Speed and Atmosphere Influences on Nanotribological Properties of NbSe2

Nanotribological properties of NbSe2 are studied using an atomic friction force microscope. The friction force is measured as a function of normal load and scan speeds ranging from 10 nm s−1 to 40 μm s−1 under two atmospheres (air and argon). At low speed, no effect of atmosphere is noticed and a linear relationship between the friction and normal forces is observed leading to a friction coefficient close to 0.02 for both atmospheres. At high speed, the tip/surface contact obeys the JKR theory and the tribological properties are atmosphere dependent: the shear stress measured in air environment is three times lower than the one measured under argon atmosphere. A special attention is paid to interpret these results through numerical data obtained from a simple athermal model based on Tomlinson approach

Experimental and theoretical investigations of friction properties of graphite intercalated compounds.

It is classically admitted that the goodfriction properties of lamellar compounds are strongly related to their anisotropic structure and especially to the existence of weak interlayer interactions through the van der Waals gap separating the basal layers. As it is also known, the presence of the van der Waals gap in the structure of lamellar compounds will allow lot of chemical species to be intercalated in the structure leading both to the expansion of structure parameters and inter layer interactions modifications. The present work is concerned with the experimental and theoretical study of friction propertiesof Graphite Intercalated Compounds (GICs) in order to better understand thetribologiclamellar compounds. In order to modulate the interlayer interactions, two types of intercalated species were used, electrophylic species (AlCl3, FeCl3, SbCl5) and nucleophilic species (Li, K, Rb)

Friction properties of fluorinated carbons

In boundary lubrication regime, friction reduction and antiwear processes are associated to the presence of additives in the lubricating oils or greases. These processes are due to the formation of protective tribofilms resulting from chemical reactions between the additives and the sliding surfaces, in the physico-chemical conditions of the sliding contact. Conventional antiwear additives mainly consist of transition metal organo phosphate or thiophosphates which present a remarkable efficiency in the case of contacts between ferrous alloys. In the case of non reacting surfaces, these additives become inactive. Recently developped lubrication strategies consist in the use of dispersion in oils of nano additives able to build the protective tribofilm in the sliding contact without reaction with the surfaces. Carbon fluorinated phases, due to their lamellar structure and their high chemical stability even at relatively high temperature (400°C) represent interesting candidates as lubricant nano-additives subjected to present friction reduction, anti wear and anti corrosion actions. This work presents the tribologic behaviour of some carbon fluorinated derivatives such as graphite fluorides, fluorinated carbon nanofibers, fluorinated carbon nanodiscs and fluorinated carbon blacks. The influence, on the tribologic performances, of the structure of the initial carbon phases, of the fluorination rate (0<F/C<1) and the structure of the fluorinated compounds is discussed

Tribological properties of low-temperature graphite fluorides. Influence of the structure on the lubricating performances

International audienceThe present work is concerned with the study of the tribological properties of various fluorinated carbon phases obtained at room temperature and then post-treated under fluorine atmosphere at different temperatures. The tribological tests evidence good intrinsic properties for all the compounds (friction coefficient in the range 0.07-0.09). Differences appear after few cycles. Friction measurements after 100 cycles and complementary Raman analyses of the tribofilm remaining in the wear scar point out that long term tribologic properties of the fluorinated compounds strongly depend on the evolution undergone by the materials under friction. The release of HF molecules, the loss of fluorine and partial rebuilding of graphitic phases are at the origin of the degradation of the friction properties. The good properties of the compounds post-treated at temperature in the range 150-300 °C are attributed to the chemical and structural stability of these compounds under friction

High resolution electron spectroscopic imaging (ESI) of reverse micelles

Overbased calcium alkylaryl sulfonate reverse micelles of core diameter 2-3 nm are investigated by means of an Energy-Filtering Electron Microscope (EFEM). For this purpose, micelles are deposited on a very thin carbon film. Both selected area EELS spectra energy loss images are obtained and discussed. Data are interpreted considering the existence of mainly four contrasts : contrast caused by elastic scattering at calcium nuclei, phase contrast due to the grainy support and the micellar shape, spectroscopic contrast due to plasmon losses in carbonaceous materials and elemental chemical contrast due to the excitation of atomic electrons, and particularly the ionization of inner-shell electrons. Depending on the operating mode of the microscope, each contrast can be emphasized and studied. The main result is that Electron Spectroscopic Imaging (ESI) with low energy losses, permits high resolution images of the calcium-rich core (2-3 nm diameter) and of the organic shell (1-2 nm thick). The specific image of the organic shell is attributed to a mass-thickness effect modulated by elastic scattering at calcium atoms. This gives a definite picture of the composite structure of overbased reverse micelles.Nous avons examiné des micelles inverses de sulfonate de calcium surbasées par Microscopie Electronique filtrée en énergie. Lataill des coeurs est comprise entre 2 et 3 nm. Les nicelles sont préalablement déposées sur une lame mince très fine de carbone amorphe. Nous avons obtenu des spectres EELS et des images filtrées en énergie, lesquelles son interprétées en considérant l'existence de quatre types de contraste : le contraste dû à la diffraction des atomes de calcium, le contraste de phase du support granuleux de carbone et de la forme sphérique de la micelle, le contraste spectroscopique dû à l'excitation des niveaux électroniques, et en particulier l'ionisation des niveaux de coeurs. En choisissant le mode de fonctionnement du microscope, chaque contraste peut être étudié en détail. Nous montrons que les images filtrées obtenues à partir des pertes faibles permettent de résoudre le coeur des micelles riche en calcium (diamètre 2-3 nm) et la couche de molécule organique l'entourant (épaisseur 1-2 nm). L'image spécifique de cette couche est attribuée à un effet d'épaisseur massique modulé par la diffraction des atomes de calcium du coeur

Les nanocarbones fluorés : des additifs prometteurs pour la lubrification

International audienceUn lubrifiant liquide est composé d'une base (huile minérale ou de synthèse) à laquelle sont ajoutés des additifs conférant au lubrifiant ses propriétés. Les additifs réducteurs de frottement et d'usure conventionnels (dialkyl dithiophosphates de métaux de transition) qui présentent une efficacité remarquable dans le cas de pièces frottantes en alliage ferreux 1 , deviennent peu efficients dans le cas de matériaux peu réactifs. De plus, les surfaces subissent une usure sévère durant la période de formation du film tribologique protecteur. Ainsi, les nouvelles stratégies de lubrification utilisent des particules colloïdales en dispersion dans la base. Ces nanoparticules sont susceptibles de constituer instantanément le film tribologique sans réaction chimique avec les substrats 3. Les phases tribo-actives choisies sont généralement des matériaux lamellaires (MoS 2 , graphite, graphites fluorés) présentant de bonnes propriétés tribologiques intrinsèques. Les travaux menés sur les carbones fluorés CF x 4 permettent d'envisager les formes nanostructurées comme une nouvelle famille de nano-additifs pour lubrifiants. Dans cette étude, les propriétés tribologiques de nanocarbones présentant différentes morphologies (sphérique (0D) : noirs de carbone (GCBs) ; tubulaire (1D) : nanofibres (CNFs) et discotique (2D) : nanodisques (CNDs)) sont déterminées en fonction du degré de fluoration des composés. Les excellentes propriétés réductrices de frottement des composés sont illustrées sur la figure 1. De très faibles coefficients de frottement sont obtenus ( 0,08) pour les trois types de nanostructures. La figure 1 met également en évidence des comportements tribologiques différents selon la structure initiale des nanocarbones. En effet, la présence de fluor améliore nettement les propriétés tribologiques des nanofibres et noirs de carbone (le coefficient de frottement diminue à mesure que le taux de fluoration augmente pour atteindre une valeur asymptotique pour un rapport atomique F/C = 0,15 pour les CNFs et 0,6 pour les GCBs) tandis que dans les cas des nanodisques de carbone, l'influence de la fluoration est nettement moins significative. La corrélation des propriétés tribologiques avec l'analyse de l'évolution structurale des nanocarbones (par MEB, MET, spectrométrie Raman) permet de mettre en évidence un processus de réduction du frottement lié à des phénomènes de surface dans le cas des CNFs et GCBs et de « bulk » dans le cas des CNDs 5. Figure 1: Evolution du coefficient de frottement intrinsèque des nanodisques (CNDs), nanofibres (CNFs) et noirs de carbone (GCBs) fluorés en fonction du taux de fluoration

Comparative modifications in bacterial gill-endosymbiotic populations of the two bivalves Codakia orbiculata and Lucina pensylvanica during bacterial loss and reacquisition

International audienceUntil now, the culture of sulphur-oxidizing bacterial symbionts associated with marine invertebrates remains impossible. Therefore, few studies focused on symbiont's physiology under stress conditions. In this study, we carried out a comparative experiment based on two different species of lucinid bivalves (Codakia orbiculata and Lucina pensylvanica) under comparable stress factors. The bivalves were starved for 6months in sulphide-free filtered seawater. For C.orbiculata only, starved individuals were then put back to the field, in natural sediment. We used in situ hybridization, flow cytometry and X-ray fluorescence to characterize the symbiont population hosted in the gills of both species. In L.pensylvanica, no decrease in symbiont abundance was observed throughout the starvation experiment, whereas elemental sulphur slowly decreased to zero after 3months of starvation. Conversely, in C.orbiculata, symbiont abundance within bacteriocytes decreased rapidly and sulphur from symbionts disappeared during the first weeks of the experiment. The modifications of the cellular characteristics (SSC - relative cell size and FL1 - genomic content) of the symbiotic populations along starvation were not comparable between species. Return to the sediment of starved C.orbiculata individuals led to a rapid (2-4weeks) recovery of symbiotic cellular characteristics, comparable with unstressed symbionts. These results suggest that endosymbiotic population regulation is host-species-dependent in lucinids

Friction Properties of Fluorinated Graphitized Carbon Blacks

International audienceThe tribologic properties of graphitized carbon blacks and their fluorinated derivatives are investigated as a function of the fluorination rate. Very low intrinsic friction coefficients are obtained for highly fluorinated compounds. The correlation of the tribologic results and structural investigations of the initial compounds by TEM strongly suggests that the friction mechanisms involve surface effects in the early stage of friction. Long-term tribologic experiments and Raman analyses point out an evolution of the structure and composition of the tribofilms during the friction process leading to similar friction properties of the tribofilms. Wear studies revealed that highly fluorinated derivatives appear less efficient than pristine and weakly fluorinated compounds