Influence of nitrogen and niobium incorporation in bcc-chromium coatings on microstructure and mechanical properties

Stoichiometric CrN and Cr:N with different nitrogen (N) content are of interest for hard coating applications. In the Cr-N material system, bcc-Cr rich coatings, containing a few percent of diluted N, provide tunability in microstructure and mechanical properties. Additionally, the incorporation of Nb into the CrN_x coatings may further tailor the materials properties. In this work, bcc-CrN_x and CrNbN_x coatings were deposited by reactive magnetron sputtering, and their mechanical and microstructural characteristics were investigated as a function of N content. Depending on the N content, the phases observed by X-ray diffractometry (XRD) varied from metallic bcc-Cr, mixed bcc-Cr/h-Cr₂N, to h-Cr₂N/CrN. X-ray reflectivity (XRR), and scanning electron microscopy (SEM) measurements show that a dense and nearly columnar-free bcc-CrN_x coatings was obtained at ~13–25 at.% of N, while CrNbN_x coatings composed of dense, column-free, and featureless microstructure at a N content of ~10–20 at.%. The dense and nearly column-free microstructure composed of dispersion of Cr₂N grains into the bcc-Cr matrix for both CrN_x and CrNbN_x coatings, as shown by high resolution transmission electron microscope analysis (HRTEM). Nanoindentation revealed a hardening of the coatings due to the grain refinement, solid solution strengthening, and variation in phase content.