Denis Mazuyer

Interfacial friction of wetted monolayers

Abstract A “molecular tribometer” has been constructed [1]that measures directly, with great accuracy, the forces and the displacements that act between metallic surfaces bearing organic layers in a liquid medium as they slide past each other. Its originality is due to its low compliances (2×10−7m N−1 for the normal direction, 2×10−6 m N−1 for the two other directions), which permit mechanical control of the thin layer. In this work, it is shown that an anisotropic thin monolayer of stearic acid adsorbed an each cobalt surface is more compliant than the contact of the cobalt surfaces and hence reduces the apparent shear moduli of the contact zone and controls the shear process. The “interfacial” friction depends on the sliding speed and is very sensitive to the very small variations of the film interface thickness. Two competitive and opposing processes occur during the sliding: crushing and lifting of the monolayer. It is suggested that the crushing appears at very low speed, when the transit time is close to that of the molecules monolayer. Our findings have direct applications for the properties of boundary lubricants and for the rheological behaviour of both surfactant and polymer molecules.

The Role of Soot Aggregation on the Lubrication of Diesel Engines

This paper deals with the role of the soot on the valvetrain wear in diesel engines. First, wear is related to the soot aggregation at the inlet, of the cam-tappet contact. Then, an experimental device using a Rayleigh pad which allows one to study the aggregation underflow of diesel soot in completely formulated oils is described. The results concerning four oil samples, chosen for their differences in formulation and aging are given and related to the wear. Finally, the aggregation is related to the interparticle interactions and, more particularly, to the dielectric constants of the soots and of the lubricant.

High-pressure lubrication with lamellar structures in aqueous lubricant

The lubrication mechanisms of water-based fluids are not yet well understood, especially when extreme load conditions are applied. The example of lamellar crystallite particles in water, provided by an emulsion-like technology, is described in this paper. It is shown, for the first time, that the presence of such nanostructures in the sliding interface can provide extreme-pressure lubrication. This water-based lubricant is particularly efficient when one of the two solid surfaces is made of brass. In this case, a multilayer is created in the contact. It is formed by a brass layer transferred to the counter face, the brass surface itself and a lamellar film adherent to the former layers. This lamellar system is able to sustain pressures up to a few GPa and to maintain a low friction coefficient.

Friction-Induced Vibration by Stribeck's Law: Application to Wiper Blade Squeal Noise

This paper is concerned with the squeal noise of a wiper/windscreen contact. It is shown that squeal noise stems from friction-induced self-excited vibrations in the context of Stribeck’s law for friction coefficient. The study is specifically focussed on the instability range of velocities and not on the amplitude of limit cycles. The studied dynamic system consists of a single degree-of-freedom mass-spring-damper oscillator submitted to a velocity-dependent frictional force which follows the Stribeck law. The local stability is analyzed by the first Lyapunov method and results in a stability criterion. Experiments have been performed on a glass/elastomer contact lubricated with water. The tribometer ‘LUG’ provides measurements of the vibrational velocity and friction force versus sliding speed. It is found that the instability appears during the transition between boundary and elastohydrodynamic regimes where the negative gradient of the friction versus velocity curve is steep. The apparition and vanishing of instability are correctly predicted by the steady-state stability criterion.

In Situ Film-Forming and Friction-Reduction Mechanisms for Carbon-Nanotube Dispersions in Lubrication

Current requirements in automotive lubrication impose extremely complex formulation. For environmental reasons, it is important to reduce or eliminate the presence of sulphur and phosphorus contained in tribological additives. For that purpose, multi-walled carbon nanotubes have been dispersed in oil in various concentrations. The lubrication mechanisms of such dispersions in mixed and EHL regimes have been investigated by means of the IRIS tribometer that allows us simultaneous contact visualization, film thickness and friction measurement under controlled contact kinematics. The lubricant film-forming capability has been determined as a function of the entrainment velocity and the nanotube content: the presence of carbon nanotubes within the contact results in a local increase in the film thickness and it can be shown that the contact acts as a filter of carbon-nanotube aggregates. Introduction of sliding results in a diminution of the number of aggregates passing through the contact. Moreover, a reduction in friction and a drift in the wear onset have been observed under controlled contact kinematics: this behaviour originates from the transient propagation of carbon-nanotube aggregates through the contact and a friction law is proposed taking into account the contact heterogeneity.

Starved elastohydrodynamic lubrication theory: application to emulsions and greases

In classical fluid lubrication the film thickness is mainly determined by entrainment velocity and oil viscosity. At high pressure, elastic deformation occurs changing the local geometry: this is the elastohydrodynamic lubrication regime (EHL). When a limited amount of lubricant is available to the contact, a component failure due to lubricant starvation can result. A new starvation model is presented, using the amount of oil on the surfaces as the key parameter controlling starvation. It is successfully applied to describe the contacts lubricated with multi-phase fluids such as emulsions and greases, which combine starvation with a very complex rheology

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.

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.

Statistical Vibroacoustics and Entropy Concept

Statistical vibroacoustics, also called statistical energy analysis (SEA) in the field of engineering, is born from the application of statistical physics concepts to the study of random vibration in mechanical and acoustical systems. This article is a discussion on the thermodynamic foundation for that approach with particular emphasis devoted to the meaning of entropy, a concept missing in SEA. The theory focuses on vibration confined to the audio frequency range. In this frequency band, heat is defined as random vibration that is disordered vibration and temperature is the vibration energy per mode. Always in this frequency band, the concept of entropy is introduced and its meaning and role in vibroacoustics are enlightened, together with the related evolutionary equation. It is shown that statistical vibroacoustics is non-equilibrium thermodynamics applied to the audio range.

On the mechanisms of abrasive wear of polyamide fibres

The abrasion resistance of fibres is a basic property in the textile industry since the fibres can be damaged during the manufacturing process. To understand the fundamental mechanisms of abrasion of 40 μm diameter polyamide fibres, a tribometer using a fibre-cylinder contact configuration immersed in water has been developed. This instrumented device controls tribological parameters such as the sliding speed, the environment and the tension applied to the fibre. A reference experimental procedure has been defined in order to run comparative experiments where the former parameters were varied. The number of rod revolutions leading to the fibre rupture has been selected as a macroscopic wear criterion. It is found to be independent of the sliding speed, in the investigated range of velocities. A simple thermal analysis of the sliding contact confirms that the contact heating is limited by the cooling effect of water. Microscope observations of worn fibres reveal abrasive scars and defibrillations on the fibre surface and are used to characterize the wear kinetic. From these observations, it is shown that the abrasive process is responsible for a continuous diminution of the fibre cross-section until the creep failure stress is achieved locally.