Influence of composition, structure and testing atmosphere on the tribological performance of W–S–N coatings

Abstract

W-S-N coatings deposited by reactive magnetron sputtering offer the possibility of ultra-low friction in unlubricated sliding. In this work, W-S-N coatings of different composition and structure have been deposited, characterised and evaluated with respect to the tribological performance and tribofilm formation. The composition was varied by changing the flow of N 2 into the deposition chamber, leading to N contents ranging from 0 to 47 at.%. W-S-N coatings deposited without substrate heating are amorphous, while substrate heating results in coatings containing nanocrystalline tungsten sulphide (WS x ) for low N contents, and nanocrystalline tungsten nitride (W y N) at a high N content. The coatings were tribologically tested against steel balls in four different atmospheres-dry N 2 , dry air, humid N 2 and humid air-to study the effects of atmospheric O 2 and H 2 O both separately and simultaneously. In dry N 2 , all coatings exhibited an excellent performance with very low friction (μ ≈ 0.02) and wear. Notably, this included the N-richest and hardest coating, containing nanocrystalline W y N and only 13 at.% of S. The friction and wear increased on changing the atmosphere, in the order of dry airhumid N 2 -humid air. In these three non-inert atmospheres, the friction and wear also increased with increasing N content of the coating. It is thus concluded that the presence of O 2 , the presence of H 2 O, and a high N content (i.e., low W and S contents) are three factors increasing the risk of high friction and wear, especially when occurring together. Raman spectroscopy mapping of the contact surfaces on the coatings and the balls showed that low friction and wear is connected to the presence of WS 2 tribofilms in the contact, and that the three previously mentioned factors affect the formation and function of this tribofilm.

On the S/W stoichiometry and triboperformance of WSxC(H) coatings deposited by magnetron sputtering

“…7h). The S3-D290 coating, whose low CoF of~0.10 in humid air was attributed to both DLC and richness in WS 2 , shows a Raman fingerprint with stretching bonds of WO 3 at 700-810 cm −1 [19] testifying to oxidation. The latter was probably produced mostly by reactions of WS 2 with H 2 O from the humid air.…”

“…Voevodin et al [6] indicated that the WS 2 /a-C composite with sulfur content < 15 at.% had a surprisingly high CoF of 0.5-0.7 in vacuum and 0.2-0.3 in dry nitrogen, consistent with the tribological behavior of singlephase unhydrogenated DLC in vacuum. A low Y/Me ratio is generally attributed to the preferential resputtering of Y due to the bombardment of energetic particles reflected on the coatings [18][19][20][21] and the reactions between MeY 2 and the residual atmosphere (e.g. H 2 , O 2 ) [17,20,22].…”

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.…”

Structure, Mechanical and Tribological Properties of MoSN/MoS2 Multilayer Films

“…In our previous work, the deposited Mo-S-N composite films [5] exhibited a much more compact amorphous structure by doping MoS 2 with nitrogen. However, high nitrogen content exceeding 17 at.% for the Mo-S-N film was unfavorable for the tribological performance improvement, since the formation of a non-lubricious metal nitrides phase was adverse for forming continuous tribofilm [5,27,29].…”