Confined fluids under time varying tribological conditions – Confluence

An experimental approach based on the simultaneous measurement of film thickness distribution and contact forces for controlled contact kinematics, is coupled to a theoretical analysis

The investigation of the ‘buried interface’ using mechanical spectroscopy is the first result. We also start to correlate friction and interfacial rheology.

The understanding of the role of various times covering a large range of time scales on the lubrication mechanisms will result in helping industrial to formulate new lubricants and conceive more appropriate mechanical systems.

- 1 invited international conference
J. Cayer-Barrioz (invited), “Local dynamics in a confined lubricated interface”, Gordon Conference on Tribology 2014, Waltham, USA, 20-25 juillet 2014.

- 3 international conferences
A. Crespo, N. Morgado, D. Mazuyer, J.Cayer-Barrioz, “Confined fluids under transient tribological conditions” (poster), Gordon Research Seminar on Tribology 2014, Waltham, USA, 19-20 juillet 2014.
A. Crespo, N. Morgado, D. Mazuyer, J.Cayer-Barrioz, “Rheology and interfacial friction of confined fluids” (poster), Lubricated Contact, Cadiz, Espagne, 12-17 avril 2015.
A. Crespo, N. Morgado, D. Mazuyer, J. Cayer-Barrioz, “Friction mechanisms at the molecular scale in a lubricated contact”, International Tribology Conference 2015, Tokyo, Japon, 16-20 septembre 2015.

- 1 French national conference
A. Crespo, N. Morgado, D. Mazuyer, J. Cayer-Barrioz, “Lubrifiants confinés sous des conditions tribologiques transitoires”, CFM 2015, Congrès Français de Mécanique, Lyon, France, 24-28 août 2015.

Nowadays, mechanical systems are optimized in steady-state conditions for which lubrication mechanisms are generally well known. However, most of friction losses and wear occur under time-varying operating conditions. The latter induce interfacial dynamic phenomena associated with the complex rheological behaviour of the confined lubricant or molecular reorganization that are not yet explained.

The objective of this ‘Basic Research’ project entitled Confluence is to identify local phenomena occurring in a confined lubricated interface under time-varying experimental conditions such as fast change in relative velocities between bodies or oscillating motion, and to characterize lubrication mechanisms taking into account the confined lubricant rheology. We will focus on the following: what are the lubrication mechanisms in terms of film formation/rupture and friction? What is the role of the rheology of the lubricant confined under pressure, that is to say, its viscoelasticity and its piezoviscosity, on the lubricating film dynamics? How the rheology or the heterogeneous flow of the confined interfaces can be related to the frictional response of the contact?

Model fluids have been chosen to reflect the impact of the rheology of lubricants and their interactions with surfaces: piezoviscous Newtonian base oils, isoviscous fluids, polymer and organized molecular phases solutions that exhibit a bulk viscoelastic behaviour and form structured adsorbed layers on the surfaces. These fluids are also well-representative of real lubricants.
Two main fields of investigation have been identified in this project. The first axis concerns the transient phenomena induced by a rapid change in velocity that result in a fast variation of both film thickness and friction response. The second focuses on the mechanisms of lubricant film formation and friction under oscillating motion.
From a practical point of view, the project Confluence implies the use of experimental tools able to combine visualization of the contact and controlled dynamics in order to describe and measure phenomena at short space and time scales. Consequently, the scientific experimental approach associated with the choice of model systems is multi-scaled, for contact pressures up to GPa, sliding velocities covering several decades from 10-10 to 1 m/s and times from 1 ms to 1 s, for both operating conditions and observed physical phenomena. It is based on the combined use of three experimental devices, developed in LTDS, that realize a sphere/plane contact at pressures that induce elastic deformation of the surfaces. This approach allows the ‘in-situ’ characterization of the contact area and confined medium for a ratio between film thickness and characteristic contact length of the order of 10-3. The film thickness itself ranges from few nm to few hundreds of nm.

In terms of fundamental results, investigating the ‘buried interface’ using mechanical spectroscopy is expected. We also attend to correlate friction and interfacial rheology. The understanding of the role of various times covering a large range of time scales on the lubrication mechanisms will result in helping industrial to formulate new lubricants and conceive more appropriate mechanical systems.