High-temperature oxidation of CrAlYN coatings: Implications of the presence of Y and type of steel

Nanolayered CrAlN and CrAlYN/CrAlN (average contents of Al ≈ 25 at.% and Y ≈ 1.6 at. %) coatings are deposited on M2 and 316 steel substrates and heated to 1000 °C in air for 2 h to study their oxidation mechanism, the thermal stability and the reactive element (RE) effect of yttrium. CrAlN on M2 develops a Cr₂O₃/Al₂O₃ passivation layer that preserves in high degree the fcc-CrAlN structure however iron ions leave the substrate and travel to the surface along the column boundaries. The CrAlYN/CrAlN coatings deposited on steels are not stable at 1000 °C, and the initial fcc-CrAlN phase is partially transformed to hcp-Al(O)N and Cr-Fe phases (M2) and Cr₂N and Al₂O₃ (316). The addition of Y changes the predominant scale growth direction. Inward oxygen diffusion becomes dominant but a reduction of the oxide scale thickness as compared to CrAlN is not observed. The advanced microstructural analysis made by transmission electron microscopy combined with electron energy-loss spectroscopy determined that yttrium migrates mainly to the oxide scale (forming mixed oxides with substrate elements - V and Mo, either as dispersed particles or segregated at the grain boundaries) in M2, and to the oxide interface and column boundaries (forming Al-Y oxides and YN, respectively) in 316 steel. The benefits of addition of Y in improving the oxidation resistance are discussed comparatively with literature data. The RE effect of yttrium is thus observed to be dependent on the substrate, film architecture and composition.