A Mechanistic Framework for Friction in Mixed Lubrication Incorporating Shear-Thinning and Tribofilm Continuity

A mechanistic framework is developed to describe friction behavior in mixed lubrication, incorporating asperity contact mechanics, shear-thinning lubricant rheology, and interfacial tribofilm effects within a unified formulation. Conventional interpretations of tribofilm performance are primarily based on film thickness; however, such an approach is shown to be insufficient to explain non-monotonic friction trends observed across ultra-low- and low-viscosity lubricants. To address this limitation, an effective continuity parameter (ξ) is introduced to represent the fraction of load-bearing asperity contacts mediated by tribofilm. The parameter is formulated within a statistical contact framework and serves as a physically interpretable descriptor of tribofilm participation at the contact scale, rather than a directly measurable geometric quantity. This enables a transition from a thickness-based to a coverage-based interpretation of tribofilm effectiveness. The proposed framework is supported by systematic tribological measurements conducted using a mini-traction machine, combined with model-based analysis. A master-equation formulation is established to decouple the relative contributions of shear-thinning and tribofilm effects to overall friction. The results demonstrate that, under ultra-low-viscosity conditions, friction behavior is governed predominantly by tribofilm continuity and interfacial shear characteristics, whereas viscosity and hydrodynamic effects play a secondary role. The framework provides a physically consistent basis for interpreting friction behavior across lubrication regimes and offers a generalizable approach for integrating lubricant formulation, contact mechanics, and interfacial phenomena in friction modeling.