Fluorine-carbon doping of WS-based coatings deposited by reactive magnetron sputtering for low friction purposes

Abstract WS 2 self-lubricant coatings were fluorine-carbon alloyed to improve the load-bearing capacity, decrease the friction and, further, explore the water/oil wettability behaviour. The WS-C/F coatings were deposited by magnetron sputtering using an Ar/CF 4 mixture. Up to a certain CF 4 flow rate, F was effectively incorporated in the coatings (maximum of 9.5 at.%) which led to both compactness and hardness increase. Further CF 4 flow rate increase led to fluorine content vanishing, while the oxygen and carbon contents increased, then hindering the mechanical properties. XPS results revealed that F and O are chemically bonded to W/S or W/C phases. The tribological performance evaluated at room temperature showed that, sporadically, the friction coefficient of the coating with 9.5 at.% F content reached lower values than WS 2 (COF = 0.04 against 0.05). Meanwhile, when it was tested at 200 °C, the same coating showed lower COF values in the overall test (0.016 compared to 0.030), this attributed to the higher interplanar distance due to F insertion in the WS 2 structure confirmed by XRD. F incorporation seemed to decrease the coating surface roughness and increased its hydrophilicity, with no effect on the oleophilicity. This study presents a further insight on the understanding of the fluorine doping of sputtered WS 2 coatings for potential mechanical application.