Microstructure evolution of NiCoCrAlY overlay coating for Ni3Al based alloy IC6 turbine vane during long term engine test

The NiCoCrAlY overlay coating 20-25 μm in thickness prepared by pulse biased arc ion plating was used for the aero-engine turbine vanes made of Ni3Al based alloy IC6. After 360 h engine test in the elevated temperature range of about 800-950 °C, the coating was still well combined with the alloy substrate. However, some black and gray spots locally appeared on the coated vane surface. The results of microstructure and chemical composition analysis by SEM, electron probe microanalysis and X-ray diffraction and mapping techniques revealed that the diffusion of the elements between the coating and alloy substrate occurred obviously, and the contents of essential oxidation elements Al and Cr in the coating decreased during the engine test, which decreased the oxidation and corrosion resistance of the coating. The results of the analysis also showed that the external elements C, Na, Ca, Si and Mg existed on the tested vane surface, which are the oil burning remnants. The further study on the spot area indicated that under the gray spots, inner oxidation occurred, forming the blocky Al2O3 and AlN particles. The formation of the gray spots was mainly due to the inner oxidation of the coating resulted from local micro-porosity of the coating and the oil burning remnants deposited on the surface.