Thermo-mechanical fatigue degradation of a nickel-aluminide coating on a single-crystal nickel-based alloy

A microstructural study of the degradation of an NiAl coating on a nickel-base single crystal subjected to thermo-mechanical (TMF) and low-cycle fatigue (LCF) has been made, using scanning electron microscopy and energy and wavelength dispersive spectrometry. Various thermal histories were investigated, and the associated coating degradation mechanisms identified. Under TMF conditions the hot part of the cycle (950-1050 degrees C) helps to stabilize the beta-phase of the coating. Surface oxidation proceeds slowly and does not affect the bulk of the coating. Evidence of enhanced outward aluminium diffusion and inward oxygen diffusion was found at long exposure times. Fatigue cracks initiate at the surface, grow intergranularly through the coating and are frequently arrested at the coating-substrate interface. In LCF conditions, massive oxidation of the surface and interdiffusion at the coating-substrate interface cause NiAl decomposition into first gamma' and later gamma phases. Cracks initiate and grow through the coating in a mode similar to the TMF case.