Structure and properties of hard and superhard Zr–Cu–N nanocomposite coatings

Zr–Cu–N nanocomposite films represent a new material of the type-nanocrystalline transition metal nitride (nc-MeN)/metal. In the present work, films were deposited onto steel substrates using unbalanced dc reactive magnetron sputtering of a Zr–Cu (62/38 at.%) target. Film structure, chemical composition, mechanical and optical properties were investigated by means of X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, wavelength dispersive electron probe microanalysis, depth-sensing microindentation and spectroscopic ellipsometry. It was found that (i) there is a strong correlation between the film structure, Cu content and film properties and (ii) either hard or superhard Zr–Cu–N films can be formed. The superhard coatings with hardness H>40 GPa are characterized by a columnar structure, a strong 111 XRD peak from ZrN grains and no diffraction peaks from Cu. These films exhibit a high elastic recovery of about 80% and contain a very low amount of Cu, approximately 1–2 at.%. In contrast, the hard (<40 GPa) Zr–Cu–N films are characterized by many diffraction peaks from polyoriented ZrN and Cu grains, a more random microstructure and a Cu content higher than 2 at.%. The optical properties of nanocomposite Zr–Cu–N films depend on the stoichiometry of the hard ZrNx compound and the content of Cu in the film.