Coupled Effects of Stearic Acid Concentration and Base‐Oil Viscosity on Bio‐Based Secondary Lubricants for Assisted Water Lubrication

To improve mixed/boundary lubrication in water‐lubricated bearings, this study investigates a small‐quantity‐assisted water‐lubrication strategy using stearic‐acid‐modified plant‐derived ester oils. Two environmentally acceptable base oils with markedly different viscosities are blended with 0–3 wt% stearic acid and evaluated under flowing‐water conditions. For the low‐viscosity oil, formulation AS‐3 reduces the steady coefficient of friction (COF) by 8.5% relative to pure water lubrication and by 3.1% relative to the neat base oil, while reducing wear volume by 4.46%. For the high‐viscosity oil, formulation BS‐1 reduces the steady COF by 7.7% relative to pure water lubrication and by 3.0% relative to the neat base oil, while reducing wear volume by 29.01%. Film‐thickness measurements, underwater wettability tests, and fluorescence visualization suggest that the primary requirement for assisted lubrication is the retention and transport of the nonaqueous lubricant toward the inlet region, where an oil‐rich reservoir can be formed and partially entrained into the contact. The large viscosity difference between the two base oils strongly affects this reservoir persistence and film‐forming behavior, whereas stearic acid acts as a secondary interfacial modifier that may influence oil/water/solid affinity and boundary‐film stability once sufficient oil is present. Comparison with pure water, neat base oils, and previous assisted water‐lubrication strategies indicates that the optimum organic friction modifier concentration depends on base‐oil viscosity rather than following a universal value. These results suggest that, for on‐demand assisted water lubrication under critical mixed/boundary conditions, secondary‐lubricant selection should consider not only environmental acceptability but also base‐oil retention capability and concentration matching of the friction modifier.