Boundary Yield Stress and Interfacial Potential Energy Barrier in Thin Film Hydrodynamic Lubrication

Boundary slip occurs in lubricated contacts if shear stress at the solid/liquid boundary attains a yield value. From a molecular perspective, the change from no-slip to slip condition signifies the liquid molecules having gained enough energy to overcome the interfacial energy barrier. This study carried out a set of thin film hydrodynamic lubrication tests using a silicon oil with additives of perfluoroheptanoic acid to examine the correlation between boundary yield stress and interfacial energy barrier. The additive changes the contact angle and contact angle hysteresis of the oil on the bearing surface. Potential energy barrier was calculated using the contact angle and contact angle hysteresis. Boundary yield stress was derived from the tests by two-dimensional hydrodynamic lubrication analyses using a simple boundary yield stress slip model. Results show that boundary yield stress and potential energy barrier are monotonically related.