Tribochemistry of Transfer Layer Evolution during Friction in HiPIMS W-C and W-C:H Coatings in Humid Oxidizing and Dry Inert Atmospheres

The experimental and theoretical investigations of transfer layers in the dry sliding contacts between steel ball and HiPIMS W-C and W-C:H coatings were performed in humid air, dry nitrogen, hydrogen and vacuum on a series of coatings with different contents of carbon and hydrogen in the matrix. Transfer layers formed on the ball in all friction tests, but their composition varied depending on the environment. In humid air, the mechano(tribo)chemical reactions necessary for the obtained phases involved oxidation of WC and Fe, water vapor decomposition and hydrogenation of carbon. Modeling indicated that humidity enhanced oxidation and carbon hydrogenation. In nitrogen, WC decomposition generating carbon was dominant, whereas, in hydrogen, it was carbon hydrogenation. In vacuum, WC decomposition producing W was found to be responsible for high coefficients of friction (COFs). COFs approaching superlubricity were obtained in the H2 atmosphere in the coatings with sufficiently high matrix C:H content. COFs seem to be controlled by the ratio of hydrogenated carbon and oxide phases in transfer layer, which depends on the reactions possible in the surrounding atmosphere.