Achieving very low wear rates in binary transition-metal nitrides: The case of magnetron sputtered dense and highly oriented VN coatings

• Wear rates of ~ 10E− 17 m3/N m obtained in the dense and highly oriented VN coatings
• Wear mechanism: layer-by-layer mode rather than fracture-dominated modes
• Key factors are strong column boundaries in addition to dense structure.

The monolithic binary transition-metal nitride coatings are easy to prepare but typically suffer from a high wear rate (typically far greater than 10− 16 m3/N m). Here we report a mechanistic investigation the wear behavior of vanadium nitride (VN) coatings which achieved wear rates on the 10− 17 m3/N m order of magnitude. The VN coatings were reactively magnetron sputtered at 773 K with relatively strong ion bombardment. The wear behavior was studied by ball-on-plate sliding tests, followed by microscopic examination using scanning electron microscopy (SEM) and atomic force microscopy (AFM), and chemical study by site-specific micro-Raman spectroscopy. Dense and highly oriented columnar VN coatings, 25–30 GPa in hardness, were obtained. Both coatings retained subsurface structural integrity after 1000 m sliding, and exhibited very low wear rates (< 5 × 10− 17 m3/N m). The worn surface was very smooth, free of grooves and cracks in SEM, while fine features such as even platelets and infrequent nano-plowing events were identified by AFM. Meanwhile, chemical analysis of the worn surface identified the formation of V2O5-rich debris. These results point to the layer-by-layer wear mechanism, for which we proposed a scenario to account for some features of our microscopic observations.