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Abstract
Carbon-alloyed transition metal dichalcogenide (TMD) coatings have great potential for providing a good tribological response in diverse operating environments. There are different ways to synthesize these coatings by magnetron sputtering, with no clear indication of the best possible route for potential upscaling. In this study, tungsten-sulfur-carbon (W-S-C) coatings were deposited by radio frequency (RF) magnetron sputtering via four different methods. All coatings were sub-stoichiometric in terms of the S/W ratio, with the bombardment of the growing film with backscattered Ar neutrals being the main mechanism governing the S/W ratio. The crystallinity of the films was dependent on the C and S contents. X-ray photoelectron spectroscopy (XPS) revealed W-S and W-C bonding in all coatings. Raman spectroscopy showed the presence of an a-C phase with predominant sp2 bonding. The hardness of the coatings may be related to the C content and the S/W ratio. A friction coefficient of 0.06–0.08 was achieved during sliding in ambient air by the coatings deposited in non-reactive mode with optimal C contents. The results indicate that sputtering in non-reactive mode should be the method of choice for synthesis of these coatings.
Highlights
Tungsten disulfide (WS2 ) is a well-known transition metal dichalcogenide (TMD) often used for tribological applications due to its lubricity
The hydrogen content of the reactive sputtered films was not considered; the results presented for the hydrogenated coatings show the sum of C, W, S and O being 100 at.%
The difference in the S/W ratio was attributed to the different levels of bombardment with backscattered Ar neutrals
Summary
Tungsten disulfide (WS2 ) is a well-known transition metal dichalcogenide (TMD) often used for tribological applications due to its lubricity. Its solid lubricant character is a result of its hexagonal crystal structure consisting of layers of transition metal (W) sheets sandwiched between two layers of chalcogen atoms (S). The bonding within the S-W-S sandwich is of the covalent type, while the sandwiches are held together by weak van der Waals forces. Because of the weak bonding between lattice layers, when shear forces are applied, interlamellar slip occurs resulting in easy sliding [1]. TMD materials have been used to improve the tribological behavior of different tribosystems. Some of the routes by which TMD materials are used include the following: coatings (thin films) [2], additives in oils [3], or tribochemically induced formation of TMD tribolayers [4]. Magnetron sputtering has often been a method of choice for deposition of TMD-based thin films because of its versatility
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