Influence of Al and Y content on the oxidation resistance of CrAlYN protective coatings for high temperature applications: New insights about the Y role

CrAlYN hard coatings with two different average Al contents: ∼16 at.% and ∼25 at.%, and Y concentration varying between 1.2 and 5.7 at.% were deposited by direct current reactive magnetron co-sputtering of mixed Cr-Al and Y targets on commercial M2 steel substrates. The samples were heated to 1000 °C in air during 2 h to study their oxidation resistance and thermal stability. The Y content is critical and the coatings present different behaviour depending on the Al content. The best oxidation resistance and thermal stability are obtained for the coating with ∼16 at.% Al and 3.4 at.% Y. The initial film microstructure and the cubic phase (fcc-CrAlN) were retained, and a thin (Cr,Al)₂O₃ oxide protective scale was formed. At lower Y content (1.2 at.%) iron, from the substrate crosses the coating, while a higher content (4.6 at.%) avoided the iron diffusion at the expense of a thicker oxide scale with new oxide phases. The coatings with higher Al content (∼25 at. %) were not thermally stable at 1000 °C. A good oxidation resistance was obtained for 2.6 at.% of Y although new phases (hcp-AlN and Cr-Fe) were formed. Higher amount of yttrium (∼5.7 at. %) led to the complete oxidation of the coating.