Prediction of Friction Coefficients in Mixed Lubrication Regime For Lubricants Containing Anti-Wear and Friction Modifier Additives

Abstract

Many laboratory tribology test machines are available for evaluating the effect of different lubricants and different operating conditions on friction. For the Mini Traction Machine (MTM) there is much published data that shows how the measured friction coefficient varies with operating conditions and lubricant type. Fully formulated lubricants containing the anti-wear additive ZDDP have often been found to have a significantly higher friction coefficient, which persists to higher speeds, compared to base oils (lubricants with no additives). Recent work has found that the surface roughness of ZDDP tribo-films can evolve to become significantly higher than that of the surfaces they are deposited on. When the measured friction coefficients of lubricants tested in the MTM machine are suitably normalized and plotted against the λ ratio (which is equal to the oil film thickness separating the moving surfaces divided by the combined surface roughness) then the curves for various different lubricants lie on a “master curve” which enables reliable friction estimates to be made for lubricated contacts in the mixed lubrication regime. A simple modification to this approach also allows for the calculation method to be extended to lubricants that contain friction modifier additives.