Al concentration affected oxidation resistance of Cr/CrAlN multilayered coatings in high-temperature steam

In this work, the multilayered Cr/CrAlN coatings with equal single-layer thickness of 500 nm but different Al concentrations from 6 to 38 at% were sputtered on Zircaloy-4 alloys to investigate the oxidation resistance and kinetics behavior in a 1000 ℃ steam. All the coated samples exhibited superior oxidation resistance to Zircaloy-4. With increasing Al content, the weight gain of multilayers decreases, with optimal oxidation resistance achieved at 38 at% Al. This behavior is attributed to the distinctive architecture of surface oxide scale, which undergoes a structural transformation from a monolithic Cr₂O₃ oxide scale in low-Al specimens to an alternating Al₂O₃/Cr₂O₃ multiple oxide layers in high-Al specimens. The oxidation mechanism was correspondingly changed from outward Cr diffusion to inward oxygen diffusion, since the sufficient Al addition promotes the formation of a continuous, dense Al₂O₃ layer, effectively suppressing the Cr depletion via outward diffusion. Moreover, a diffusion barrier model was developed to elucidate the oxidation kinetics of Cr/CrAlN multilayers with modulated oxide barriers.