Independent Testing of WS2 Nanoparticles Functionalized by a Humin-Like Shell as Lubricant Additives
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Received: 2 November 2017; Accepted: 3 January 2018; Published: 4 January 2018
Abstract: Nanoparticles of transition metal dichalcogenides (TMDC) have been known to reduce friction and wear when added to oil-type liquid lubricants. Aggregation limits the ability of the nanoparticles to penetrate into the interface between the two rubbing surfaces—an important factor in friction reduction mechanisms. Doping has been successfully used to reduce agglomeration, but it must be done in the production process of the nanoparticles.
The use of surface-functionalized nanoparticles is less common than doping. Nonetheless, it has the potential to reduce agglomeration and thereby improve the reduction of friction and wear. In this study, we present the results of preliminary tribological ball-on-flat tests performed with WS2 nanoparticles functionalized by a humin-like conformal shell, as additives to polyalphaolefin-4 (PAO-4) oil. We tested WS2 inorganic nanotubes (INTs) and two grades of inorganic fullerene-like nanoparticles (IFs). The shell/coating was found to improve friction reduction for IFs but not for INTs through better dispersion in the oil. The thicker the coating on the IFs, the less agglomerated they were. Coated industrial-grade IFs were found, by far, to be the best additive for friction reduction. We suggest the combination between reduced agglomeration and poor crystallinity as the reason for this result.
Keywords: lubricant additive; oil; nanoparticles; WS2; functionalization
1. Introduction
In the last few decades, nanoparticles (NPs) have been widely studied and used to reduce both friction and wear. Inorganic NPs with lamellar anisotropic structure are good candidates for lubrication. They provide high compression strength and low shear strength. Examples of such materials are chlorides, borates and oxides of transition metals. Within this group, the transition metals dichalcogenides (TMDC) excel at tribological applications. TMDCs are sulfides, selenides and tellurides of the transition metals tungsten, molybdenum, tantalum, titanium, and niobium. Molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are by far the most studied in tribology, somewhat in their nanotube (INT) form, but much more extensively in their fullerene-like particle (IF) form. These polyhedral onion-like nanostructures were first discovered by Tenne et al. in 1992 [1] and have constantly shown the ability to improve the tribological properties of different systems ever since. Lower friction and wear can be achieved by adding TMDC NPs to oils [2–8] or by incorporating them in solid matrices [6,9–12] and in coatings [13–17].
WS2 particle source, researchgate.