Effect of the oxygen content on the structure, morphology and oxidation resistance of Cr–O–N coatings

Cr–O–N coatings were produced by reactive cathodic arc deposition at different N2/O2 flow ratios onto cemented carbide substrates. The structure, and mechanical properties of the coatings depend strongly on their oxygen content. The increase of the oxygen content leads to a decrease of the mean crystallite size of the nanocrystalline cubic (B1 structure) Cr–O–N phase and to an enhancement of the (002) preferred orientation. At O/(O + N) ratios > 0.7 the coatings crystallize in the rhombohedral Cr2O3 structure. The morphology of the samples, as studied by scanning electron microscopy (SEM), is columnar. The hardness of the coatings increases up to a maximal value of 28 GPa with increasing the oxygen content. Higher oxygen contents lower the hardness of the coatings. X-ray powder diffraction (XRD) studies were performed in situ at high temperatures, in vacuum and in air. The crystallite growth at elevated temperatures, both in vacuum and in air, is hindered significantly by the presence of oxygen in the coatings. The Cr–O–N coatings with the B1 structure, annealed in vacuum and in air, provide an improved thermal stability, with no evidence of oxidation or formation of the Cr2N phase up to 900 °C.