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Triboemission – i.e. the emission of photons, electrons and other charged particles that arise from a sliding contact – may play a key role in tribochemical processes, such as lubricant degradation. However, the mechanisms that give rise to this type of emission are not well understood. For the first time, we present spatially resolved measurements of electron emission, obtained as a range materials are worn. These are obtained from scratch tests, carried out under vacuum conditions (10−5 Torr), in which microchannel plates coupled to a phosphor screen are used to image electron emission. The results show that electron emission occurs at specific locations on the worn surface and, depending on the conductivity of the material, these sites remain active and decay with a time constant of up to several seconds. SEM images of the worn surface at these sites reveal that either surface fractures or grain defects are present. This suggests that fractoemission mechanisms are at least partially responsible for triboemission (however, the possible contribution of tribocharging mechanisms are also discussed). Specifically, this provides evidence to support the theory that triboemission results from the imbalance of charge on opposing faces of wear cracks and that this generates an electric field sufficient to accelerate molecular fracture products, which then bombard the surface leading to secondary emission. The strong geometric correlation between damage topography and electron emission distributions shows the potential of using this technique to monitoring wear and crack formation in real time and under high (30x) magnification.

We present a study on the simultaneous evolution of the electron emission and surface charge accumulation that occurs during scratching tests in order to monitor coating failure. Steel discs coated with a diamond-like-carbon (DLC) film were scratched in both vacuum (~10−5 Torr) and atmospheric conditions, with electron emission and surface charge being measured by a system of microchannel plates and an electrometer, respectively. The results highlight a positive correlation between emission intensity values, surface charge measurements and surface damage topography, suggesting the effective use of these techniques to monitor coating wear in real time.

Counter-rotation angled injection employed for aerostatic hybrid bearings reduces the cross coupling stiffness that may lead to whirl-whip instabilities at high rotation speeds. The benefits of counter-rotation injection have been known for years. Theoretical investigations were performed for water or air fed hybrid bearings but experiments were conducted only for water fed bearings. The present work is the first effort dedicated to angled injection in air fed hybrid bearings. The tests were performed for a simple rotor supported by two identical hybrid bearings. The hybrid bearings are provided with small size, shallow pockets and are fed with air via counter-rotation oriented orifice type restrictors. An impulse turbine fed with air entrains the rotor. An impact gun applies dynamic excitations and the rotordynamic coefficients are identified from the equations of motion of the rotor. Different air feeding pressures are tested as well as high rotational speeds. Compared to the dynamic characteristics of radial injection hybrid bearings, the direct stiffness of counter-rotation injection bearings has slightly lower values and the direct damping is higher but the main impact is the drastic reduction of the cross-coupling stiffness that may have even negative values.

This research is aimed at understanding the mechanisms that give rise to friction induced noise in automotive windscreen wipers, with a focus on frequencies between 500 and 3500 Hz. To study this phenomenon, experimental friction, sound, and high-speed video measurements are combined with finite element modeling of a rubber wiper/glass contact. In agreement with previous research, simultaneous sound and friction measurements showed that wiper noise in this frequency range results from the negative damping effect caused by the dependence of friction on speed in the mixed lubrication regime. Furthermore, during sliding, the friction induced noise recorded by the microphone occurred in one of two frequency ranges (close to 1000 Hz and between 2000 and 2500 Hz). These coincided closely with the eigen-frequencies of first two bending modes, predicted by finite element modeling. Experimental observations also showed the wiper to be oscillating backward and forward without any torsional motion and that the thickness of the glass had no effect on the emitted noise. These observations highlight how friction induced noise—although caused by conditions within contact—has characteristics that are determined by the structure of the excited component. A number of additional findings are made. Most importantly, both experiment and finite element modeling showed that the presence of water in contact with the wiper modulates the frequency and amplitude of the emitted noise by effectively adding mass to the vibrating system. While this is occurring, Faraday-like standing waves are observed in the water. In addition to this, friction induced vibration is shown only to occur for glass surfaces with intermediate surface energies, which is possibly due to high contact angles preventing water reaching the contact. Based on the understanding gained, a number of suggestions are made regarding means of reducing windscreen wiper noise.

Surface temperature measurements within sliding contacts are useful since interfacial heat dissipation is closely linked to tribological behavior. One of the most powerful techniques for such measurements is in-contact temperature mapping whereby a sliding contact is located beneath an infrared microscope. In this approach, one of the specimens must be transparent to infrared and coated such that radiation components can be distinguished and isolated from background values. Despite its effectiveness, a number of practical constraints prevent this technique from being applied to rough surfaces—a research area where temperature maps could provide much needed two-dimension input data to inform mixed and boundary friction models. The research described in this paper is aimed at improving the infrared temperature mapping technique in terms of validity, robustness, and spatial resolution, so that measurements of rough surfaces contacts can be made. First, Planck's law is applied in order to validate the use of surface coating as a means of removing background radiation. Second, a refined method of calibration is put forward and tested, which negates the need for a soft aluminum coating and hence enables rough surfaces to be measured. Finally, the use of super-resolution algorithms is assessed in order extend spatial resolution beyond the current limit of 6 μm.

International audience; In this paper, we present a technique to simultaneously measure Young's modulus E and Poisson's ratio ν of an isotropic material at local scale in a single experiment. Using several flexural modes of vibration of the scanning microdeformation microscope, it is possible to decouple the contributions of E and ν from the first two resonant frequencies, thereby providing access to both the elastic parameters. The proposed approach is applied to SU8 thin films deposited on silicon substrates and provides values consistent with those from the literature.

Colloque avec actes et comité de lecture. Internationale.; International audience; Ce papier présente une étude de caractérisation mécanique dynamique de polymères (PDMS et résine SU8) en utilisant l'analyse mécanique dynamique (échelle macro), le microscope acoustique à pointe vibrante (échelle micro) et la nanoindentation (échelle nano). Les modules de stockage et de dissipation sont mesurés pour une plage de fréquences allant de 0.01 Hz à quelques kHz. Les techniques concordent entre elles et permettent d'exhiber les comportements viscoélastiques très différents des 2 polymères. Un modèle d'Havriliak-Negami est utilisé pour obtenir les modules relaxés des matériaux.

The principal aim of this paper is to deal with friction-induced self-excited vibrations in the context of Stribeck law for friction coefficient. More precisely, the theoretical dynamic system under study consists of a single-degree-of-freedom mass-spring-damper oscillator subjected to a velocity-dependent frictional force as it slides on a conveyor belt, following a Stribeck law. We have analyzed the local stability of the static equilibrium and described the created periodic cycle using the averaging method. Numerical continuation with Matcont software is also used for qualitative analysis of this non linear system. In order to validate the theoretical analysis, an original experimental device, the named Lug test rig developed in our laboratory, was employed. For a glass/elastomer contact lubricated with water, friction force and dynamic displacement have been measured. The appearance of the instabilities is explained in relation to the friction measurements.

In this paper, specific geometries of resonant cantilevers for scanning force microscopy aimed to reduce sliding between tip and sample have been presented. These cantilevers have been designed for the scanning microdeformation microscope. Flexural and torsional vibration modes have been observed and compared to finite element simulations. Static deflections and dynamic sliding have been studied and the results have confirmed the efficiency of the cantilevers to keep the tip vertical during contact and to reduce the displacement of the tip on the surface.

National audience

International audience; The SMM is a well known scanning acoustic probe technique. Recently in the last years in order to optimize this metrological instrument a sensitivity study was carried out to adapt the stiffness of the microcantilevers to the encountered contact stiffnesses. The accuracy of the measurement is so optimized for the elasticity of the sample to characterize. Problems coming from the sliding of the tip on the surface and their effects were exhibited. New specific geometries of microcantilevers were conceived to reduce these perturbations. Their use reduced significantly the slip and so led to a better determination of the resonance frequencies, even for high amplitudes of vibration. In a last part a study of mechanical characterization was realized on polymers using DMA, SMM and nanoindentation. The use of different techniques enables to obtain complementary measures (viscoelastic characterization for several decades)