Microstructure, micro-mechanical and tribological properties of the nc-WC/a-C nanocomposite coatings magnetron sputtered on non-hardened and oxygen hardened Ti–6Al–4V alloy

Nanocomposite nc-WC/a-C coatings with hydrogen-free amorphous carbon a-C matrix have been deposited onto the surface of a baseline (non-hardened) and oxygen hardened Ti–6Al–4V alloy by means of a reactive magnetron sputtering. Oxygen hardening of the Ti–6Al–4V alloy was performed by means of oxygen atoms soaking from the Ar + O2 atmosphere at 1173 K with assistance of plasma glow discharge. Investigations of a micro/nanostructure as well as micro/nanohardness and tribological properties of the baseline and surface treated alloys were performed. The results of micro/nanostructural analyses showed that the coatings are composed of different tungsten carbides nanocrystals (up to 6 nm in size) embedded in an amorphous matrix. Scratch tests revealed a better adhesion of the coatings to the oxygen hardened substrate, than to the baseline alloy. Oxygen hardening in plasma glow discharge effectively improved the surface properties of the investigated alloy. It was established that the nc-WC/a-C coating on oxygen hardened alloy has much better tribological properties than the same coating produced on baseline alloy. The applied duplex surface treatment combining the oxygen hardening with subsequent deposition of nc-WC/a-C coatings decreases the friction coefficient as well as essentially improves microhardness and wear resistance in dry sliding of the coated Ti–6Al–4V alloy. The results are promising from the point of view of potential applications of such nanocomposite coatings in aeronautics and automotive industry for the components exposed to sliding wear.