Ag Surface Diffusion and Out-of-Bulk Segregation in CrN-Ag Nano-Composite Coatings

CrN-Ag nanocomposite coatings are deposited on Si(100) wafers and 20MnCr5 steel disks in amixed Ar+N2 atmosphere by reactive magnetron sputtering. Structure, composition and morphologywere investigated by Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy (AES),X-ray Photoemission Spectroscopy (XPS), X-ray Diffraction (XRD) and Focused Ion Beam (FIB)cross sectional analysis. The as deposited film matrix is mainly composed by CrN phase (78%), buta relevant part (28%) is composed by Cr2N. Ag agglomerates in the CrN matrix forming elongatedgrains 200–400 nm wide and 50–100 nm high, which extends on the top of CrN columns. At thesurface Ag aggregates into two different structures: large tetrahedral crystalline clusters, with typicaldimension ranging from 200 to 500 nm, and smaller Ag nanoparticles with diameter of 15–25 nm.The annealing in N2 atmosphere up to 500 C does not affect size and distribution of the Aggrains in the sub-surface region, while it induces a size increase of the bigger Ag clusters onthe surface, mainly related to Ag surface diffusion and clusters coalescence. Annealing at highertemperature leads to an evident Ag out-of-bulk segregation, generating Ag depleted voids in thenear-surface region, and further increasing of the Ag clusters size at the surface. Tribological testson as deposited CrN-Ag film reveal a coefficient of friction against a steel ball reduced with respectto CrN film, probably related to the presence of Ag which acts as solid lubricant, but the coatingis removed after a very short sliding distance. The poor mechanical properties of the realized Agbasedcoatings are confirmed by lower hardness and Young modulus values with respect to pureCrN.