Abstract
MCrAlX (M: Ni or Co or both, X: minor elements) coatings have been used widely to protect hot components in gas turbines against oxidation and heat corrosion at high temperatures. Understanding the influence of the X-elements on oxidation behavior is important in the design of durable MCrAlX coatings. In this study, NiCoCrAlX coatings doped with Y + Ru and Ce, respectively, were deposited on an Inconel-792 substrate using high velocity oxygen fuel (HVOF). The samples were subjected to isothermal oxidation tests in laboratory air at 900, 1000, and 1100 °C and a cyclic oxidation test between 100 and 1100 °C with a 1-h dwell time at 1100 °C. It was observed that the coating with Ce showed a much higher oxidation rate than the coating with Y + Ru under both isothermal and cyclic oxidation tests. In addition, the Y + Ru-doped coating showed significantly lower β phase depletion due to interdiffusion between the coating and the substrate, resulting from the addition of Ru. Simulation results using a moving phase boundary model and an established oxidation-diffusion model showed that Ru stabilized β grains, which reduced β-depletion of the coating due to substrate interdiffusion. This paper, combining experiment and simulation results, presents a comprehensive study of the influence of Ce and Ru on oxidation behavior, including an investigation of the microstructure evolution in the coating surface and the coating-substrate interface influenced by oxidation time.
Highlights
MCrAlX coatings (M for Ni and/or Co, X for minor elements) are widely used to protect superalloys against high-temperature oxidation and heat corrosion in gas turbine engines, either as overlays or as bond coats for TBCs (Thermal barrier coatings) [1,2,3]
The protection offered by MCrAlX coatings against high-temperature oxidation relies on the performance of a thermally grown oxide (TGO) scale formed at the coating-gas interface [4,5]
Because the simulation is based on the combination of diffusion and phase equilibrium calculation, the effect of Ru on the interdiffusion was further investigated by phase equilibrium calculation using
Summary
MCrAlX coatings (M for Ni and/or Co, X for minor elements) are widely used to protect superalloys against high-temperature oxidation and heat corrosion in gas turbine engines, either as overlays or as bond coats for TBCs (Thermal barrier coatings) [1,2,3]. The protection offered by MCrAlX coatings against high-temperature oxidation relies on the performance of a thermally grown oxide (TGO) scale formed at the coating-gas interface [4,5]. The growth of the alumina scale at the coating surface and substrate interdiffusion depletes the Al-rich β phase in MCrAlX coatings, which serves as an Al reservoir during oxidation. By modifying these two processes, the β phase depletion rate can be reduced; improved coating oxidation performance can be achieved by doping elements, such as Y [6,7], Hf [8,9], Ta [10], Re [11], and so on. The addition of various reactive elements (Y, Hf, or Ce) to alloys and coatings can remarkably improve TGO spallation resistance during cyclic oxidation by creating an anchor effect [13]
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