A High-Resolution TEM/EELS Study of the Effect of Doping Elements on the Sliding Mechanisms of Sputtered WS2 Coatings

It has been shown many times that cosputtering low-friction coatings of molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) with other elements can improve the structural, mechanical, and tribological properties. To achieve the lowest friction, MoS₂ or WS₂ should be doped with element(s) improving the hardness and density of the coatings. On the other hand, such elements, or their compounds, should not be present in the outermost molecular layers at the sliding interface. This article suggests that there are important differences between how MoS₂ and WS₂ coatings respond to or react with doping elements, despite the almost identical structure and behavior of the undoped materials. Two systems have been investigated by high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) electron energy loss spectroscopy (EELS), W-S-C-Cr and W-S-C-Ti, and showed significant amounts of oxides, which typically formed a layer just underneath the crystalline WS₂ top layer. Further, carbon was almost completely absent in the tribofilms, despite the fact that the as-deposited coatings contained as much as 40–50 at% C. An interesting observation here is that WS₂ basal planes surround or embed Fe wear particles, suggesting a relatively strong adhesion or a Fe-S chemical bonding between iron/steel and WS₂. The result of this is that the wear particles become pacified and remain in the contact as low-friction material.