Mechanism of Antiwear Property Under High Pressure of Synthetic Oil-Soluble Ultrathin MoS2 Sheets as Lubricant Additives.

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 MoS2 sheets is synthesized and investigated as lubricant additives between steel surfaces. It is found that, with the ultrathin MoS2 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 MoS2. Therefore, it is believed that, because of the good oil solubility and ultrasmall thickness, the ultrathin MoS2 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 MoS2. 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 MoS2 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 MoS2 sheets as lubricant additives and provides an inspiration to develop a novel generation of lubricant additives with high-strength ultrathin 2D materials.