Colloidal lubrication of DLC contacts : from steady state to transient state : Application to the piston - rings - cylinder contact

摘要

Ecological issues related to global warming, and more generally the reduction of pollution, have lead to a major revolution in the field of transport. Considerable research work has been carried out during the past decades in order to improve the mechanical efficiency of internal combustion engines. In Diesel engines, almost 40 % of total engine energy losses due to mechanical friction occur in the Piston rings-Piston-Cylinder contact (PPC). The overall framework of this PhD thesis is Diesel engine lubrication in presence of soot and this work focuses more particularly on Diamond-Like Carbon (DLC) lubricated contacts for PPC region. Unique tribometry tools are used to reproduce the particular contact kinematics involved in the piston assembly. This thesis aims to identify the influence of an aged lubricant on the lubrication and friction mechanisms of DLC hard coatings for all lubrication regimes in steady-state and transient conditions. DLC coatings developed during this thesis significantly reduce the boundary friction in steady-state and transient conditions. The lubricant destructuring due to aggregate formation, generated by the passage of soot, or by a temporary vanishing of the entrainment speed, are governed by the triplet, lubricant, surface and contact kinematics. Finally, the analysis of the tribological response of the lubricated interface in steady-state and transient conditions leads to the theoretical modeling of the friction during a complete cycle of sliding at variable velocities.