Effects of Lubrication Mode on Friction and Heat Partition Coefficients at the Tool-Work Material Interface in Machining

The quantification of friction coefficient along the tool-work material interface in machining remains an issue in tribology. This article aims at identifying the evolution of friction coefficient for a large range of sliding velocity during the machining of an AISI 4140 steel (290 HB) with a TiN-coated carbide tool. The influence of various lubricants (straight oil or emulsion) and lubrication modes (flow or mist) is investigated and compared to a dry sliding situation (dry machining). It has been shown that, in dry machining, the friction coefficient decreases with the sliding velocity until reaching a lower limit around 0.2. On the contrary, the presence of a straight oil significantly decreases friction coefficients to a value around 0.1. Emulsion enables a significant decrease of friction coefficient to around 0.2 for low sliding velocities, whereas its action is absent for higher sliding velocities. Oil mist exhibits an intermediate behavior. Finally, it has been shown that all kinds of lubrication lead to a large decrease of heat flux transmitted to cutting tools, even if heat flux is almost similar irrespective of the nature of oil and of its application mode. Straight oils decrease the heat flux transmitted to cutting tools due the decrease of friction coefficient, whereas they are not able to modify the heat partition coefficient at the interface. On the contrary, emulsions limit the transmission of heat flux to cutting tools due to their lower heat partition coefficient, even if their influence on friction is more limited.