Characterization of oxide scales to evaluate high temperature oxidation behavior of Ni–20Cr coated superalloys

Abstract

Modern thermal spray processes such as plasma spraying are usually considered to deposit high-chromium, nickel–chromium coatings onto the superalloys to enhance their high temperature oxidation resistance. The purpose of these coatings is to form long-lasting oxidation protective scales. In the current investigation, Ni–20Cr alloy powder was deposited on three Ni-base superalloys; Superni 75, Superni 600 and Superni 601 by shrouded plasma spray process. Oxidation kinetics was established for the uncoated as well as the coated superalloys in air at 900 °C under cyclic conditions for 50 cycles by thermogravimetric technique. Each cycle consisted of 1 h heating followed by 20 min of cooling in air. All the coated superalloys nearly followed the parabolic rate law of oxidation. X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray (SEM/EDAX) techniques were used to characterise the oxide scales. The coating was found to be successful in maintaining its integrity with the superalloy substrates in all the cases. The oxide scales formed on the oxidized coated superalloys were found to be intact and spallation-free in general. The XRD analysis revealed the presence of phase like NiO, Cr 2O 3 and NiCr 2O 4 in the oxide scales. The XRD results were further supported by the SEM/EDAX analyses.