Microstructure and oxidation resistance of nanocrystalline 304 SS-Al coatings

The long-term oxidation behavior of nanocrystalline 304 SS and 304 SS-4Al coatings has been investigated. The coatings were deposited on 304 SS samples using a plasma enhanced magnetron sputtering process. Cyclic oxidation tests were conducted on the coated and uncoated samples at a peak temperature of 750 °C for up to 1000 one-hour thermal cycles between the peak and room temperatures. The crystal structure of the as-deposited 304 SS coating consisted of σ phase in a αFe matrix. The addition of 4%Al to the 304 SS coating stabilized the αFe structure and prevented the formation of the σ phase. Thermal cycling exposure led to formation of external oxide scale of mixed oxide on the uncoated sample, Cr2O3 on the 304 SS coating and Al2O3 on the 304 SS-Al coating. The coatingʹs fine grain structure had promoted selective oxidation of Cr and Al in the 304 SS and 340 SS-4 wt.% Al coatings, respectively. The external oxide scale on the coated samples exhibited good spallation resistance compared to the scale on the uncoated samples. After thermal cycling exposure, both coatings exhibited internal oxidation along the columnar boundaries. Inward diffusion of Al into the substrate during thermal cycling resulted in precipitation of iron-aluminide particles in the interdiffusion zone. The Al content in the coating dropped from 4 wt.% to 1.8 wt.% after 990 thermal cycles due to the inward and outward diffusion of Al. The improvement in oxide scale spallation resistance and accelerated depletion of aluminum are believed to be related to the fine grain structure of the coating.