Dry sliding wear of magnetron sputtered TiN/CrN superlattice coatings

TiN/CrN superlattices, consisting of alternating nanometer-scale TiN and CrN layers, were deposited by the reactive magnetron sputtering technique and characterized using X-ray diffraction, nanoindentation, pin-on-disc testing and SEM/EDS analysis. The hardness of the superlattices first increases rapidly with decreasing bilayer period Λ, followed by a continuous decrease, giving rise to a maximum at Λ≈10 nm. Higher hardness corresponds to larger H3/E*2 ratio, which is an indication of higher plastic deformation resistance. The coefficients of friction of the superlattices against WC–Co counterpart are in the range of 0.75–0.95, lower than those for a commercial TiN hard coating (1.0–1.20). The TiN/CrN coatings slid against WC–Co pins exhibit much higher wear resistance than TiN and the wear rate decreases with an increase in hardness, with the highest hardness (H=35.4 GPa) corresponding to a wear rate less than 9% of the wear rate of a TiN coating. When superlattices act as a disc counter-face, the loose wear debris generated during dry sliding comprises a mixture of heavily oxidized pin and coating materials. A W-rich patchy-like tribo-layer is present on the wear track, indicating the occurrence of materials transfer during the sliding process.