Frictional characteristics of molecular length ultra-thin boundary adsorbed films

Abstract

The paper presents measurements of friction of any ultra-thin film entrained into the contact of a pair of very smooth specimen subjected to entrainment in a converging micro-wedge of a special-purpose micro-tribometer. An ultra-thin film is expected to form at the boundary solids through adsorption of boundary active molecules. Fluids with linear and branched molecules are used in the investigation. It is found that the frictional characteristics of these films can be adequately described through use of Eyring thermal activation energy and a potential energy barrier to sustain conjunctional sliding motion. The combined experimental measurement and the simple activation energy approach shows that the thin molecular adsorbed films act like hydro Langmuir–Blodgett layers, the formation and frictional characteristics of which are affected by the competing mechanisms of adsorption, forced molecular re-ordering and discrete-fashion drainage through the contact by the solvation effect. This process is a complex function of the contact sliding velocity as well as a defined Eyring activation density (packing density of the molecules within the conjunction). It is shown that the contribution of solvation to friction is in the form of energy expended to eject layers of lubricant out of the contact, which unlike the case of micro-scale hydrodynamic films, is not a function of the sliding velocity.