Viscoelastic effects in boundary lubrication

The static friction of steel under boundary lubricated conditions was investigated both experimentally and theoretically. The theoretical model was developed using the assumption that during the application of a tangential load to a friction couple, the real area of contact grows in a viscoelastic manner until a critical shear stress is reached. Using this model, it was possible to distinguish the effect of static and dynamic contact time on area growth and thus to show why the traditional "time dependence of static friction" theories have limited validity. The model predicts that µs, the static friction coefficient, is a function of the rate parameter θ, and that a relaxation time can be assigned to a given interface. Subsequent experimental work using steel surfaces in vacuum as well as steel surfaces lubricated by various surface films showed that surface conditions play a large role in determining the exact µs - θ relationship for a given friction couple. Over the range of θ investigated the static friction coefficient of steel is constant if certain surface films are present; for other films the static friction coefficient vs θ curve shows an upper and lower asymptote. In the latter case a relaxation time was assigned to each boundary lubricant. For given asymptotes these relaxation times can be used to predict whether the film will be a useful lubricant at a particular θ. A subsequent investigation showed that the relaxation times are strongly affected by temperature. Since raising the substratum temperature results in smaller relaxation times, it is obvious that a particular lubricant may become ineffective as the substratum temperature changes. Both the experimental and theoretical work clearly demonstrate that the static friction of steel can be significantly modified by the application of appropriate boundary lubricants.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat