Lubrication Mechanisms and Surfaces
Introducing a liquid to separate two solid surfaces in contact is not a novel idea. The thin fluid film is capable to bear a load, hence reducing energy losses, protecting the surfaces from wear and corrosion, and evacuating the wear debris and the heat. Lubrication mechanisms can then be discussed in terms of film forming capability and in terms of friction dissipation.
A lubricated contact is a complex system: its behavior and its evolution are affected by the macro- and micro-scale geometry of the solid surfaces, the working conditions, the lubricant rheology. It can also play the role of a “mechanochemical reactor” inducing the reaction of additives on the surfaces, the tribofilm formation and/or the adsorption of adsorbed boundary films on the surfaces.
The inlet zone conditions the lubricant flow, and controls the generation and maintenance of lubricant thickness in the contact high-pressure zone. Friction stresses are controlled by the rheology of the lubricant under pressure in the contact zone, and by lubricant/surface interactions.
In a first historical approach, different lubrication regimes were defined, from boundary lubrication regime to elastohydrodynamic/hydrodynamic regimes, depending on the contact kinematics, on the physical properties of the fluid and its volume, and on the surface properties.
In this presentation, we describe the main features of each lubrication regimes, followed by the mechanisms involved. We discuss the influence of the contact kinematics, steady-state or varying with time (reciprocating conditions for instance). As reducing friction dissipations is a promising way to contribute to the global welfare, the actual trend is to use low-viscosity fluids – leading to thinner and thinner films – and the second one is to modify the surfaces in contact. We illustrate a combined approach consisting in investigating the surface effects on film forming and friction in contacts lubricated with low-viscosity fluids, over all the lubrication regimes, with film thickness ranging from nanometer to micrometer.