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Nacre (so-called mother-of-pearl ) is a natural organic/inorganic nanocomposite displaying exceptional high mechanical properties to weight ratio and a good biocompatibility with the human bones. Consequently, it becomes an attractive material for prosthesis design. These properties are ascribed to its highly ordered layered ‘ bricks and mortar ’ microstructure. This work studies the tribological behaviour of sheet nacre, rubbing on itself in dry conditions and in the presence of a liquid medium, in relation with its multiscale structure. The results show that the coefficient of friction, rather high under dry conditions (0.45), is even increasing under wet conditions (0.78). Aside a significant dissymmetry between pin and disc, the wear rates were shown to display the same trends. These changes are associated with marked differences in surface topography and in the extent of third body. The systematic study of the microstructure (by AFM, image analysis, SEM and TEM) of the worn surfaces and third bodies allows us to propose a particle detachment model where the role of the inter- and intracrystalline organic material and their interactions with the environment are emphasized.

In order to account for the influence of environment on the frictional properties of graphite, a new model of tribocontact considering the confined volume under the pin as a triboreactor is proposed. The heart of the reactor is the mechanical production of debris displaying fresh prismatic surfaces. Both the rate of formation and the rate of deactivation of these latter are influenced by the environment and are mutually influencing each the other. The model combines three distinct mechanisms, seldom considered before: (a) the one step embrittlement of graphite governed by the amount of moisture within the contact; (b) the kinetics of water adsorption outside the contact, considered to obey the Elovich equation; (c) the triboreactions between the water and the active surfaces associated with the prismatic surfaces of the graphite debris. The validity of the model is supported by various experimental results.

We present experimental results on the friction force exerted by a network of poly(dimethylsiloxane) (PDMS) moving on a solid substrate coated with the same polymer. Two different coatings are compared: a surface, densely grafted with short chains (which can be seen as a model impenetrable surface) onto which a well-defined number of long chains (connectors) can be gradually added and adsorbed PDMS layers. Increasing the sliding velocity between 10-5 and 10-1 m/s suggests a transition between “liquidlike” and “solidlike” frictional behavior. Increasing the molecular weight of adsorbed or grafted chains shifts this transition to higher sliding velocities. Increasing systematically the areal density (Σ) of connectors yields two opposite trends: (i) At our lowest velocities, an increase of Σ results in higher friction. This increase of friction is due to the pull-out process of the grafted chains from the network. (ii) At higher velocities, the same connectors lead to a reduction of friction (lubrication e...

Although particle detachment is a common phenomenon associated with most tribological processes, it seldom occurs that each piece of elemental debris can be considered as the result of a single event. Such an association has been revealed by the systematic study of a specific system, where a pin of graphite is made to rub against thoroughly polished steel. While the discontinuous nature of the transfer film allows a quantitative assessment of the volume of transfer he to be made by 3D optical-profilometry, the linear dependence of the rate of particle detachment dhe/dn (n=number of rubbing cycles) with the logarithm of sliding speed v strongly suggests the existence of a particular type of stick–slip, where each stick may lead to the detachment of a debris particle. The variations in size of these debris with environment as revealed by AFM, further suggest that the global rate of particle detachment is of the form: dhe/dn=Nxei, where N is the number of stick–slip events per rubbing cycle, x the proportion of stick events leading to a cohesive rupture, and ei the mean volume of an elemental particle. While this relation is apparently supported by most experimental results, its actual validation can only be made by experiments at the level of single (nanoscale) asperities, carried out under well-controlled experimental conditions.

An interpretation of the results of inverse gas chromatography, at infinite dilution, is proposed for the determination of the London or dispersive component of the surface free energy and surface enthalpy of a solid, starting with the knowledge of the adsorption behavior of a –CH2– group of linear long-chain hydrocarbons. These values are theoretically discussed to provide meaningful results from the chromatographic technique at infinite dilution.

Material transfer is a common phenomenon in dry friction, generally associated with a change in surface topography and in frictional interactions. In previous work, the actual influence of adhesion, cohesion and interfacial shear stresses on the extent of transfer (the transfer being assessed experimentally by image analysis) was established both on a theoretical and experimental basis, but at constant sliding speed. In this work emphasis is put on the influence of sliding speed. Both the frictional forces and the transfer characteristics (i.e. the area fraction as well as the particle density), as determined after identical sliding distances, decreased linearly with the logarithm of the sliding speed. The possible existence of stick-slip phenomena suggested by these experimental relationships was investigated further by studying the influence of various parameters affecting either the mechanical characteristics of the tribometer (stiffness, inertial moment) or the physicochemical characteristics of the compacted graphite (granulometry of the graphite powder prior to compaction, crystallinity of graphite, relative humidity during sliding, etc.). The results are interpreted by introducing a complex time of stick accounting for the coexistence of both macrostick-slip and microstick-slip, and by introducing this time dependence into the various adhesive interactions involved in the previous model of transfer stability.

Often in dry friction, material transfer occurs as a discontinuous film consisting of discrete particles, strongly adhering to the substrate. On smooth surfaces this leads to the formation of asperities interacting both mechanically and physico-chemically with the counterface. A simple analysis of the forces acting on a single asperity allows the definition of the conditions of stability and hence of transfer particle existence. The model developed is illustrated by transfer particle density variations (assessed by image analysis) as a function of experimental conditions for various materials whose surface energies have been determined by inverse gas chromatography or wetting contact angles. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Lahaina, Hawaii, October 16–20, 1994

3D-C/C-SiC model composites are used to study the influence of the addition of small amounts of SiC on friction properties of C/C composites. Samples are prepared by pressure-pulsed CVI (P-CVI) from toluene and MTS/H 2 precursors. SiC is present in matrix as thin layers (1 to 15 layers, 100-170 nm thick, with 0.4 < V S i C < 6.2%) at different distances from fiber surface. Friction properties, as assessed from pin-on-disk (of same nature) experiments in controlled atmosphere at a given temperature, are presented. For V S i C < 3%, the addition of SiC does not change markedly the friction properties whatever the localization of a SiC single layer with respect to fiber surface. Conversely, for 3 < V S i C < 6%, the addition of SiC (as 3 to 15 layers) increases the coefficient of friction and the wear rate, both being linearly inter-related. The effect of SiC on the friction properties is tentatively discussed.

This work was conducted on Pinctada maxima nacre (mother of pearl) in order to understand its multiscale ordering and the role of the organic matrix in its structure. Intermittent-contact atomic force microscopy with phase detection imaging reveals a nanostructure within the tablet. A continuous organic framework divides each tablet into nanograins. Their shape is supposed to be flat with a mean extension of 45 nm. TEM performed in the darkfield mode evidences that at least part of the intracrystalline matrix is crystallized and responds like a ‘single crystal'. The tablet is a ‘hybrid composite'. The organic matrix is continuous. The mineral phase is thus finely divided still behaving as a single crystal. It is proposed that each tablet results from the coherent aggregation of nanograins keeping strictly the same crystallographic orientation thanks to a hetero-epitaxy mechanism. Finally, high-resolution TEM performed on bridges from one tablet to the next, in the overlying row, did not permit to evidence a mineral lattice but crystallized organic bridges. The same organic bridges were evidenced by SEM in the interlaminar sequence.

International audience; Sheet nacre is a promising natural bioceramic, which consists on the internal lustrous "mother of pearl" of many molluscan shells, e.g. pinctada, our model. The aim of this work is to study the structure of the flat polygonal tablets of nacre, in order to understand the multi-scale organization of this composite material and the role of the organic template during the growth of the biocrystal. We studied the organic matrix, in situ with techniques such as darkfield transmission electronic microscopy (TEM) on small cross-sections of nacre of Pinctada maxima, or intermittent-contact atomic force microscopy coupled with phase imaging on samples of nacre of Pinctada margaritifera polished parallel to the surface of the tablets. In this study, we demonstrate the continuity of the organic framework and the crystallographic orientation in the biocrystal at 2 relevant levels : nano- and micro-scale.

This work is part of a systematic investigation concerning the mechanisms of transfer film formation in dry friction and their influence on frictional forces. This paper deals mainly with discontinuous films and is characterised by an experimental approach using image analysis to quantify the discrete particles formed on smooth surfaces, combined with the analysis of the parameters controlling transfer particle stability. This work aims to complete the previous investigations in the case of polymers, and check to what extent adhesion (measured by peel test) affects transfer particle density. It is based on the use of four well characterised films of polyethylene, differing mainly by the amount of grafted acrylic acid (respectively 0.0, 0.5, 1.0, and 2.0%). The variations of particle density with normal load, displaying either positive or negative slopes are of great interest. In agreement with previous hypotheses, these variations may be interpreted in terms of shear stresses τ, provided that they include the well known pressure dependence of τ.

Velocity accommodation across rubbing surfaces lubricated with thick films is well understood and can be determined from fluid dynamics theory. The situation is not the same in “dry” friction as the question is not usually formulated in the same terms. This paper shows that in “dry” friction, velocity can be accommodated through 20 different mechanisms (5 sites and 4 modes per site) known as velocity accommodation mechanisms. The situation is therefore more complex than that found in thick film lubrication where only one such mechanism (shear) exists. Friction and wear are shown to depend on the acting mechanism. Examples are given for each mechanism. The factors that control these mechanism are identified. Visualisation studies show that more than one mechanisms can act simultaneously and that the mechanisms can change during a test. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Baltimore, Maryland, October 16–19, 1988

Free cationic polymerisations were initiated in bulk by field ionisation with a positive tip. Dilatometric measurements obtained with ‘dry’ and ‘ultra-dry’ styrene, a-methylstyrene and isobutylvinylether were in agreement with kinetic equations derived by assuming that in the absence of deactivating impurities the space charge is built up only by growing macromolecules. Determination of the space charge by independent physical methods suggests that the kp values of the literature need to be re-evaluated. It was also shown that charge injection enhances the degree of dryness (measurable by this process) and is therefore useful in ultimate purification of monomers.

The surface of finely divided solid lubricants such as MoS2, (CFx)n and graphite is modified by grafting of different polymers. Study of the tribological properties of the grafted products under conditions of dry sliding showed that the coefficient of friction, as well as surface properties such as film formation and adhesion, are improved. The effect is especially marked with grafted MoS2 and depends on the nature and chemical stability of the grafted polymer. The apparent joint effect of easier plastic deformation suggests that this may be related to a Rehbinder effect (superficial plasticization of the metal). With MoS2 and (CFx)n suspended in paraffin oil, grafting improves the load-carrying capacity.

The influence of molecular weight, crystalline structure, and surface energy of high-density polyethylene (HDPE) upon initial friction and transfer behavior was studied on a pin-and-disk tribometer, at low speed (1.6 mm/s) 20°C and 12 percent relative humidity, using: (a) Composite PE pins made from thin film (< 400 μm) of defined molecular weight, Mw (5.104, 4.105, 4.106 g/Mole) and crystalline structure, controlled by the temperature of crystallization (120°C < Tc < 132°C), stuck onto a PE support (6 × 6 × 20 mm) (b) Thoroughly polished, plasma-cleaned steel disks (Ra < 0.02 μm) It was shown, that any increase in the PE surface energy (by plasma or chemical treatment) greatly influences the initial friction and transfer behavior. Depending on the superficial cross-linking density, which is dependent upon the surface treatment, either transfer of polymer onto the metal, or the formation of metal-containing transfer particles on the polymer is promoted during the first few rotations. The influence of the ...

Wear has been presented recently as a third-body flow process. All flow processes are governed by rheology or more generally by deformation mechanisms and by boundary conditions. Earlier work has shown that, in dry friction, a significant part of the difference in velocities at contact can be accommodated at the surface as boundary slip, the rest being taken up in the bulk as deformation, or along rupture lines. While large boundary slip can be put to advantage in some engineering components. It is not always the case and control of that phenomenon is desirable. This paper attempts to isolated some of the parameters that control boundary slip.

Modifications of the fibre reinforcement (fibre nature or architecture) and/or high temperature posttreatments of CIC composites are the most commonly used ways to control their tribological properties. In this study, the role of the matrix has been more especially investigated: (i) by controlling the nature of the pyrocarbon prepared by Chemical Vapour Deposition (CVD) either from propane or toluene precursors and (ii) by doping the ex-toluene matrix with silicon carbide, in variable amounts and microstructures (C/C-SiC). The tribological tests performed on such materials (pin-on-disc apparatus) have shown a direct relationship between the wear and friction coefficients. The friction coefficient is apparently proportional to the number of active sites generated du ring the formation of wear particles. The ability of composites to produce active sites is influenced by the turbostratic arrangement of pyrocarbons. Polynucleated smooth laminar pyrocarbons would lead to a less easier formation of wear particles than mononucleated rough laminar pyrocarbons. Silicon carbide operates as an abrasive, with a continuous creation of wear particles and associated with more elevated wear and friction coefficients than those for CIC composites. Moreover, the addition of SiC allows a gradualincrease of the friction coefficient with the sliding speed, whereas CIC composites exhibit an abrupt transition between a low and a high friction coefficient., Pour agir sur les propriétés tribologiques de composites CIC, les méthodes les plus courantes consistent à modifier le renfort fibreux (architecture ou nature des fibres) et/ou à imposer un traitement à haute température aux composites. Dans cette étude, c'est au niveau des matrices qu'ontprincipalement porté les modifications : (i) en réalisant des CIC à matrices de pyrocarbone légèrement différentes obtenues par densification chimique en phase vapeur à partir des précurseurs propane ou toluène et (ii) en dopant la matrice ex-toluène par du carbure de silicium introduit en quantité et sous formes variables (C/C-SiC). L'étude tribologique conduite sur ces matériaux (géométrie pion/disque) a montré une relation directeentre l'usure et le coefficient de frottement. Il semble que ce dernier soit proportionnel au nombre de sites actifs issus de la formation des débris. En altérant l'arrangement turbostratique des pyrocarbones, il est possible d'influencer l'aptitude des composites à produire des débris. Les pyrocarbones laminaires lisses polynucléés généreraient moins facilement ces débris qu'un pyrocarbone laminaire rugueux mononucléé. Le carbure de silicium agit comme un abrasif, il génère en permanence des débris donnant lieu à des usures et coefficients de frottement plus élevés que ceux obtenus à partir des C/C. Mais surtout il permet un passage progressif du coefficient de frottement entre une valeur basse et une valeur élevée alors que les C/C sont caractérisés par deux états de frottement distincts ; bas et haut.