Molecular Contact Pressure in Tribology

This paper is a part of a long-term study of the physics of boundary lubrication. In this regime, the shape and pressure of the film are controlled by two phenomena: first, the elastic or plastic deformation of the substrate, and second, the physical state of the film. Three physical states can be detected: the condensed solid, granular and colloidal states. A new piece of apparatus, allowing the continuous and simultaneous measurement of static and dynamic forces, displacement and current voltage between a sphere and a plane, is used to determine surface forces and rheological properties of the film. The viscous and elastoplastic behavior of the interface is given by a transfer function. The lubricant we studied was calcium dialkylbenzenesulfonate solution containing encapsulated calcium carbonate and dissolved in pure dodecane (volume fraction 5.10-). While a pure dodecane solution behaves like a Newtonian fluid, with its bulk viscosity, the colloidal solution shows an increase of the viscosity near the steel surfaces suggesting adsorption. After 8 hours, each steel surface is covered with a layer, which includes an almost complete “monolayer” of calcium carbonate particles, with some incomplete layers on top of it. During indentation of the layer the repulsive forces vary exponentially with the thickness of the layer. This result is consistent with two effects: the consolidation of the layer during the process and scaling factors such as the Hill number (a/D).