Effect of molybdenum addition on oxidation behavior and secondary protection mechanism of FeCrAl coatings

Abstract

FeCrAl alloy coatings are widely used for their excellent oxidation resistance at high temperatures, but they can suffer from rapid material degradation due to breakaway oxidation of the coating. Composition manipulation is an essential way to prompt the oxidation resistance of the coatings. In this study, we prepared Fe-13Cr-7Al (at.%) alloy coatings with varying Mo contents (Mo = 0, 1.5, 3, 6 at.%) by magnetron sputtering and investigated their oxidation behaviors at 550 °C. The surface and cross-sectional morphologies of the coatings before and after oxidation were examined using electron microscopy. The results showed that the crystalline grain and particle size of the coatings decreased significantly with the increase of Mo content, leading to a notable increase of grain boundaries and further enhancing element diffusion during the oxidation tests. As a result, a protective and stable oxide scale rapidly grew on the surface of FeCrAlMo coatings during oxidation. Moreover, FeCrAlMo coatings demonstrated improved secondary protection mechanisms with increasing Mo contents. This was attributed to the formation of an internal oxidation layer of a spinel barrier (M3O4, M = Fe, Cr, Al, Mo) beneath the outer oxidation layer. The spinel barrier impeded inward diffusion of oxygen ions and outward diffusion of metal cations, resulting in enhanced oxidation resistance of the spinel-type oxide. The thermodynamic calculations were used to evaluate the oxidation mechanisms of the coatings. These results provide a deeper understanding of the effect of Mo addition on the oxidation behavior and underlying protection mechanisms of FeCrAl coatings, and can be beneficial for designing and developing materials for use in oxidation environments.