Frictional behaviour of a greased contact under low sliding velocity condition

In greased contacts, the complex rheology of a grease, depending on thickener, base oil and additives, becomes a key point for understanding and controlling the evolution of friction coefficient at extremely low sliding velocities. Such velocity range (from 0 to few mm/s) is of main relevance for several issues, such as unstable friction-induced vibrations, which can occur as well in the case of lubricated interfaces. In this paper, a greased contact at low sliding velocities is then reproduced and analysed on a dedicated tribometer, under well controlled boundary conditions. Static and dynamic friction coefficients, as function of velocity, are obtained, considering either dry and lubricated contact, with both the grease and its base oil. The effect of DLC surface coating is analysed too. The contributions of the lubricant components on the frictional and dynamic response have been then discussed, showing a non-trivial trend of friction, with respect to speed, within the investigated velocity range.