Thermodynamic analysis of lubrication considering solid-liquid interface interaction

Abstract

Friction or lubrication process is a typical process of the energy dissipation. It can be reasonably described and speculated by using the entropy increase and dissipative structure theory of the non-equilibrium thermodynamics. In this paper, we model and analyze the typical thin-film lubrication mechanism based on the theory of thermodynamics, by using the interfacial disjoining pressure to characterize the dominant role of the solid-lubricant interaction on a microscale and establishing the lubrication Stribeck curve based on thermodynamic concepts. The concept of entropy production is adopted to describe the lubrication system, which is defined as the sum of multiplications of the thermodynamic forces and flows. Then the variations and the competing relations between the pairs of thermodynamic forces and flows could be used to reveal the different factors dominated in the lubrication system, such as the solid-liquid interaction, the sliding velocity, and the normal load. In this paper, we assume that all the dissipated energy caused by the viscous resistance of lubricant is converted into heat, then the total entropy increase per surface area at the frictional interface is considered, affected by interfacial disjoining pressure and the one-dimensional heat flow. With the entropy increasing analysis of lubrication process, we find that when the entropy production in the steady state becomes minimum, the total energy dissipation due to friction also becomes minimum, which directly indicates the lowest friction coefficient point at the lubrication Stribeck curve. Moreover, when a lubrication system loses its stability slightly from the equilibrium state, self-organization may occur at the solid-lubricant interface, thus resulting in partially ordering interfacial structures, which may indicate the interfacial structures when tribosystem turns from hydrodynamic lubrication phase into thin-film lubrication phase. In the experimental aspect, the location of the lowest friction coefficient point at the Stribeck curve has a very good correspondence to the minimum entropy point predicted by our thermodynamic model, and the lubrication transition process from hydrodynamic phase to thin-film phase can be explained quite well by the theory of dissipative structures when the system loses its stability. Furthermore, a calculation model of the friction coefficient for thin-film lubrication is obtained when considering the dominant contribution of the solid-lubricant interfacial interaction through an equivalent force method. The calculation data correspond well to the experimental results. In summary, thermodynamic model could effectively characterize the lubrication process in mechanism by revealing the involved multi-scale effect, multi-physical effect and nonlinear coupling effect.