Microstructural evolution and failure characteristics of a NiCoCrAlY bond coat in “hot spot” cyclic oxidation

Microstructural evolution and the onset of failure in a NiCoCrAlY bond coat (BC) on a single-crystal superalloy substrate have been investigated in a newly developed “hot spot” apparatus that imposes a temperature gradient along the length of a coated, cylindrical specimen. Local spallation events were observed in the “hottest spot” of the coated specimens after 50–60 oxidation cycles with a peak temperature of 1050 °C. The thickness of the thermally grown oxide (TGO) was in the range of 2–3 μm when spalls were initiated. The failure surface contained a significant density of embedded oxides and the remnant TGO exhibited surface cracks indicative of a buckling delamination failure mode. The failed surface morphology and the cross-section microstructure of the BC have been characterized, including morphological imperfections in the TGO. The role of these microstructural features in the failure process is addressed.