Cyclic oxidation behavior of Pt-modified aluminide coating treated with ultrasonic nanocrystal surface modification (UNSM) on Ni-based superalloy

Abstract

The effect of ultrasonic nanocrystal surface modification (UNSM) on Pt-modified aluminide coatings on Ni-based superalloy was investigated. UNSM was applied to make the grain size finer and to release compressive stress on the Al oxide film by reducing surface roughness. A Pt layer, with a thickness of 10–11 μm, was coated on a superalloy and transformed to a Ni-Pt alloy layer by annealing, at 1050 °C for 3 h. Before pack aluminizing, the surface of the Ni-Pt alloy layer was shocked by UNSM. The grain size of this UNSM-shocked Ni-Pt alloy layer was finer than the grain size on the untreated specimen. During pack aluminizing, the treated Pt-modified aluminide coating had more Al uptake and greater thickness than the untreated Pt-modified aluminide coating, because many grain boundaries and volume increase were incurred by UNSM. Furthermore, the treated coating displayed a smoother surface before, and after, pack aluminizing. The treated coating showed superior cyclic oxidation resistance. The decrease of surface roughness in the treated coatings diminished compressive stress, which caused spallation of the thermally grown oxide (TGO) and faster depletion of Al. Faster Al depletion in the untreated coating led to a phase transformation, from β-NiAl to γ′-Ni 3Al, and then to changes in volume and solubility of the alloying element, Cr. The increase of the surface and interface roughness was a result of the change in volume as well as the increased stress and strain between the TGO and the coating. Additionally, the increasing solubility of the alloying element, Cr, in the coating, resulted in the formation of a large amount of Cr- and Ni-related oxides, which are unstable in the TGO during cyclic oxidation. Spallation of the TGO caused an accelerated rate of Al depletion in the untreated coating, and a faster degradation rate than in the treated coatings.