An effective friction energy density approach to predict solid lubricant friction endurance: Application to fretting wear

Abstract

Bonded MoS2 solid lubricant coatings are extensively used in tribology to reduce friction coefficient and wear rate. This coating strategy is particularly appreciated in aeronautical applications to limit fretting wear damage. A major question, however, concerns prediction of endurance for such palliatives (Nc: µ>µc). Focusing on a MoS2-bonded solid lubricant coating fretted against a Ti–6Al–4V counter-body, an extensive fretting wear analysis coupling a large spectrum of contact pressures, sliding amplitudes, contact sizes and contact geometries was undertaken. The study showed that different friction responses could be activated depending on contact pressure and sliding amplitude conditions. Low pressures and small sliding amplitudes induced a lubrication plateau friction response (I) whereas high pressures and large sliding amplitudes induced continuously rising friction combined with titanium transfer (II). The transition from friction response (I) to friction response (II) could be formalised using an “effective” pveff factor. Moreover, bonded-MoS2 coating endurance could be predicted by an “effective friction” energy density parameter expressed as the difference between the nominal friction energy density imputed in the interface and an energy contribution related to the titanium transfer activation. Using this φ–N chart representation, all the experimental friction endurance values followed a single endurance master curve, formalised as an inverse function of the “effective” friction energy density parameter. An equivalent “effective” Archard work density approach is also introduced. Based on a pressure work density parameter, this formulation is easier to apply but displays a wider scatter because friction fluctuations are not taken into consideration.