Influence of Ti addition on the structure and properties of low-friction W–S–C coatings

Transition metal dichalcogenides, such as WS2 and MoS2, are known for their layered structure and lubricating properties. When deposited as thin coatings, however, their use as solid lubricants is limited by their low hardness and load-bearing capacity. The addition of another element, such as carbon, can improve the mechanical properties, although the hardness of for example W–S–C coatings is still rather low. In this work, Ti has been added to W–S–C coatings in order to further increase the hardness by carbide formation. W–S–C and W–S–C–Ti coatings were deposited by non-reactive magnetron sputtering, and characterized with regard to composition, structure and mechanical and tribological properties. It was found that the addition of Ti leads to the formation of a new carbide phase, and a significant increase in hardness for coatings with moderate carbon contents. The friction properties of W–S–C–Ti coatings were found to be comparable to that of W–S–C coatings, with friction coefficients down to μ ≈ 0.02 and similar wear rates against steel in a dry atmosphere. Formation of WS2 in the wear track of W–S–C–Ti was confirmed by transmission electron microscopy. It has thus been shown that the addition of Ti to W–S–C coatings can increase the hardness, while still maintaining WS2 lubrication.