Evaluating the influence of adding Nb on microstructure, hardness and oxidation resistance of CrN coating

In this study, Cr-Nb-N coatings with a 0-1.7 at.% Nb addition were fabricated by magnetron co-sputtering deposition in an Ar-N2 mixture atmosphere. The influence of Nb addition to the microstructure, indentation hardness and oxidation resistance of the CrN coating has been investigated. The results reveal that the Nb ions substitute the Cr ions in the Cr-N lattice, forming the solid solution-type coatings. All the cross-sectional morphologies of the Cr-Nb-N coatings illustrate a dense and columnar microstructure, showing independent of Nb content. Nevertheless, incorporating the Nb into the CrN provides a favorable condition for the grain size refinement of the coating. In addition, improving the Nb content promotes the preferred growth of the (1 1 1) grains. Nanoindentation tests reveal an enhanced hardness of the coatings from 15.2 ± 0.6 to 19.1 ± 0.7 GPa with increasing the Nb content from 0 to 1.7 at.% due to both the solid solution strengthening and the grain size refinement. The oxidation results demonstrate that the most beneficial addition of the Nb to improve the oxidation resistance of the CrN coating is 0.5 at.%. An excessive addition of Nb deteriorates the oxidation resistance. The outward diffusion of the Cr ions will be effectively blocked by the preferential transport of the larger Nb ions, resulting in the improved oxidation resistance.