Depth-Dependent Tribofilm Formation in Biodiesel-ZDDP Lubrication: Linking Interfacial Chemistry to Friction and Wear

Biodiesel fuel dilution can modify the tribochemical response of zinc dialkyldithiophosphate (ZDDP) in engine lubricants, yet **the way** real multicomponent biodiesel alters the depth distribution of ZDDP-derived tribofilms remains unclear. Here, reciprocating tribological tests were combined with three-dimensional non-contact profilometry, SEM–EDS, FTIR and Ar+ sputter depth-resolved XPS to examine biodiesel (BD) and ZDDP, separately and in combination, in polyalphaolefin-6 (PAO6). Increasing BD concentration progressively reduced friction and wear; at 5 wt% BD the corrected wear scar diameter decreased from 292.3 to 195.3 μm and mean friction coefficient from 0.12 to 0.06. Adding 1 wt% ZDDP reduced wear scar diameter to 150.5 μm. The 9 wt% BD/1 wt% ZDDP/PAO6 formulation gave smallest wear scar (131.4 μm) and lowest measured wear volume (45,616 μm³), 28.6% below that obtained with ZDDP alone. Depth-resolved XPS showed that ZDDP/PAO6 tribofilm retained P-, S- and Zn-containing reaction products over broad sputtered-depth range. In contrast, BD/ZDDP/PAO6 tribofilm displayed pronounced depth-dependent compositional gradient: the outermost surface was enriched in C–C/C–H and ester-related oxygenated carbon species, whereas P, S and Zn signals increased sharply in subsurface region. High-resolution spectra assigned P 2p primarily to phosphate/polyphosphate-related species and S 2p to reduced sulfur species. These results indicate that biodiesel does not simply increase amount of ZDDP-derived inorganic products. Instead, it reorganizes their spatial distribution, producing organic-rich near-surface region over ZDDP-derived reaction-rich subsurface region. This graded architecture provides plausible basis for improved balance between friction reduction and wear protection.