In this paper, a new kind of oil-soluble ultrathin MoS2 nanosheets is prepared through a one-pot process. A superior extreme pressure property, which has not been attained with other nano-additives, is discovered when the nanosheets are used as lubricant additives. The as-synthesized MoS2 nanosheet is only a few atomic layers thick and tens of nanometers wide, and it is surface-modified with oleylamine so it can be well dispersed in oil or lubricant without adscititious dispersants or surfactants. By adding 1 wt% ultrathin MoS2 nanosheets, at the temperature of 120 °C, the highest load liquid paraffin can bear is tremendously improved from less than 50 N to more than 2000 N. Based on the tribological tests and analysis of the wear scar, a lubrication mechanism is proposed. It is believed that the good dispersion and the ultrathin shape of the nanosheets ensure that they can enter the contact area of the opposite sliding surfaces and act like a protective film to prevent direct contact and seizure between them. This work enriches the investigation of ultrathin MoS2 and has potential application in the mechanical industry.

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Wear occurs between two rubbing surfaces. Severe wear due to seizure under high pressure leads to catastrophic failures of mechanical systems and raises wide concerns. In this paper, a kind of synthetic oil-soluble ultrathin MoS sheets is synthesized and investigated as lubricant additives between steel surfaces. It is found that, with the ultrathin MoS sheets, the wear can be controlled under the nominal pressure of about 1 GPa, whereas the bearable nominal pressure for traditional lubricants is only a few hundred megapascals. It is found that when wear is under control, the real pressure between the asperities agrees with the breaking strength of ultrathin MoS. Therefore, it is believed that, because of the good oil solubility and ultrasmall thickness, the ultrathin MoS sheets can easily enter the contact area between the contacting asperities. Then, the localized seizure and further wear are prevented because there will be no metal-to-metal contact as long as the real pressure between the asperities is below the breaking strength of ultrathin MoS. In this way, the upper limit pressure the lubricant can work is dependent on the mechanical properties of the containing ultrathin two-dimensional (2D) sheets. Additionally, ultrathin MoS sheets with various lateral sizes are compared, and it is found that sheets with a larger size show better lubrication performance. This work discovers the lubrication mechanism of ultrathin MoS sheets as lubricant additives and provides an inspiration to develop a novel generation of lubricant additives with high-strength ultrathin 2D materials.