The tribological behaviour of graphene oxide nanosheets in mineral oil was investigated under a wide spectrum of conditions, from boundary and mixed lubrication to elastohydrodynamic regimes. A ball-on-disc setup tribometer has been used to verify the friction reduction due to nanosheets prepared by a modified Hummers method and dispersed in mineral oil. Their good friction and antiwear properties may possibly be attributed to their small structure and extremely thin laminated structure, which offer lower shear stress and prevent interaction between metal interfaces. Furthermore, the results clearly prove that graphene platelets in oil easily form protective film to prevent the direct contact between steel surfaces and, thereby, improve the frictional behaviour of the base oil. This evidence is also related to the frictional coefficient trend in boundary regime. 1. Introduction In the field of tribological applications, nanoparticles as additives in base oil have been extensively investigated. These studies refer to synthesis and preparation of nanoscale particles and their tribological properties and friction reduction mechanisms. It has been found that when the nanoparticles were added to base oil, the extreme pressure property and load-carrying capacity were improved and friction coefficient was decreased. In the past few years, nested spherical supramolecules of metal dichalcogenide have been synthesized by the reaction of metal oxide nanoparticles with H2S at elevated temperatures. Because of their nested fullerene-like structure, these species are known as inorganic fullerene-like (IF) nanoparticles. The IF nanoparticles exhibited improved tribological behaviour compared to the microscale platelets for their robustness and flexibility. The tribological properties of fullerene-like nanoparticles as additives to liquid lubricants were studied in [1–4]. Recently, due to high load-bearing capacity, low surface energy, high chemical stability, weak intermolecular, and strong intramolecular bonding, nanocarbon materials have received a great attention by tribology researchers. In recent years, graphene platelets due to their unique structure and remarkable properties have been the focus of interest in studies on practical applications. However, few studies on the tribological applications of graphene platelets have been reported so far. A number of researchers have reported that graphite [5] and some graphite derivatives [6, 7] as well as other lubricant materials [8–10] together have the above desirable properties. Lin et al. [11] investigated the
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