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In order to improve the description of a contact submitted to friction, a new approach has been developed which takes into account the fact that wear occurs on a microscopic scale and is time dependent. It results from the coupling of an experimental cyclic tribometer and a numerical imager. Since the results obtained appear as numerical images, each of them corresponding to the variation in various physical data measured during the test, we have called it triboscopy. A triboscopic diagram contains a great amount of information, both qualitative and quantitative. In this paper we present various reduction techniques for triboscopic diagrams, separately considering both cycle dependence and position dependence. One application is given here where the wear behaviour of an antiseizure polymeric layer deposited on a grooved metallic substrate is precisely described. We interpret the processed data in terms of local abraison and local seizure.

Analytical tribology involves the recording of chemical, electronic and strctural information from wear debris and surfaces. Ultimately, the analysis can be performed in situ from the working interface, considerably increasing the level of interpretation of mechanisms. Three analytical techniques have been developed, namely, surface analysis (AES/XPS), Raman and absorption spectroscopies (XAS). Each approach is dedicated to specific studies in the field of dry friction, thin layer lubrication and boundary lubrication, respectively. The example of XAS is highlighted, to show how valuable information on friction-induced tribochemistry in the presence of anti-wear additives can be obtained. In this latter case, the approach has allowed some basic aspects of the anti-wear mechanisms of zinc dithiophosphate to be elucidated.

International audience; This study deals with a method which allows identification of velocity-dependent and velocity-independent friction contributions, from energy decay curve of the free response of a damped single degree-of-freedom oscillator, including tribological contact. An original device is built and experimental dynamic responses confirm that contact friction is characterized by both velocity-dependent and velocity-independent contributions. Simulations from identified values agree with experimental data. Successive peaks of the free response are very well fitted. Estimated duration of the signal before its extinction is very consistent. The methodology appears powerful, especially for low friction coefficients leading to longer signal duration before extinction.

The effect of hyperthermal atomic oxygen (AO) exposure on the surface properties of Si‐doped diamond‐like carbon (DLC) was investigated. Two types of DLC were tested that contain approximately 10 at% and 20 at% of Si atoms. Surface analytical results of high‐resolution x‐ray photoelectron spectroscopy using synchrotron radiation (synchrotron radiation photoemission spectroscopy; SR‐PES) as well as Rutherford backscattering spectroscopy (RBS) have been used for characterization of the AO‐exposed Si‐doped DLC. It was identified by SR‐PES that a SiO2 layer was formed by the hyperthermal AO exposure at the Si‐doped DLC surface. RBS data indicates that AO exposure leads to severe thickness loss on the undopedd DLC. In contrast, a SiO2 layer formed by the hyperthermal atomic oxygen reaction of Si‐doped DLC protects the DLC underneath the SiO2 layer

The aim of this study is to assess the tribological behavior of pure crystalline zinc orthophosphate under boundary lubrication in order to model zinc phosphate-based anti-wear additives. Boundary films were generated from α-Zn3(PO4)2 powder dispersed in poly-alpha-olefin oil, at ambient temperature, by means of a steel sphere-on-flat contact in reciprocating motion. Electrical contact resistance and friction coefficient evolutions enable an understanding of the tribological behavior of crystalline zinc orthophosphate at the sliding interface. A conductive atomic force microscope (C-AFM) equipped with a current sensing setup, Raman spectroscopy, and nano-indentation were used to characterize the resulting film. When involved in a tribological contact, zinc orthophosphate powder forms a continuous patchy adherent film, changing its structure to amorphous orthophosphate, on both sliding steel surfaces. Morphological and mechanical properties of the film are discussed with respect to the ZDTP tribofilm models.

Nanoparticles are potential additives for the improvement of lubricant properties, because of the structural modifications they undergo under high pressures in mechanical contacts. The behavior of inorganic fullerene-like WS2 nanoparticles (IF-WS2) under high isotropic pressures of up to 20GPa generated in a diamond anvil cell was studied and compared to the response of the lamellar 2H phase of WS2. The same materials were then subjected to static uniaxial pressures in a Hertzian contact in the GPa range. The evolution of the particles as a function of pressure was studied by in situ Raman spectroscopy and transmission electron microscopy at the end of the test. Data analysis shows that IF-WS2 particles resist high hydrostatic pressures well, but they are totally exfoliated by uniaxial compression in a Hertzian contact under low pressure. These results explain the excellent tribological properties at ambient temperature of IF-WS2 nanolubricant that have previously been attributed to the nested nanospheres...

International audience; Femtosecond pulsed laser deposition (PLD) has been performed using a mode-locked Ti:sapphire laser including an amplification stage (150 fs, 800 nm, 1 kHz) to deposit tetrahedral -amorphous carbon films (ta-C) on AISI 316L stainless steel and ultra high molecular weight polyethylene, in perspective to extend the wear resistance of materials used in hip joints. Ta-C films have been elaborated in high vacuum conditions at room temperature. The diamond-like coated silicon substrates exhibit high wear resistance (in the 10-8)-10-9 mm3 (N m)-1 range) with moderate hardness (in the 20-30 GPa range), which may be favorable for the accommodation motion between contacting surfaces in a hip joint. In situ sputter cleaning of the orthopaedic substrates in argon plasma prior to carbon deposition has been investigated, leading to the enhancement of the adhesion of the films onto the stainless steel substrates. The adhesion properties of films deposited in various conditions on metallic substrates have been studied by tensile tests. The tribological behavior of the coatings deposited on cleaned substrates have been widely investigated in a pin-on-flat configuration in ambient air and Ringer solution. Finally, a DLC thin film with an homogeneous thickness has been deposited on hemispherical surface of 22.2 mm in diameter of a stainless steel femoral head of a hip prosthesis, whose wear behavior will be quantified using a hip joint simulator during one million of cycles (corresponding to the human activity during one year).

The Ti3SiC2 compound is a thermodynamically stable nano-layered ternary carbide, that belongs to a family of over 50 ternary carbides and nitrides, so-called MAX phases. These phases are a new class of solids that possess a unique combination of properties: they are readily machinable, relatively soft for ceramics, but elastically stiff, and electrically and thermally conductive. However, only very few studies are dedicated to their tribological behavior. This paper presents some results on the tribological behavior of Ti3SiC2. During dry sliding on a Ti3SiC2 plane, a first regime of low friction and mild wear can be observed, with the build-up of a tribofilm on the counterface. Then, while in an apparently steady state, the tribological behavior suddenly turns to higher friction and severe wear of both counterfaces. Combining friction experiments and topographical observations of contacting surfaces during the first regime, we propose some explanations for such drastic transition in tribological behavior. While tribofilm thickness and roughness stabilize in the mild wear regime, the covering of the ball surface still increases. Thus, matter accumulation on the ball finally leads to seizure of the contact.

Tested in boundary lubrication, inorganic fullerene-like WS2 nanoparticles used as additives in oil present interesting friction reducing and anti-wear properties. A dispersion with only 1 wt% of particles leads, from a contact pressure of 0.83 GPa, to a drastic decrease of the friction coefficient below 0.04 and to very low wear. High resolution transmission electron microscopy (HRTEM), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Raman Spectroscopy and video imaging were used to explain the lubrication mechanisms. A structural modification of fullerene-like nanoparticles into sheets during the friction test was evidenced to be the main effect at the origin of these properties.

A method involving the grafting of long chain molecules on hydroxylated ceramic surfaces such as silica is proposed to improve the lubrication of microactuators. Preliminary experiments show that one monolayer of C18 or C22 molecules, chemically bonded, leads to an effective improvement in silica tribological properties, the friction coefficient decreasing from 0.5 to 0.1.

In this study, the rheology and durability of tribofilm has been investigated. A ZDDP tribofilm (6×8 mm) has been partially removed by Ar+ ion etching (5keV), using an aluminum mask with a comb shape. A striped sample was obtained with a lateral alternation of etched zones and ZDDP tribofilm areas, respectively (about 1 mm width). On this striped sample, a friction test was performed using a reciprocating pin-on-flat friction machine. Tests were carried out in different lubricant environments at ambient temperature with the maximum contact presure of 0.5 GPa. The friction signal and the Electrical Contact Resistance (ECR) were plotted as a function of position and number of cycles. The results can be explained by a mechanical process in the tribofilm zone (its removal) and chemical reaction in the etched zone.

The MAX phases are new, thermodynamically stable, nanolayered ternary carbides and nitrides. These materials have a big potential in tribological applications due to their structure, similar to graphite or molybdenum disulfide. For example, the friction coefficients of the basal planes of Ti3SiC2 have been shown to exhibit very low (-3) friction coefficients. The aim of this study is to better understand the tribological behavior of polycrystalline Ti3SiC2 against stainless steel. Experiments were conducted on a ball-on-flat tribometer (∼ 25°C and ∼ 30% relative humidity) that simultaneously measures friction coefficients and electrical contact resistance. Different stainless steel ball diameters and normal loads were used and resulted in contact pressures between 0.35 and 1.25 GPa. Two different tribological behaviors were observed, both with relatively low friction coefficients for ceramics. The first behavior, referred to as type I, is characterized by low friction coefficients (≍ 0.15); low wear and a transfer film containing titanium and carbon formed on the ball. The other behavior, type II, is characterized by friction coefficients that starts at ≍ 0.15, and then increases to about 0.4. At the end of the experiment, the ball is worn, and compacted wear debris containing iron can be found on the plane. The two behaviors seem to be independent of contact pressure, but are rather sensitive to normal applied load. The transition between these two regimes will be discussed.

ZDDP additives have been used for their antiwear and antioxidant properties in lubricating systems for many years. In case of boundary lubrication, this additive generates a protective solid film on the surfaces called tribofilm. This work focuses on the rheological properties and durability of such films. In a previous paper, the feasibility of a particular experimental analysis of the rheology of ZDDP tribofilms has been demonstrated. This test procedure under realistic conditions (macrotribology) permits the visualization and quantification of the film rheology and its durability. It consists of different steps. First of all, a large ZDDP tribofilm (6×7 mm 2 ) is generated using a Cameron-Plint tribometer in cylinder-on-flat configuration under standard lubricated conditions (PAO and ZDDP). An aluminum mask with a comb shape is positioned on the tribofilm, which is then etched with an Ar + ion beam (5 keV), in order to remove the tribofilm where the mask does not cover the sample. Finally, a striped sample is obtained with a lateral alternation of etched zones and the original ZDDP tribofilm area (of approximately 1 mm width). On this striped sample, a Dual Zone Friction Test (DZFT) experiment is performed. It is in fact a pin-on-flat experiment run across a strip of tribofilm. The friction signal and the Electrical Contact Resistance (ECR) are plotted as a function of the position and the number of cycles. The pin-on-flat experiments are carried out dry (open air, room temperature) and under ultra high vacuum and the results were compared. Special attention is given to the evolution of the chemical composition inside the track during each experiment: this, in order to study the kinetics of the tribofilm disruption by coupling friction experiments with Auger Electron Spectroscopy analysis (AES) at intervals of the experiment. Both mechanical and chemical mechanisms seem to be involved in the removal processes of the ZDDP tribofilm.

The wear resistance of thin MoS2 layers has been extensively studied in the literature. The occurrence of material transfer, mechanical and tribochemical wear has been recently evidenced. Nevertheless, the high durability of this type of solid lubricant is still a matter of controversy. One explanation for this phenomenon is that friction and contact resistance are both controlled by material transfer processes. Then, the in vivo monitoring of the active interface, i. e. during sliding, is found as being a major technical issue. In this paper, we present some results concerning the monitoring of the interface by the means of electrical contact measurements in a reciprocating sphere on plane friction test. Thanks to spatially-resolved mechanical and electrical measurements, we give evidence for debris removal and recirculation through the contact. Based on a simplified model of interface, we correlate the evolution of interfacial thickness and remaining material, as analyzed by EDS. The dependence of the tribological behavior to environmental change is then probed. We precise some limits of this method in the case of moisture.

Les couches minces Diamond-Like Carbon (DLC) permettent d’obtenir des frottements et usures faibles dans les contacts secs. Il est aussi possible de controler les proprietes electriques du contact en realisant des couches nanocomposites metal/DLC. Dans ce travail, nous nous interessons a la caracterisation mecanique des surfaces en contact a l’issue d’essais de frottement lineaires alternatifs entre un plan recouvert d’une couche nanocomposite Cu/DLC et une bille en acier. En particulier, dans certaines conditions, ces essais conduisent a la formation d’un tribofilm sur les surfaces, du aux echanges de matiere entre les surfaces en contact lors du frottement. L’observation des surfaces frottees permet de correler l’evolution du coefficient de frottement et de la resistance electrique de contact a la formation ou non d’un tribofilm adherent sur la bille. Lorsqu’un tribofilm est forme, celui-ci est conducteur, et la couche mince ne subit qu’une usure progressive. Neanmoins, le coefficient de frottement evolue au cours de l’essai. Afin de mieux comprendre cette evolution, nous avons mesure les proprietes mecaniques par nanoindentation dans et hors des traces d’usure sur le plan et sur la bille, pour des essais realises a differents nombres de cycles. Les proprietes elastoplastiques du tribofilm se revelent etre differentes de celles des materiaux initiaux et semblent pouvoir etre correlees a l’evolution du coefficient de frottement.

The ultra-low friction coefficient (typically 10-2range) of MoS2-based coatings is generally associated with friction-induced orientation of “easy-shear” planes of the lamellar structure parallel to the sliding direction, particularly in the absence of environmental reactive gases and with moderate normal loads. We used a AES/XPS ultra-high vacuum tribometer coupled to a preparation chamber, thus allowing to elaborate oxygen-free MoS2 PVD coatings and to perform friction tests in various controlled atmospheres. Friction of oxygen-free stoichiometric MoS2 coatings deposited on AISI52100 steel has been studied in ultra-high vacuum (UHV, 5.10-8 Pa), high vacuum (HV, 10-3Pa), dry nitrogen (105 Pa) and ambient air (105 Pa). “Super-low” friction coefficients below 0.004 were recorded in UHV and dry nitrogen, corresponding to calculated interfacial shear strength in the range of 1 MPa, about ten times lower than standart coatings. Ultra-low friction coefficient of about 0.013-0.015 were recorded in HV, with interfacial shear strength in the range of 5 MPa. Friction in ambient air leads to higher friction coefficients in the range of 0.2. Surface analysis performed inside the wear scars by Auger Electron Spectroscopy show no trace of contaminant, except after friction in ambient air where oxygen and carbon contaminants are observed. In the light of already published results, the “super-low” friction behavior (10-3 range) can be attributed to a particular case of superlubricity, e.g. a combination of crystallographic orientations, surface chemistry and absence of contaminant allowing a strong decrease of the interfacial shear strength.

All the usual characterization of an interface submitted to shear is generally achieved thanks to an experiment using a pair of two loaded solids in continuous sliding. So, the friction coefficient value can be determined. But, only this friction value is quite poor information and is not able to reveal the complexity of the interfacial processes occurring in the sliding contact. A novel method has been developed at LTDS, allowing us to identify both the velocity-dependent and the solid friction contributions. It is based on a contact submitted to constant normal load, one solid being supported by a 1D mechanical oscillator, under the form of an elastic bi-blade, able to return to its equilibrium position through damped oscillations. The way the amplitude is decaying during this elastic recovery is recorded. A mechanical model has been developed, in order to fit experimental data, allowing us to identify two different contributions: the velocity-dependent contribution (typically viscous damping), and the solid-like contribution (typically solid friction). This technique has been applied here to a lubricated sphere-on-plane contact. The contacting surfaces are made of AISI 52100 polished steel. Two model products are tested: pure glycerol and 1,3-buthylene glycol. First results are presented, showing the capabilities of this technique for giving a new insight on the mixed lubrication regime description.

The effect of the preadsorption of HCl on the subsequent adsorption on alumina of carboxylic acids, ethanol and pyridine has been examined by inelastic electron tunnelling spectroscopy. The spectra are interpreted in terms of adsorption sites which have been previously identified on the alumina surface of tunnel junctions.

We wish to discuss the application of IETS to the observation of electronic, rather than vibrational transitions. This extension of the method seemed interesting for several reasons.

The inelastic tunneling of electrons in a metal-insulator-metal junction has been shown to be a spectroscopic method for studying the vibrational modes of the whole system. In the present paper we consider the possibility of deducing precise information from this spectroscopy. The low-voltage part of the spectrum (i.e., the $\frac{{d}^{2}I}{d{V}^{2}}\ensuremath{-}\mathrm{v}\mathrm{s}\ensuremath{-}V$ characteristic) gives information about the phonons of the electrodes. The phonon density, which is deduced for a Mg electrode, is critically compared with the density deduced from neutron scattering. The range of this phonon probe is then studied by tunneling into multilayer electrodes. The 40-90-meV range of the characteristic of a Mg-Pb junction exhibits a specific structure due to the lattice vibrations of the insulator. This structure is compared with the infrared spectrum and the phonon density of states of MgO, as well as with a theoretical calculation of the tunneling current in the transfer-Hamiltonian formalism. From the fit obtained, it is deduced that the 30-\AA{}-thick insulator, grown on Mg, is an oxide, in contrast with the insulator grown on Al, which was previously deduced to be a hydroxide. At higher energies (100-500 meV), the vibrational spectrum of molecules contained in the insulator region is observed. The identification of the lines is shown to be accurate and it gives precise information on these molecules, especially about their chemical binding with the insulator. This last point could be important in the future for studying the problem of adsorption on solid surfaces.

Mechanical failure of integrated circuits and micro-electro-mechanical systems (MEMS) demands new understanding of friction in small devices. In present research, we demonstrated an in situ approach to measure sliding friction of a patterned surface composing multi-materials and structures. The effects of materials and surface morphology on friction and electrical contact resistance were investigated. The material transfer at the interface of dissimilar materials was found to play dominating roles in friction. The current work provides important insights from the fundamentals of friction that benefit the design of new micro-devices.

Fluorinated diamondlike carbon (FDLC) films have been deposited on Si wafers by rf plasma-assisted chemical vapor deposition, under a variety of conditions. The films have been characterized by FTIR and index of refraction measurements, RBS and FRES analysis for determination of film composition, and stress measurements from the bending of the wafers by the deposited films. Friction and wear measurements have been performed using pin-on-flat and pin-on-disk testers in ambient air, at maximum Hertzian contact pressures ranging from 320 to 1100 MPa. By adjusting the deposition parameters, the properties of the FDLC films could be changed from soft films, with no significant wear resistance, to films containing more than 20% F and having wear resistance comparable to unfluorinated DLC. The tribological properties of the FDLC films are discussed in relation to their physical properties, as determined by the deposition conditions.

An insulating layer is produced on aluminum by glow discharge in CF4. This substrate, used as the insulator of metal-insulator-metal diodes is characterized by Inelastic Electron Tunneling Spectroscopy as an aluminum trifluoride AlF3. The adsorption of ammonia, benzylamine. formic and propiolic acid on this substrate is investigated. A comparison between spectra of samples obtained on this insulator and on alumina for the same dopant molecules shows significant differences which are interpreted as revealing an enhanced Lewis acidity of AlF3 relative to Al2O3, determined — at least in part — by an enhanced electronegativity of surface Al+ cations of AlF3.