Effect of Sandblasting Process on the Oxidation Behavior of HVOF MCrAlY Coatings

Abstract

Surface characteristics, such as topography and roughness (R a), are important in affecting the isothermal oxidation behavior of MCrAlY coatings. In this study, the effect of sandblasting on the oxidation behavior of HVOF thermal-sprayed MCrAlY coatings was investigated. Oxidation tests were conducted under isothermal conditions at 1050 °C for different periods. The R a of the bondcoat was approximately 12 μm. Before and after oxidation, the thermally-grown scale composition and morphology were analyzed using scanning electron microscope (SEM/EDX) and X-ray diffraction. The results showed that the oxide scale that formed on the as-sprayed coatings was a mixture of Al2O3 and spinel, while the oxide formed on the sandblasted coating was composed mainly of Al2O3. In addition, with increasing R a of the bondcoat, the oxidation rate progressively decreased and the amount of Al2O3 increased compared with the as-sprayed coating. Scanning electron microscope analysis showed that the thickness of the scale layer on the sandblasted CoNiCrAlY coating at 1050 °C was much thinner than that on the as-sprayed one. Isothermal oxidation kinetics of HVOF CoNiCrAlY coatings with different surface modifications at 1050 °C. This figure indicates the effect of the surface process on the oxidation kinetics of the CoNiCrAlY coating at the temperature of 1050 °C. As it can be observed, in both samples, oxidation rate is initially high and gradually lowers with the passage of time. The reason for this reduction in the longer times is the formation of a uniform and dense oxide layer at the beginning of the process within which the rate of oxygen penetration reduces and gets harder with lasting oxidation time. The type of oxidation kinetics of these coatings is parabolic. According to the results obtained, the oxidation rate constant (Kp) for the coating in the as-sprayed and sandblasted states equals 1.75 × 10−13 and 0.63 × 10−13 mg/cm2, respectively. The reduction in the oxidation rate signifies the effect of the surface morphology on the oxidation behavior of these coatings.