The influence of synthetic air flow on the properties of arc evaporated Al-Cr-O-N coatings

Highlights: • Monolithically grown (Al_xCr_1-x)(O_1-yN_y) using synthetic air. • Formation of crystalline single-phase cubic (Al_xCr_1-x)(O_1-yN_y). • Increasing thermal stability with increasing oxygen fraction. -- Abstract: Various (Al_xCr_1-x)(O_1-yN_y) oxynitrides with different nitrogen non-metal fractions y and two different multilayers thereof were developed by varying the synthetic air flow (without any other gas addition) during arc evaporation of powder-metallurgically prepared Al_0.70Cr_0.30 cathodes. The (Al_xCr_1-x)(O_1-yN_y) oxynitride coatings crystallize with a single-phase face centred cubic (fcc) (Al,Cr)N-based structure for y ≥ 0.8, and with a phase composition of fcc-(Al,Cr)₂O₃-like and corundum type α-(Al,Cr)₂O₃-based structures for y < 0.8. Two different multilayers, one with a steep and one with a smooth transition between the constituting (Al_0.66Cr_0.34)(O_0.85N_0.15) and (Al_0.62Cr_0.38)(O_1.0N_0.0) layers are prepared by periodically varying the synthetic air flow while arc evaporation the Al_0.70Cr_0.30 cathodes, leading to a bilayer period of ~20 nm. The hardness of our as-deposited (Al_xCr_1-x)(O_1-yN_y) oxynitrides, decreases from ~33 to 15 GPa with decreasing y from 0.8 to 0, while both multilayers demonstrate hardnesses of ~26 GPa. The onset temperature T_onset for thermally induced dissociation of Cr–N bonds of our oxynitrides, to finally form Cr under N₂-release, significantly increases from ~1000 to 1300 °C with decreasing N-content y from 0.8 to 0.15. The multilayer with a smooth layer-transition shows no mass loss up to the maximum temperature of 1500 °C, whereas that with a steep layer-transition shows a mass loss due to N₂-release with T_onset ~1250 °C. Dynamic oxidation treatments of our coatings in synthetic air up to 1500 °C indicate increased structural stability with decreasing nitrogen content y. Based on our results we can conclude, that by varying the synthetic air flow during arc evaporation of Al–Cr cathodes (here, Al_0.70Cr_0.30), (Al_xCr_1-x)(O_1-yN_y) oxynitrides with y ≤ 0.8 can be prepared, having excellent mechanical properties and thermal stabilities.