Breakdown Processes of Boundary Films Formed by Oiliness Additives

Breakdown processes of boundary films formed by lubricating oils containing oiliness additives were studied in a steel-mercury interface under quasi-static loading. Simultaneous measurements of the thickness and breakdown ratio of the films were carried out using an electrical impedance method. Different breakdown processes were obtained for different lubricating oils. At a concentration of 10-3 mass%, the lubricating oils formed residual films with a thickness of a few tens of nanometers that support a normal load without any hydrodynamic effects. A possible mechanism for this is the osmotic pressure created by the concentration difference between the bulk oil and residual film. Friction coefficients of a steel-on-steel contact lubricated by the lubricating oils in a boundary lubrication regime were related to the breakdown processes in the steel-mercury interface using multiple regression analysis. Lubricating oils that showed low-rate breakdown processes showed low friction coefficients. This indicates that the breakdown rate of boundary films is an important factor in the mechanism of boundary lubrication