Highly oil-dispersed functionalized reduced graphene oxide nanosheets as lube oil friction modifier

rGO/GO Nanosheets in Tribology: From the State of the Art to the Future Prospective

“…This method seems to be superior with respect to other methods when considering dispersion in oils, due to the possibility of grafting alkylated modifiers on the sheets. As also mentioned previously, organic moieties and octadecylamine were used to produce alkylated rGO [56,68], and the tribological results indicated the formation of a relatively stable dispersion in commercial engine oil. They also exhibited a stable suspension for a controlled period of one month with a rGO concentration of 0.02 mg mL −1 .…”

“…In particular, Zhao et al [67] report the preparation of mildly thermally reduced graphene oxide at 700 • C for 5 h, which shows an optimal concentration of 0.5 wt%, beyond which additive aggregation under lubrication conditions highly occurs, leading to a reduced permeation of the as-obtained rGO aggregates into the rubbing surfaces. This agglomeration phenomenon at higher concentrations also occurs with chemically modified rGO [68] and surfactant-stabilized graphene nanosheets [70]: beyond the optimum concentration value, the higher the sheets aggregation (which could be caused by additive instability in oil), the more likely that the uninterrupted supply to metal surfaces could be not provided. In addition, aggregation can also start at the concentration at which lubricating surfaces become saturated with additive nanosheets [70].…”

“…In addition, aggregation can also start at the concentration at which lubricating surfaces become saturated with additive nanosheets [70]. In [68], the optimum concentration of the synthesized functionalized reduced graphene in base oil is 0.01 wt%, and the authors clearly explain the reason to choose this optimum amount, since: (1) at lower concentrations (0.005 wt%), rGO can easily disperse, though, due to the shortage of additive, the additive sheets cannot cover the whole metal surfaces with their protective film; (2) at higher concentration (0.015 wt%), the excess sheets will act as debris, producing abrasive-like wear. A similar trend in additive concentration and a similar corresponding explanation can also be formulated for the basal plane octadecylamine functionalized GO [56] and the stearic/oleic acid-modified graphene platelets [20].…”

“…With this method, stability in oil was guaranteed for 25 h because of the reduced number of oxygen groups of the rGO compared to the pristine GO. According to Ismail et al [68], click coupling between alkyne-functionalized GO and azido decane leads to the formation of triazole ring-decorated, reduced graphene oxide, which, in virtue of its hydrocarbon tail and after sonication, shows one-month stability in base oil. In the study by Mungse et al [56], hydroxyl and epoxide groups of graphene oxide were functionalized with alkyl chains constituted by octadecylamine.…”

The Influence of Hydroxyl Groups on Friction of Graphene at Atomic Scale

“…However, research on the friction of graphene oxides and how their related functional groups influence friction is relatively little. In a previous experimental work, graphene oxides, as lubricant addictive, were found to reduce friction and wear [10]. The role of oxygen functional groups of graphene oxides for lubrication were also discussed by measuring the friction coefficient and the wear of two reduced graphene oxides terminated with hydroxyl and epoxy-hydroxyl groups and polyethylene glycol [4].…”