Application Feasibility of Graphene as Additive in Two Kinds of Lubricating Oil

A large number of studies have proved the good lubricating performance of graphene in base oil, but there are few studies on its performance in lubricating oil. Whether graphene can enter the additive system of lubricating oil and achieve good synergy with each component is the key problem to give full play to its various excellent characteristics and improve the comprehensive performance of the system. To explore the influence of graphene on the original additive system of lubricating oil and judge its feasibility as an additive, graphene dispersions were prepared by using multilayer graphene and multiple lubricating oil. The tribological properties of graphene dispersions under different ways of contacting were tested using MS-10A four-ball friction tester and TE77 long-stroke high-frequency reciprocating friction tester. The oxidation stability of lubricating oil was measured by Robot Bath 15200-5 rotary oxygen bomb tester of SETA company and HP DSC1 high pressure differential scanning calorimeter of METTLER TOLEDO company, and the sample was evaluated by standard method SH/T0193-2008. The high temperature cleaning effect of lubricating oil was tested by C-9 coking tendency tester of Shanghai METIS INSTRUMENT company, and the coking plate test of lubricating oil and graphene dispersions was carried out according to the standard method SH/T0300. The corrosion resistance of graphene dispersions was qualitatively determined by copper strip corrosion method GB/T5096. The dispersion testing results showed that the graphene failed to maintain long-term stability of dispersion in lubricating oil, while it kept agglomerated throughout the experiment. Therefore, it can be inferred that in practical application, graphene dispersions can not be stored and used for a long time, and the serious aggregation and sedimentation problems would make inevitable negative effects on its performance. The physicochemical properties showed that, graphene did not significantly improve the anti-oxidation, clean dispersion and corrosion resistance of the lubricating oil, while even reduced the original performance of the oil under some conditions. Concurrently, the tribological tests showed that graphene can not improve the tribological properties of the lubricating oil. Different contact modes had different effects on the performance of graphene dispersions. Under the point-to-point and point-to-face contact modes, the effect of graphene on the antifriction performance of lubricating oil was not obvious, and the friction coefficient increased gradually with the extension of testing time. In the line-to-surface and surface-to-surface contact modes, the addition of graphene significantly reduced the antifriction performance of lubricating oil. The analysis showed that the adopted graphene perturbed the balance and stability of the original system, and was unable to play a synergistic role with the additive system of lubricating oil. Meanwhile, the current limited preparation technology restricted the production quality and efficiency of high-quality graphene with even size and uniform thickness. Various performance indicators were far below the laboratory level, and the cost of large-scale preparation of graphene was much higher than that of commonly used lubricating additives. Hence, the above results indicated that, graphene materials were not feasible to be widely used as additives in lubricants at present, and the problems involved deserved further studying.