Improved hardness and oxidation resistance for CrAlN hard coatings with Y addition by magnetron co-sputtering

• Promoted hardness of CrAlN coatings with Y addition due to solid solution strengthening and Hall–Petch effect.
• 0.3 and 0.7 at.% Y additions improve the oxidation resistance of the CrAlN coatings.
• More than 1.3 at.% Y addition deteriorates the oxidation resistance of the CrAlN coatings.

This study aims to investigate the influence of Y content on microstructure, hardness and oxidation resistance of the quaternary CrAlYN hard coatings. The CrAlYN coatings were deposited by magnetron co-sputtering using Cr50Al50 composite and pure Y targets. The Y content increases from 0 to 2.3 at.% with Y target power increasing from 0 to 150 W while Cr50Al50 target power keeping constant at 250 W. Electron probe microanalysis (EPMA) and X-ray diffraction (XRD) results indicate that Y atoms substitute Cr and/or Al atoms in CrAlN lattice forming the solid solution CrAlYN coatings. The surface and cross-sectional morphologies of the CrAlYN coatings exhibit tapered grains and columnar structure respectively. Moreover, the tapered grain sizes and the column widths decrease with increasing Y content. Nanoindentation result reveals a promoted hardness of the CrAlYN coatings from 16.9 ± 0.8 GPa to 24.1 ± 1.0 GPa with enhanced Y content from 0 to 2.3 at.% due to both of the solid solution strengthening and Hall–Petch effect. The oxidation result demonstrates that the Y content beneficial for the oxidation resistance of the CrAlYN coatings is between 0.3 and 0.7 at.%. Excess Y addition (≥ 1.3 at.%) significantly deteriorates the oxidation resistance of the CrAlYN coatings as a result of the formation of porous and non-protective oxide scales.