Tribological performance of hybrid surfaces: dimple-shaped anodized Al alloy surfaces coated with WS-CF sputtered thin films

Abstract

This work is on the study of the synergetic effect of anodized Al alloy surfaces, with thin tungsten disulphide films alloyed with carbon and fluorine (WS-CF) deposited by magnetron sputtering. The surface roughness and surface chemistry were changed to tune the surface wettability and improve the frictional behaviour of the hybrid surfaces. The SEM analysis of these surfaces showed that, although the film growth has led to the progressive pore narrowing, pores were still visible after deposition of an ~ 200 nm thick film. Even though the film deposition has induced the surface smoothening, the surface free energy (SFE) of the hybrid surfaces was not substantially altered, when compared to bare anodized surface with large dimple (LD). Accordingly, the contact angles measured with water (WCA) and α-bromonaphthalene (OCA) on the hybrid surfaces suggested that the film surface chemistry was the main responsible for the wettability results, having the bottom dimple-shaped surface roughness just a low effect. LD surface coated with pure WS2 coating (LD + WS2) showed a remarkable high hydrophobicity combined with a high oleophilicity. Pin-on-disc testing carried out at room temperature in dry conditions showed much lower coefficient of friction (COF) values for the coated samples (hybrid surfaces), comparatively to uncoated ones. Furthermore, the friction performance of the hybrid surfaces was better for the F-alloyed thin film (sample named as WS-CF5 and having 9.5 at% of F) than for the WS2 film. In lubricated testing with a polyalphaolefin oil (PAO-8), the soft uncoated surfaces presented lower COF values, since the rapid smoothening of the contact surface turned the lubrication regime from mixed to hydrodynamic-like. For the LD surfaces coated with the F-alloyed WS film (named as LD + WS-CF5), the higher hardness of WS-CF5 was responsible for keeping the lubrication regime in partial lubricated conditions, leading to higher initial COF. Afterwards, a WS2-based tribolayer is formed in the solid-solid contacts which, together with the progressive film smoothening, change the lubricated regime toward hydrodynamic. Therefore, the friction of LD + WS-CF5 was reduced with the elapsed time and approached the one of the uncoated samples (LD). Concluding, both these hybrid surfaces can have a role on oil-water separation processes, self-cleaning of building outdoors or on microfluidic and printing applications.