The effect of air humidity on tribological behaviours of W–C:H coatings with different tungsten contents sliding against bearing steel

Friction and wear behaviors of W–C:H coatings with different tungsten contents sliding against bearing steel balls at different air humidities were investigated. The worn out surfaces of steel balls and coatings were analyzed with the aid of scanning electron microscopy (SEM) and Raman spectroscopy. A tribolayer composed of a graphite-like material mixed with tungsten and iron oxides was observed on the friction surfaces of the steel balls. The chemical and phase compositions of the tribolayer, which depend both on the tungsten content in coatings and air humidity, determine the tribological properties of the W–C:H coating in a frictional contact with bearing steel. At average air humidity (50%), those coatings that contain less than 10 at% of tungsten in a frictional contact with steel exhibit favorable tribological properties. The friction coefficient of frictional contacts under test reaches a low value (f∼0.01) at a low air humidity and increases with humidity of up to ca. 0.2. The best tribological properties in a wide range of air humidity (5–90%) have been found for W–C:H coatings with the tungsten content between 2 and 5 at%.