute to the degradation of thermal barrier coating system (TBCs). The oxidation and interdiffusion of TBCs on single-crystal superalloys were comparatively studied in this work at 950 °C, 1050 °C and 1100 °C over a duration of 1500 h. The results indicated that the thickness of the thermally grown oxide (TGO) progressively increased with the oxidation time and temperature, following a parabolic law. The discontinuous TGO gradually developed into a double-layer structure: the upper layer was a mixed oxide layer, while the lower layer predominantly comprises an Al₂O₃ layer. Moreover, an interdiffusion zone (IDZ) is formed at the interface between the BC layer and the substrate, with its thickness increasing in response to both temperature and oxidation time. The Cr-rich phases appeared in the IDZ, where Al initially diffused from the BC layer to the substrate, followed by a reverse diffusion process. Simultaneously, Ni and refractory elements diffused from the substrate into IDZ, resulting in the precipitation of the topologically close-packed (TCP) phases and the formation of a secondary reaction zone (SRZ). It has further been shown that Cr-rich phases further diffused into SRZ. Higher oxidation temperatures accelerated the growth of the SRZ and facilitated the transformation of the TCP phases from a blocky to a needle-like morphology. Ultimately, the continuous growth of IDZ and SRZ led to the evolution of the substrate from a monocrystalline to a polycrystalline structure. This study provides valuable data on the microstructural evolution of TBCs and the assessment of their service life.

Graphical Abstract