Ultra-low friction W–S–N solid lubricant coating

Abstract

W-S-N films were deposited by reactive magnetron sputtering from WS2 target in Ar/N2 atmosphere. Besides the standard evaluation of composition, structure, morphology, hardness and cohesion/adhesion, the core objective of this paper was to analyze coating tribological behavior. The chemical composition was 34 at.% N, 12 at.% O, 29 at.% W and 25 at.% S and the as-deposited films were completely amorphous. The film thickness was 2.3 µm including the approximately 300 nm thick adhesion improving titanium interlayer. The friction coefficient was lower than 0.003 when sliding in dry nitrogen. The coating showed remarkable wear resistance surviving more than 2 million laps on pin-on-dics. The excellent friction properties were attributed to the formation of a thin tungsten disulfide tribofilm on the top of the wear track of the coating and on the counterpart surface. Moreover, the coating showed ability to replenish damaged areas with solid lubricant. We demonstrated that a

Amorphous W–S–N thin films: The atomic structure behind ultra-low friction

“…As demonstrated in this work, amorphous W-S-N is an intricate elemental mixture, with a good supply of W and S atoms to diffuse to wherever the contact spots are found in a given point in time, allowing for easy WS 2 formation at the surface. This is in line with the reports from Gustavsson et al, who showed that after sliding, the outermost surface on a W-S-N coating with 34 at.% N consisted of well-aligned WS 2 [16]. It is highly important that the added element does not hinder the formation of a lubricating tribofilm by, for example, remaining in the contact or forming other compounds which give high friction or wear.…”

“…For the theoretical simulations a composition of W 0.33 S 0.33 N 0.34 was chosen, similar to the N-31 film, and to W-S-N with 34 at.% of N for which ultra-low friction was observed [16]. The simulations consisted of 66 W, 66 S and 68 N atoms, giving a total of 200 atoms.…”

“…Such nanocomposite, or amorphous, W-S-X coatings have been shown to be successful low-friction coatings due to the formation of WS 2 tribofilms. Among these, the amorphous W-S-N coatings have displayed exceptionally good lubricating properties in dry conditions [10,16]. There is thus no reason to assume that the presence of crystalline or nano-crystalline WS 2 in the as-deposited film is a prerequisite for formation of a wellfunctioning WS 2 tribofilm.…”

“…For optimizing the benefits of WS 2 lubrication, non-metallic nitrogen should thus be a good candidate as the added element, and Nossa and Cavaleiro [10,[12][13][14][15] first demonstrated low-friction W-S-N films. The concept was further investigated in a recent study by Gustavsson et al, who demonstrated ultra-low friction (l < 0.003) for an amorphous W-S-N film [16]. http This has potential in opening up new ways to find lowfriction surfaces and is a small breakthrough in the field of solid-state lubrication.…”

Structure and Properties of Titanium Doped Tungsten Disulfide Thin Films Produced via the Magnetron Co-Sputtering DC Technique

“…The rapid damage of the doped coatings was caused by their increased fragility, which resulted from the incorporation of Ti atoms into the structure between the planes formed by sulfur [30]. This phenomenon limits the sliding of the planes, thereby reducing the plasticity of the film and thus the lubricant effect; nevertheless, reports have indicated that highly beneficial tribological properties of WS 2 films can be achieved by doping them with Ti and/or other compounds [8,22,[30][31][32][33][34][35][36]. In the case of the films studied in this work, their poor tribological properties can be attributed to the low coatings thickness (507.2 nm), which leads to rapid wear and prevents the films from being able to bear sufficient load.…”