The Interplay between Polar Base Oils and Ester Additives Influences ZDDP Adsorption and Tribofilm Formation

To reduce the environmental impact of shock absorber fluids, bio-based ester oils are being considered as alternatives to conventional mineral oils.Due to their high polarity, ester base oils can influence adsorption and tribofilm formation.In this study, the effects of pentaerythritol dioleate (PDO) and pentaerythritol tetraoleate (PTO) on the adsorption behavior of zinc dialkyldithiophosphate (ZDDP), tribofilm formation, and friction characteristics in the polar ester base oil 2-ethylhexyl oleate (EHO) were investigated.Friction tests, attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy coupled with energydispersive X-ray spectroscopy (SEM-EDX), and quartz crystal microbalance with dissipation monitoring (QCM-D) measurements were conducted.In EHO, the PDO-based lubricant exhibited lower friction than PTO.ATR-FTIR and SEM-EDX analyses indicated that PDO altered the composition and spectroscopic characteristics of the ZDDP-derived tribofilm, whereas PTO had less influence.QCM-D measurements revealed that EHO formed a residual adsorption layer on iron oxide surfaces and that PDO and PTO exhibited different adsorption behaviors in EHO.The combined ZDDP-PDO system exhibited adsorption characteristics distinct from those of the individual additives.These results suggest that, in polar base oils, the base-oil-derived adsorption layer may modify additive adsorption behavior, which may in turn influence tribofilm formation and friction characteristics.