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. On the other hand, we showed that the coverage ratio of the monolayer associated with the shear elastic modulus of the boundary film governed the friction level. Changing the surface chemistry while keeping the roughness constant controls the final organization in the boundary layer, the correlated mechanical properties, and the level of friction dissipation.

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 0.5 N m, with an accuracy of 0.05 mN m; temperature; and the position of the shaft for velocities up to 12 000 rpm. To illustrate the capability of the Osiris tribometer, thin hydrodynamic film measurements were performed on smooth surfaces. The results are presented here, and the thermal effects, acceleration, and inertia contributions are discussed. Finally, the influence of the surface topography using textured surfaces was demonstrated and the role of adsorbed layers on the surface due to fluid formulation was highlighted.

Exploring mechanochemical reactions at the nanoscale: theory versus experiment.

Mechanochemical reaction pathways are conventionally obtained from force-displaced stationary points on the potential energy surface of the reaction. This work tests a postulate that the steepest-descent pathway (SDP) from the transition state to reactants can be reasonably accurately used instead to investigate mechanochemical reaction kinetics. This method is much simpler because the SDP and the associated reactant and transition-state structures can be obtained relatively routinely. Experiment and theory are compared for the normal-stress-induced decomposition of methyl thiolate species on Cu(100). The mechanochemical reaction rate was calculated by compressing the initial- and transition-state structures by a stiff copper counter-slab to obtain the plots of energy versus slab displacement for both structures. The reaction rate was also measured experimentally under compression using a nanomechanochemical reactor comprising an atomic-force-microscopy (AFM) instrument tip compressing a methyl thiolate overlayer on Cu(100) (the same system for which the calculations were carried out). The rate was measured from the indent created on a defect-free region of the methyl thiolate overlayer, which also enabled the contact area to be measured. Knowing the force applied by the AFM tip yields the reaction rate as a function of the contact stress. The result agrees well with the theoretical prediction without the use of adjustable parameters. This confirms that the postulate is correct and will facilitate the calculation of the rates of more complex mechanochemical reactions. An advantage of this approach, in addition to the results agreeing with the experiment, is that it provides insights into the effects that control mechanochemical reactivity that will assist in the targeted design of new mechanochemical syntheses.

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. The existence of contact junctions and their friction dynamics are studied through coupled experimental and computational approaches. The former provides detailed measurements of how the friction force decays, after the stiction peak, to a steady-state value over a few nanometers of sliding distance, while the latter demonstrates how this memory distance is related to the evolution of the number of cross-surface attractive physical links, within contact junctions, between the molecules adsorbed on the rough surfaces. We also show that roughness is a sufficient condition for the appearance of structural aging. Using a unified model for friction between rough adsorbed monolayers, we show how contact junctions are a key component in structural aging and how the infrajunction molecular motion can control the macroscopic response.

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. The adsorption kinetics, the packing of the self-assembled monolayers, and the coverage rate depend on the molecular architecture of the fatty acids and lead to various mechanical behaviors under confinement.

Friction setup and real-time insights of the contact under controlled cold environment: The KORI 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 KORI tribometer allows one to reach negative temperatures until -20 °C and to perform tribological experiments for velocity from 50 µm s-1 to 1 m s-1 under load up to 50 N and to simultaneously measure resultant forces until 30 N and visualize the contact in real-time. In parallel, an ice manufacturing unit and a specific protocol were developed to grow a transparent ice disc with a controlled initial roughness and surface state. Real-time and simultaneous visualization of the ice-rubber contact provides additional data, such as the apparent contact area and the mean size of a real contact spot during friction, after adequate and dedicated image processing. To illustrate the capability of the KORI tribometer, rubber-ice friction measurements were performed at -10 °C and the results are presented here, as a function of time and velocity.

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. Interestingly, in the lamellar phase domain, we observe the presence of defects upon dilution, which SAXS shows to correspond to intrabilayer correlations. NMR and FF-TEM techniques suggest that these defects are related to an increase in the spontaneous curvature of the molecule monolayers in the lamellae. ATR-FTIR spectroscopy was also used to investigate the degree of ionization within these assemblies. The successive morphological transitions are discussed with regards to possible molecular mechanisms, in which the interaction between the acid surfactant and the amine counterion plays the leading role.

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. It appears that the Amontons' proportionality between frictional and normal stresses does not hold here: the higher the contact pressure, the lower the friction. Besides, the sliding stress is strongly dependent on the velocity: it increases with the sliding velocity. Using a model based on the kinetics of formation and rupture of adhesive bonds between the two shearing surfaces theoretically accounts for the behavior of this system. This approach allows us to correlate the frictional properties to the molecular organization on the surfaces.

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. Moreover, the behavior of these systems is theoretically accounted for using a model based on the kinetics of formation and rupture of adhesive bonds between the two shearing surfaces with an additional viscous term.