Abstract
Graphene (0.5 wt.%) was dispersed in the hydrophobic room-temperature ionic liquid 1-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (IL) to obtain a new non-Newtonian (IL + G) nanolubricant. Thin layers of IL and (IL + G) lubricants were deposited on stainless steel disks by spin coating. The tribological performance of the new thin layers was compared with those of full fluid lubricants. Friction coefficients for neat IL were independent of lubricant film thickness. In contrast, for (IL + G) the reduction of film thickness not only afforded 40% reduction of the friction coefficient, but also prevented wear and surface damage. Results of surface profilometry, scanning and transmission electron microscopy (SEM and TEM), energy dispersive analysis (EDX), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were discussed.
Highlights
The excellent performance of ionic liquids (ILs) as lubricants has been a subject of study for the last two decades [1,2,3,4]
As has been previously described [5], the load carrying ability of ILs in metal–metal and metal–ceramic contacts is enhanced by increasing alkyl chain lengths
An octyl-methyl-imidazolium cation was selected following previous results for (OMIM]BF4 + graphene dispersions and because ionic liquids with longer alkyl chain cations could be solid at room temperature, while the hydrophobic nature of bis(trifluoromethanesulfonyl) imide ([TFSI]) was one of the factors considered for its selection
Summary
The excellent performance of ionic liquids (ILs) as lubricants has been a subject of study for the last two decades [1,2,3,4]. This was attributed to stronger van der Waals interactions as the number of carbon atoms in the lateral chain increases This parallel orientation could favor the shearing of graphene sheets at the sliding contact, enhancing the lubricating performance of a hybrid graphene–[OMIM][TFSI] lubricant [28]. An intermediate concentration value of 0.5 wt.% was selected to develop the new (IL + G) lubricant studied in the present work Another relevant factor in the selection of the 0.5 wt.% graphene proportion is the recently described [30] influence of graphene proportion on the variation of viscosity with temperature for graphene dispersions in ionic liquids. The dispersion with 0.5 wt.% graphene content showed the best tribological performance
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