Structural and mechanical properties of Cr–Al–O–N thin films grown by cathodic arc deposition

Coatings of (CrxAl1−x)δ(O1−yNy)ξ with 0.33 ⩽ x ⩽ 0.96, 0 ⩽ y ⩽ 1 and 0.63 ⩽ δ/ξ ⩽ 1.30 were deposited using cathodic arc evaporation in N2/O2 reactive gas mixtures on 50 V negatively biased WC–10 wt.% Co substrates from different Cr and Al alloys with three different Cr/Al compositional ratios. For N2 < 63% of the total gas, ternary (Cr,Al)2O3 films containing <1 at.% of N forms; as determined by elastic recoil detection analysis. Increasing the N2 fraction to 75% and above results in formation of quaternary oxynitride films. Phase analyses of the films by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy show the predominance of cubic Cr–Al–N and cubic-(Cr,Al)2O3 solid solutions and secondary hexagonal α-(Cr,Al)2O3 solid solution. High Cr and Al contents result in films with higher roughness, while high N and O contents result in smoother surfaces. Nanoindentation hardness measurements showed that Al-rich oxide or nitride films have hardness values of 24–28 GPa, whereas the oxynitride films have a hardness of ∼30 GPa, regardless of the Cr and Al contents. Metal cutting performance tests showed that the good wear properties are mainly correlated to the oxygen-rich coatings, regardless of the cubic or corundum fractions.