Achieving macroscopic superlubricity on the steel surfaces with a functionally designed Ce‑based emulsion

This study explored the achievement of macroscopic liquid superlubricity using a Ce-based emulsion at the steel/steel interface. The Ce-based emulsion was prepared with Poly-L-lysine (PLL), oleic acid (OA), and a cerium-based emulsifier. After an initial running-in period of approximately 300 s, a superlubricity state characterized by a minimum coefficient of friction (COF) of 0.001 and a maximum contact pressure up to 197 MPa was achieved. The superlubricity originated from the synergistic adsorption of OA, PLL, and the emulsifier, driven by the C=C/COOH groups in OA, the amino-rich chains of PLL, and the Ce³⁺ containing head of the emulsifier. This cooperative adsorption promoted the formation of a hierarchical tribofilm that effectively separated the sliding surfaces under boundary lubrication. The study demonstrated the potential of emulsion-based lubricants for achieving ultra-low friction and wear in mechanical engineering applications.