Achieving Superlubricity of Ricinoleic Acid in the Steel/Si 3 N 4 Contact Under Boundary Lubrication

To improve fuel efficiency and minimize environmental pollution, the interest in organic friction modifiers has resurged, especially fatty acids. Efforts have been made to enhance fatty acids’ performance by tuning their saturation state and adding OH lateral groups in the alkyl chain. Here, we compared lubrication properties in the boundary regime for oleic acid (OA), ricinoleic acid (RA), and linoleic acid (LA) under the same viscosity for a steel/Si 3 N 4 tribo-pair. The presence of the OH group in the OA chain reduces not only friction coefficient (CoF) by ~ 70% but also prevents scratches from generating on both surfaces. In the meantime, it determines local friction distribution by controlling top surface terminations. Even at 100 °C, the steady-state friction coefficient of steel/Si 3 N 4 lubricated by RA is only 0.014. A temperature decrease to 80 °C leads to a superlubricity regime (CoF < 0.01) resulting from the cooperation of nanometer-thick liquid film (possibly containing water) and low friction of contact asperities terminated by OH hydroxyl groups provided by RA. In summary, steel/Si 3 N 4 contact lubricated by ricinoleic acid gives unsurpassed tribological performances reaching superlubricity under a wide range of operating conditions.