A Molecular Dynamic Approach to Nano-Scale Lubrication: Surface Nature & Molecular Effects
Résumé
In molecular lubrication problems, lubricants are confined to molecular scale thicknesses. Experiments have shown that in such confinements the structure and dynamics of lubricants are greatly influenced by the nature of the confining surfaces. The well-organized surface structure is reflected, through the interfacial interactions, by a set of potential valleys that lubricant molecules are attracted to occupy. Any surface, no matter how geometrically smooth, manifests a sort of a foot-print over the neighboring lubricant layers. The surface interaction is a combination of two components: adsorption and corrugation potentials. In a Lennard-Jones model for the surface interactions, the adsorption potential increases with the surface density and with the well-depth energy of the surface-lubricant interaction. The corrugation potential, on the other hand, depends on the in-plane potential variations which are due to the crystalline structure and density of the surfaces. A simple method is proposed which is inspired by this need for quantitative measurements of adsorption and corrugation potentials. The method is initially used to compare the properties of different surface types. These surfaces are then used to confine a lubricant film in molecular lubrication conditions. The results for lubricant structuring, boundary flow and friction are analyzed according to the measured adsorption and corrugation. The major effects of lubricant molecular nature and shape in confined films between wetting and non-wetting surfaces are then studied. In terms of global friction it was found that corrugation is the most influencing factor through its strong relationship with boundary slip and thus surface wettability. In comparison between geometrically smooth surface models, low-friction surfaces can be identified as those with the lowest potential corrugation. In pure confined lubricants, the increase of chain length and branching results in an attenuation of the layering phenomena. The structure of the film becomes bulk-like homogeneous in branched molecules at only three molecular diameters distance from the confining surfaces. Long and branched chains encounter an increased level of inter-layer bridging which results in a stronger internal lubricant cohesion. When no boundary slip occurs as in the case of wetting surfaces, molecular length and branching increase friction because of the enhanced effective viscosity of the film. However near non-wetting surfaces, the internal film cohesion in long and branched chains may become stronger than their cohesion with the surfaces. This results in a larger boundary slip and an attenuation of friction in these films. Finally, hexadecane and ZDDP polar anti-wear additive were simulated. In agreement with experiments, the presence of ZDDP resulted in a higher friction compared to pure hexadecane oil. It was found that the molecular migration and adsorption of ZDDP molecules at the surfaces are the cause of this phenomenon. When ZDDP stick to the non-wetting surfaces, they significantly reduce the boundary slip which normally occurs with pure hexadecane oil. Moreover in the case of wetting surfaces, the whole adsorbed layer becomes locked to the surfaces. This results in a global increase of the average shear rate in the middle of the film. The central hexadecane film becomes sheared at a higher rate giving rise to a higher shear stress and friction, in agreement with experimental findings.