Inthisstudy,wecharacterizedtheadsorptionofafewphosphorus-typeanti-wearagentsonanironoxidesurfaceinanester-based stock (diethyl sebacate, DES). In addition, the adsorption properties were compared with those in a non-polar base stock. The phosphorus-type anti-wear agents employed in this study were tricresylphosphate (TCP), tributyl phosphate (TBP), and diphenyl phosphite (DPPhi). Molecular simulation revealed that the absolute adsorption energy increased as TBP, TCP ＜ DES ＜ DPPhi. The combination of experimental results based on frequency modulation atomic force microscopy and quartz crystal microbalance with dissipation monitoring technique suggested that competitive adsorption of DES and the anti-wear agents occurred on the iron oxide surface. DES molecules formed a layered structure on the iron oxide, and the upper layers (except the first layer adsorbed directly on the iron oxide surface) were suggested to be replaced by the anti-wear agents. The competitive adsorption also affected the coefficient of friction.

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