Numerical Rheological Modeling of Thin Lubricating Oil Films Considering Polymolecular Adsorption and ZDDP
DOI:
https://doi.org/10.31891/2079-1372-2026-121-3-19-26Keywords:
thin-film lubrication, polymolecular adsorption, ZDDP, pseudo-solid adsorption layer, effective viscosity, rheological modeling, friction coefficientAbstract
A numerical rheological model of a thin lubricating oil film was developed and verified to account for polymolecular adsorption, pseudo-solid adsorption layers, and the influence of zinc dialkyldithiophosphate (ZDDP) on the rheological properties of the lubricating film. The model adapts the concept of the Polanyi adsorption potential to describe the viscosity distribution across the film thickness, incorporating pseudo-solid adsorption layers adjacent to both friction surfaces, while the effective viscosity is determined by harmonic averaging with numerical integration performed using the trapezoidal rule (n = 200) implemented in Microsoft Excel. Shear-thinning of the bulk lubricant is described by the Carreau–Yasuda model. Model parameters were identified by fitting calculated friction coefficients to experimental data obtained for base oil I-20A and oils containing 1.25 and 2.5 wt% ZDDP.
The developed model represents a novel numerical approach that combines polymolecular adsorption, pseudo-solid adsorption layers and non-Newtonian lubricant behaviour for thin-film lubrication, suggesting that—beyond tribochemical processes—an increase in the effective viscosity of the polymolecular adsorption layer makes a significant contribution to friction reduction in the mixed lubrication regime. The proposed Excel-based methodology enables identification of model parameters and comparative prediction of lubricant tribological performance without specialized numerical software, and may be used as an engineering tool for assessing the influence of ZDDP concentration on the tribological performance of lubricating oils for plain bearings.
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