In Situ Chemical Mapping of the Thin-Film Lubricated Contact.

In this article, we propose a direct measurement of the distribution of two different chemical species in a thin-film lubricated contact under pure rolling conditions. The two chemical species were a carbonyl from an ester bond of a polyalphametacrylate (PAMA) polymer and an ether bond of a polyalphalycol (PAG) molecule, formulated in low-viscosity fluids using a mineral polyalphaolefin (PAO). A full chemical mapping of the lubricated contact region was provided thanks to the Bastet tribometer consisting of a ball-on-disk tribometer placed under a microscope coupled with an infrared spectrometer.

Effect of Surface Chemistry on the Squeeze-Thin Film and Friction of Boundary Films.

An approach combining adsorption characterization and lubricity effectiveness of amine-based friction modifier molecules has been performed using chemically controlled surfaces, coated either with cobalt or carbon, while keeping the surface roughness constant and sub-nanometric. Through squeeze measurements and numerical modeling, we have identified the mechanical properties of both adsorbed amine films, as a function of the surface on which they were formed. On the one hand, we were able to evidence that the fluid structuring at the vicinity of the adsorbed boundary film differed as a function of the latter mechanical properties, directly resulting from its molecular organization.

Thin films in hydrodynamic lubrication regime: The Osiris friction setup.

A model hydrodynamic lubrication tribometer consisting of two hydrodynamic journal bearings working under thin film conditions was developed to investigate the mechanisms of hydrodynamic friction with low-viscosity fluids and the role of surface effects. A small nominal radial clearance of about 5 µm was considered between the two surfaces. This fully instrumented setup provides in situ information on the sheared fluid film in terms of simultaneous measurements of film thickness; localization and extension of the cavitation zone, with a resolution of 30°; nominal friction torques up to 0.

From Molecular to Multiasperity Contacts: How Roughness Bridges the Friction Scale Gap.

The tangential force required to observe slip across a whole frictional interface can increase over time under a constant load, due to any combination of creep, chemical, or structural changes of the interface. In macroscopic rate-and-state models, these frictional aging processes are lumped into an ad hoc state variable. Here we explain, for a frictional system exclusively undergoing structural aging, how the macroscopic friction response emerges from the interplay between the surface roughness and the molecular motion within adsorbed monolayers.

Friction setup and real-time insights of the contact under controlled cold environment: The KŌRI tribometer for rubber-ice contact application.

A friction setup combining real-time ice-rubber contact visualization, force measurement, and a compact controlled cold environment system was developed in order to investigate ice-rubber contact complex tribological response and the various contributions to friction, such as viscoelastic deformation, ice surface melting, adhesion, ice creep, or quasi-liquid layer effect. The cold system was based on a cryogenic bath circulator, an air convection circuit, and several thermal insulation combinations such as silica aerogel and expanded polystyrene. The KŌRI tribometer allows one to reach negative temperatures until -20 °C and to perform tribological experiments for velocity from 50 m s to 1 m s under load up to 50 N and to simultaneously measure resultant forces until 30 N and visualize the contact in real-time.

Effect of Unsaturation on the Adsorption and the Mechanical Behavior of Fatty Acid Layers.

Adsorption, self-organization, and mechanical properties of different fatty acid layers under different confinement states have been investigated as a function of the presence and conformation of one unsaturation in their aliphatic chain. In situ characterization, at the molecular level, was performed with the ATLAS molecular tribometer, in terms of rheology, forces, and thickness of confined fluid. The results demonstrate that the fatty acids adsorb on the surfaces by weak interactions and form viscoelastic films with a thickness of about 15 Å on each surface.

Polymorphism of natural fatty acid liquid crystalline phases.

We study the phase behavior in water of a mixture of natural long chain fatty acids (FAM) in association with ethylenediamine (EDA) and report a rich polymorphism depending on the composition. At a fixed EDA/FAM molar ratio, we observe upon dilution a succession of organized phases going from a lamellar phase to a hexagonal phase and, finally, to cylindrical micelles. The phase structure is established using polarizing microscopy, SAXS, and SANS.

Frictional rheology of a confined adsorbed polymer layer.

The sliding dynamics of a confined adsorbed polymer layer is investigated at the nanoscale. A combined mechanical and physical approach is used to model the rheology and structure of the adsorbed layer. The confinement at short distances governs the nanotribological behavior of the polymer layer formed close to the surface.

Friction dynamics of confined weakly adhering boundary layers.

The nanotribological behavior of self-assembled monolayers is investigated. The latter accommodate friction through transient relaxation and dilatancy effects whose kinetics depends on the structure of the confined layers. Thus, the molecular ordering onto the surfaces controls the level and the stability of the friction coefficient.