Potential-Controlled Boundary Lubrication Using MoS2 Additives in Diethyl Succinate

The active control of friction in oil-based lubricants was realized in the present study with the use of MoS2 particle additives and the application of an electric field. By modifying the surface charging state of the MoS2 particles, the dependence of potential-controlled boundary lubrication behavior on the electrical properties of the particles was demonstrated. For a diethyl succinate lubricant containing negatively charged MoS2 particles, the coefficient of friction (COF) was reduced by 60–70% when a positive potential was applied to a lower friction pair of copper plates. After modification with poly(diallyldimethylammonium chloride), the particles were positively charged, and the COF was reduced with the application of a negative potential. The mechanisms underlying the potential control of the COF were investigated by observing the distributions of the particle additives and characterizing the tribofilms formed at different potentials. Most of the charged particles were locally concentrated near the opposite pole, and this was reversed when the electric field changed. For locally high concentrations of MoS2 particles, a MoS2/MoOx tribofilm with a thickness of 100–500 nm and a loose structure formed on the lower friction pair, which significantly decreased the shear force during the friction process.