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Colloque avec actes et comité de lecture. Internationale.; International audience; Les essais de nanoindentation instrumentée permettent d'identifier les propriétés mécaniques des matériaux telles que le module d'Young ou la dureté. Il est proposé, dans ce travail, une nouvelle méthode pour obtenir ce résultat. Celle-ci repose sur l'utilisation de la seconde harmonique de la réponse vibratoire. Cette mesure originale permet de déterminer précisément les propriétés mécaniques des matériaux à de faibles enfoncements (inférieurs à 100nm) sans référence directe à la mesure d'enfoncement. Les premiers essais effectués sur la silice et du PMMA sont présentés.

Zinc dialkyldithiophosphate (ZDTP) is an additive extensively used in motor oil because of its good anti-oxidant and anti-wear properties under boundary lubrication conditions. Its anti-wear action is associated to the formation of a thin protective solid film onto the rubbing surfaces, due to a tribochemical reaction. Because ZDTP is pollutant, the knowledge of its anti-wear mechanisms is a crucial point in order to help to reduce its use or to design substitution additives. On a mechanical point of view, the efficiency of ZDTP tribofilms results from the equilibrium between film formation and wear rates, associated with appropriate mechanical properties. Their measurement contributes to better understand how they work, but the patchy nature of the tribofilms and their low thickness, hundreds of nanometers, make them difficult to measure. Nanoindentation tests have been performed at several temperatures (24 to 80°C) and several strain rates (0.003 to 0.1 s-1) on a ZDTP tribofilm using a Nanoindenter XP® entirely set into a climatic chamber. For all tests, an increase of the elastic modulus was observed from a threshold contact pressure value. This effect is similar to the Anvil effect observed on polymers: in confined geometry, the elastic modulus increases versus hydrostatic pressure. For the tribofilm, this effect is enhanced at high temperature and low strain rate. Furthermore, when the temperature increases, a change in the rheological behaviour of the tribofilm is observed. Up to about 50°C, the tribofilm exhibits a viscoplastic behaviour - the hardness increases versus strain rate - and above 50°C, the hardness decreases versus strain rate (“shear thinning-like” behaviour).

Conference held at Kanazawa, JAPAN, 16-19 October 2006. Presentation was "Best Poster" Awarded; International audience; Among Diamond-Like Carbon (DLC), some hydrogenated amorphous carbon (a-C:H) coatings may exhibit superlow friction behavior (i.e. friction coefficient in the millirange) under Ultra-High Vacuum environment. Hydrogen content, environment and some mechanical properties have been identified as key-requirements to achieve such low coefficient of friction. Nevertheless, this exceptional friction regime may last only few hundreds of cycles depending on the film nature. It is believed that adhesive phenomena may account for such drastic friction increase. In order to better understand the tribological mechanisms governing the achievement and loss of superlow friction, comparison of the wear tracks at different stages of friction with pristine surfaces have been performed by Atomic Force Microscopy (AFM) in contact mode. While surface of a-C:H films appears smooth by conventional means of observation, AFM revealed a nanostructured surface with pads of about 10 nm height and 100 nm wide. The wear of these pads during sliding seems to be correlated with the loss of superlow friction. The relations between topography and superlubricity of such materials will thus be discussed.

First the nanomechanical properties of ZnDTP antiwear tribofilms were measured by nanoindentation at ambient temperature. As the tribofilms are very thin, the elastic properties of the substrate influence the measurements. A simple model was used to extract the elastic modulus of the ZnDTP tribofilm from the composite measured modulus. During the loading stage of the nanoindentation test, it was observed that the mechanical properties of the tribofilm significantly increase. Additional measurements performed after local nano-machining of the tribofilm have shown that this phenomenon does not correspond to a gradient of properties throughout the film but to an accommodation process in response to the applied pressure, which permits to describe the ZnDTP tribofilms as "smart" materials. Because of the patchy structure of the tribofilms, it was tried to go further by having an estimation of the properties of the tribofilms from a very high number of tests. A 10 µm x 10 µm cartography of the stiffness from 1600 points of measurement was realized. It was clearly visible that the lowest stiffness values were obtained in the valleys and the highest were obtained on the pads. This work was extended to the measurement of the nanomechanical properties of the tribofilm at temperatures up to 80°C (near service conditions) using a Nanoindenter XP® entirely set into a climatic chamber. The effect of applied pressure on the elastic properties was also observed at 80°C. The results show that the elastic modulus of the tribofilm remains constant in the studied temperature range. The use of the F/S2 parameter, independent of contact geometry, permitted to point out a significant decrease in hardness while the temperature increases. Furthermore, AFM observations of the residual pile-up pointed out an evolution of the film deformation process with temperature. Pile-up seems to have been formed by successive pronounced steps at ambient temperature whereas at 80°C, it seems to result from a more continuous flow of matter. This point was investigated by extending the temperature range to lower values and the viscous-plastic properties of the tribofilm were studied by varying the deformation rate during the nanoindentation tests.

Diamond-Like Carbon (DLC) films have been shown to demonstrate various tribological behaviors: in ultra-high vacuum (UHV), with either friction coefficients as low as 0.01 or less and very mild wear, or very high friction coefficients (>0.4) and drastic wear. These behaviors depend notably on gaseous environment, hydrogen content of the film [1], and on its viscoplastic properties [2,3]. A relation between superlow friction in UHV and viscoplasticity has indeed been established for a-C:H films and confirmed for a fluorinated sample (a-C:F:H). In this study, nanoindentation and nanoscratch tests were conducted in ambient air, using a nanoindentation apparatus, in order to evaluate tribological behaviors, as well as mechanical and viscoplastic properties of different amorphous carbon films., Conference was held in Kobe, JAPAN 29th of May to 2nd of June 2005