The wettability and tribological behaviour of thin F-doped WS2 films deposited by magnetron sputtering

This work aims to study the influence of the thickness on the wettability behaviour and on the tribological performance of thin WS2-C coatings doped with fluorine (F) deposited by magnetron sputtering. The morphology of the coatings did not change with the thickness decrease, revealing an increase on both the compactness and on the smoothening of the coating surface with F increase. Such an evolution led to more hydrophilic and less oleophilic surface behaviour. XRD results showed that the coatings start growing with an alignment of the (002) planes parallel to the substrate, which changed for randomly oriented crystals with the thickness increase. F insertion induced a decrease in the crystallinity, more evident in the case of thinner coatings. This result was corroborated by Raman analysis which showed a broadening of the main peaks assigned to WS2. Tribological tests carried out at room temperature without lubrication showed lower friction coefficient for F-doped coatings, especially for the thinner one (in the overall test, average 0.03). Regarding the lubricated tests performed with pure PAO (polyalphaoleophin) oil, both thin coatings presented similar friction coefficients (similar to 0.07) in the steady-state regime. The tribolayer based on oriented W-S layers formed on the wear track for the F-doped coating can justify the very low friction achieved when testing in non-lubricated conditions.