A Micro-Contact Model for Boundary Lubrication With Lubricant/Surface Physiochemistry

Abstract

A model is developed to study the tribological behavior of sliding micro-contacts. It provides a building block to the modeling of tribo-contacts in boundary lubrication. Three contact variables are calculated at the asperity-level by relating them to the state of contact and the state of asperity deformation. These variables include micro-contact friction force, load carrying capacity and flash temperature. The deformation of the contacting asperity is either elastic, elasto-plastic, or fully plastic. Furthermore, the asperity may be covered by the lubricant/additive molecules adsorbed on the surface, protected by a surface oxide layer or other chemical reaction films, or in direct contact with no boundary protection. The possibility of the contact in each of these three states is represented by a corresponding contact probability. A numerical method is developed to determine the contact state and contact variables in the course of an asperity-to-asperity collision. The asperity flash temperature, which governs the kinetics of lubricant/surface adsorption/desorption, is first calculated by integrating the Jaeger equation over the contact area and in time. Then, the probability of contact covered by an adsorbed film is determined using the Volmer adsorption isotherm, and the probability of contact protected by the oxide layer is estimated using a classical wear theory. For elastic/elasto-plastic deformation of the asperity, the friction coefficient is given by the linear combination of the friction coefficients of the three contact states with their contact probabilities as the weighting factors. For fully plastic deformation of the asperity, the contact pressure and friction force become dependent of each other. The shear stress is approximated by a linear function of the contact probabilities, and the contact pressure and friction coefficient then calculated. Meanwhile, the influence of fresh surface generation due to plastic flow on the contact probabilities is also modeled. Insights are provided into the asperity collision through numerical studies of a sample problem. In addition, parametric studies are carried out to analyze the effects of lubricant and surface parameters on the micro-contact severity and its load capacity.