Mechanism of the oxide scale formation in thermally-sprayed NiCoCrAlY coatings modified by CeO2 nanoparticles

Abstract

The increased operating temperatures of next-generation gas turbines require high-performance coatings with improved oxidation resistance. NiCoCrAlY coatings have significant potential for achieving this goal however improved insights into their oxidation performance are required. In this study, a high-velocity oxy-fuel (HVOF) process was used to compare NiCoCrAlY/nano-CeO2 with NiCoCrAlY conventional coatings. High-temperature oxidation behaviour of the modified coatings was investigated and compared with the conventional coating. Free-standing coatings were then subjected to short- and long-term isothermal oxidation test at 1000 °C. The microstructural features of the modified powders and coatings before and after oxidation were characterized using a Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive Spectroscopy (EDS), Transmission Electron Microscope (TEM), Raman spectroscopy and X-ray Diffraction (XRD). The oxide growth rate of the coatings was also examined and modeled using various diffusion-based calculations. Results showed that, the modified NiCoCrAlY-1.0 wt% nano-CeO2 coating had the highest oxidation resistance (26 % higher than the conventional NiCoCrAlY coatings). The higher oxidation resistance of the modified NiCoCrAlY-1.0 wt% nano-CeO2 coating was attributed to its higher density as well as its lower structural porosity and oxide growth rate after the short- and long-term oxidation tests. Moreover, acceptable conformity of parabolic rate behaviour was obtained for all types of the conventional and modified coatings. The dominant mechanism of oxidation improvement for NiCoCrAlY-1.0 wt.% nano-CeO2 coating was found to the controlling the oxide scale growth rate. As a consequence, the findings indicated that the modified NiCoCrAlY-1.0 wt.% nano-CeO2 coating can be a candidate for the protection of hot sections of gas turbines in the future.