Metal Substrate-Dependent Tribological Performance of Environmentally Acceptable Ester-PAO Lubricant Blends

Environmentally acceptable lubricants (EALs) are increasingly required in high-risk oil-to-sea interfaces, yet their boundary lubrication performance remains difficult to predict because it depends strongly on both lubricant chemistry and metal substrate reactivity. In this work, we compare the tribological response of stainless steel (AISI 316L) and bearing steel (AISI 52100) lubricated with a low-viscosity polyalphaolefin (PAO4), a polar pentaerythritol polyol ester (PE), and their 1:1 blended base oil, formulated with an environmentally acceptable ionic liquid, tributyl(ethyl)phosphonium diethyl phosphate (PEP), and with ZDDP as a benchmark additive. Ball-on-disc tests under identical boundary lubrication conditions show systematically lower friction and wear on bearing steel than on stainless steel, evidencing a strong substrate dependence. On stainless steel, a Cr-rich passive film limits additive-driven tribochemistry, and wear is therefore governed by base-oil polarity and additive-base lubricant competition for surface sites, which promotes abrasion when polarity increases. In contrast, the Fe-dominated chemistry of bearing steel enables tribochemical film formation, yielding lower wear across formulations. PEP delivers the most robust antiwear response, including cases of no measurable wear, and its performance is controlled by both adsorption and dispersion state: in PAO4, PEP forms an emulsion that deposits a thick viscoelastic layer suppressing wear but increasing friction, whereas in the base oil blend and PE, reduced emulsion stability and stronger surface competition shift the response toward phosphate-rich tribofilms and abrasive contributions. QCM-D reveals stronger/less reversible adsorption for PEP than for ZDDP, while ToF-SIMS confirms phosphorus-rich films as central to PEP protection and mixed P/S chemistry for ZDDP on Fe-reactive surfaces. These findings underline that the EAL formulation must be codesigned with substrate chemistry and base lubricant polarity to ensure reliable boundary performance.