Isothermal and Cyclic Oxidation Behavior of HVOF-Sprayed NiCoCrAlY Coatings: Comparative Investigations on the Conventio
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Isothermal and Cyclic Oxidation Behavior of HVOF-Sprayed NiCoCrAlY Coatings: Comparative Investigations on the Conventional and Nanostructured Coatings F. Ghadami1
•
A. Sabour Rouh Aghdam1 • S. Ghadami1
Submitted: 28 July 2020 / in revised form: 11 September 2020 / Accepted: 11 October 2020 ASM International 2020
Abstract Conventional and nanostructured NiCoCrAlY coatings were deposited using the high-velocity oxy-fuel thermal spraying technique. The nanostructured NiCoCrAlY powder feedstock for the coatings was produced by the ball-milling method. The microstructures of the as-received and nanostructured powders as well as their developed coatings were investigated by x-ray diffraction, a field-emission scanning electron microscope equipped with energy-dispersive x-ray spectroscopy, and a transmission electron microscope. Williamson–Hall measurements were also carried out to estimate the crystalline size of the powders and coatings. For the evaluation of the oxidation kinetics, the free-standing coating specimens were subjected to short- and long-term isothermal and cyclic oxidation at 1000 and 1100 C, respectively, under a laboratory air atmosphere. The results indicated that the asreceived NiCoCrAlY coating had parabolic oxidation behavior in short- and long-term exposure tests. For the nanostructured NiCoCrAlY coating, in contrast, the longterm oxidation kinetics deviated from parabolic behavior and showed instead sub-parabolic rate behavior. The obtained results also revealed that the nanostructured NiCoCrAlY coating had greater oxidation resistance to both isothermal and cyclic conditions on account of the formation of a dense and slow-growing Al2O3 layer on the coating surface.
& F. Ghadami [email protected]; [email protected] & A. Sabour Rouh Aghdam [email protected] 1
Department of Materials Engineering, Tarbiat Modares University, Tehran, P.O. Box: 14115-143, Tehran, Iran
Keywords NiCoCrAlY high-energy ball-milling HVOF deposition nanostructured coatings oxidation behavior
Introduction Recently, investigations have focused on the development of advanced materials and coatings for which high-temperature corrosion and oxidation attacks have been increased (Ref 1–6). For the case of gas turbine and aeroengine systems, it is well known that the hot corrosion and oxidation resistance of industrial MCrAlY-coated parts (where M = Ni, Co, or both) depends on the composition, growth rate, and microstructure of the oxide scale formed on the coating during high-temperature exposure. In addition, the powder feedstock preparation method, primary structure of the coating, post-treatment process, and also deposition method play important roles in the improvement of high-temperature oxidation resistance of MCrAlY coatings (Ref 7–9). In most high-temperature isothermal oxidation tests, the oxidation kinetics of MCrAlY coatings is diffusion-controlled, which follows a parabolic rate law (Ref 9–16). There are very limited investigations focused on deviation from parabolic
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