A Quantitative Friction-Based Approach of the Limiting Shear Stress Pressure and Temperature Dependence

Friction in highly loaded lubricated contacts (where the pressure is greater than 1 GPa) may present a plateau at intermediate slide-to-roll ratio, known in the literature as the limiting shear stress (LSS) plateau. Its physical origins and its dependence to the operating conditions are still unclear, that is why predicting friction in such contacts still remains an issue. Apart from the nature of the lubricant, the two main parameters influencing the friction plateau value are pressure and temperature. The literature provides several empirical expressions of the LSS which either consider the pressure influence only, or both pressure and temperature but almost always with coupled terms. Therefore, the published LSS values derived from friction measurements can be considered as the macroscopic consequence of the influence of pressure and temperature but also of shear heating that occurs in sliding highly loaded contacts. In this paper, the contribution of each parameter was studied separately, i.e. through experiments accrued out under nominal isothermal conditions, but conducted at different temperatures and pressures on two lubricants: a synthetic ester (benzyl benzoate) and a turbine mineral oil. A new LSS model was derived, based only on the mechanical (i.e. shear) contribution to the LSS. Surprisingly, a simple linear dependence of the LSS with both pressure and temperature was found, revealing that the influence of each of the two parameters is decoupled from the other. As far as we know, this is the first time that such an uncoupled LSS model in pressure and temperature is reported. This work offers a better quantification of the response of lubricants submitted to very high pressure and high shear: it should help to improve friction prediction in highly loaded lubricated contacts.