Contribution to the study of mechanical properties of nanoscale friction -reducing and wear movies

La tribologie est définie comme la science qui étudie les phénomènes de frottement, d’usure et de lubrification. Elle intervient à chaque fois que deux surfaces sont en mouvement relatif l’une par rapport à l’autre. On la rencontre donc principalement dans les moteurs, les instruments de coupe, les assemblages mécaniques etc. Le frottement et l’usure sont les causes principales des pertes de performances et de durabilité des systèmes. L’usure coute 3 % du PIB en France et on estime que 10 % à 30 % de l’énergie produite dans le monde est consommée par frottement, c’est dire l’importance que porte le monde industriel à cette science.C'est la lubrification qui a pour rôle de minimiser le frottement et de protéger les surfaces de l'usure. Il existe différents régimes de lubrification, cependant les travaux effectués au cours de cette thèse de doctorat s’intéressent exclusivement au régime de lubrification limite. Dans ce régime de lubrification, l’épaisseur du film de lubrifiant est du même ordre de grandeur que la hauteur des aspérités des surfaces en contact et le frottement et l'usure des surfaces peuvent être très sévères. La stratégie mise en place pour lutter contre ces phénomènes consiste à ajouter des additifs à l'huile lubrifiante, qui formeront au cours du frottement un film protecteur (le tribofilm). Il apparait donc primordial de bien caractériser ces tribofilms afin de mieux comprendre les mécanismes de réduction de frottement et de l’usure, qui restent encore à ce jour mal connus.Les nanotechnologies permettent aujourd'hui d'apporter un nouvel éclairage sur cette problématique et nous les avons mises à profit pour étudier le comportement mécanique des tribofilms à l’échelle nanométrique.L’objectif de ce travail est de comprendre les processus de réduction de frottement observés à l’échelle macroscopique lorsque des particules de composés lamellaires sont associées à un liquide de faible viscosité. Les tribofilms étudiés sont formés à partir de trois composés lamellaires différents, le graphite, le thiophosphate de nickel NiPS3 et le bisulfure de molybdène MoS2. Pour ces trois composés, deux types de tribofilms ont été étudiés : ceux formés à partir des particules seules (les tribofilms secs) et ceux formés en présence de liquide (les tribofilms éther).L’analyse des propriétés tribologiques mesurées à macro et nano échelle, nous ont permis de mieux comprendre la contribution des effets individuels et collectifs sur les mécanismes réducteurs de frottement. Ces expériences ont permis de mettre en évidence que la réduction du coefficient de frottement observée à macro-échelle n'est pas due à la nanostructuration des tribofilms ni à une baisse des interactions entre les feuillets mais plutôt à un effet collectif des particules et du liquide.Tribology is the science of friction phenomena, wear and lubrication. It takes part every time two surfaces are in relative motion. It performs meanly in engine and mechanical systems. Friction and wear are the mean reasons of loose of performance and durability of mechanical systems. Wear costs 3% of the GDP in France and about 10% to 30% of the produced energy in the world is consumed by friction.The aim of lubrication is to reduce friction and protect the surface against wear. There are four different types of lubrication regimes but this study is focused on the boundary regime. In this regime, lubricant thickness is of the same order of the magnitude of the sliding surfaces roughness, leading to a high-level of friction and wear. Additives are added to the lubricating base oil, in order to build a protecting tribofilm. Characterization of the tribofilms appears essential to better understand the friction and wear reduction mechanisms.Nanotechnology allows to bring a new insight on this issue through the study of the mechanical behavior of the tribofilms at nanoscale.The aim of this work is to better understand the process of the friction reduction observed at macroscale when lamellar particles are associated to a low viscosity liquid. The tribofilms are formed from three different lamellar compounds, graphite, nickel thiophosphate NiPS3 and molybdenum disulfide MoS2. Two different kinds of tribofilms are studied: those formed with the solids particles only (dry tribofilms) and those formed in presence of liquid (wet tribofilms).The analyses of the tribological properties measured at nanoscale allow us to better understand the contribution of the individual and collective effects on friction reduction mechanims. We show that the reduction of the friction coefficient observed at macroscale is not related to the surface nanostructuration of the tribofilms neither to the decrease of the interaction energy between the sliding surfaces, but rather to collective effects of the liquid and the particles in the contact

Determination of the mechanical properties of red blood cells in sickle cell disease by means of AFM technique

International audienceSickle cell disease (SCD) is the most frequent genetic disease in the West Indies and in France. This disorder is caused by polymerization of the abnormal hemoglobin S which results from the substitution of an acid glutamic by a valine at codon 6 of the beta globin chain. Polymerization of deoxygenated hemoglobin S induces formation of long stiff rod-like fibers which force the red blood cells to take over a wide variety of irregular shape. SCD patients exhibit life-threatening complications such as chronic anemia and vaso-occlusion events due to the loss of red blood cell (RBC) deformability and increase of RBC adherence to endothelial cells [1]. In this study, the mechanical properties of the red blood cells are investigated by atomic force microscopy. For this purpose, the cells are immobilized on glass lamella coated with poly-l-lisine in order to increase RBC adherence. All the experiments are performed in presence of phosphate buffered saline (PBS). Red blood cells were imaged with silicon nitride probe with nominal spring constant of 0.06 N/m whereas the mechanical measurements were performed with silicon tips colloidal particle with nominal spring constant of 0.035 N/m. More than 150 mechanical tests have been performed on each sample in order to have reliable statistical data. All force/deformation curves were analyzed with PUNIAS (Protein Unfolding and Nano-Indentation Analysis Software), a custom-built semi-automatic processing and analysis software. To calculate the young modulus, we used Sneddon’s modification of the Hertz model for the elastic indentation of a flat and soft sample by a stiff sphere [2]. Four different groups of patient were analyzed, AA, AS, SS and SS with hydroxyurea treatment. The results show that the SS red blood cells are stiffer than AA cells, whereas we do not observed any difference between AS and SS cells. We also observed a decrease of the young modulus for the patients treated with hydroxyurea.REFERENCES[1] Microelasticity of red blood cells in sickle cell disease, J. L. Maciaszek, B. Andemariam, and G Lykotrafitis, J. Strain Anal. Eng. Des. 46(5), 368–379 (2011)[2] Elasticity measurement of living cells with an atomic force microscope: data acquisition and processing, Carl P. and Schillers H., Pflugers Arch - Eur J Physiol, 457:551–559 (2008

Tribological Performances of Graphite and Hexagonal Boron Nitride Particles in the Presence of Liquid

International audienceAbstract This article focused on the effects of the presence of liquid on tribological performances of graphite and hexagonal boron nitride (hBN) particles at ambient atmosphere. Two tribological tests conditions have been evaluated with AISI 52100 steel materials. In dry conditions for lubrication tests with pure particles and liquid conditions a drop of liquid is added before or during the friction tests. In dry conditions, graphite and hBN have two different tribological performances, while whatever was the type of liquid in liquid conditions, they are similar: μpure graphite ≈ 0.09 and μpure hBN ≈ 0.42, while μgraphite+liquid ≈ μhBN+liquid ≈ 0.06. Raman spectroscopy and scanning electron microscopy (SEM) investigations bring out weak influence of sliding mechanical constraints for tribofilms built in the presence of the liquid. Different hypotheses have been investigated; tribo-intercalation, influence of adsorption, lubrication by hydrogen, and action of the liquid. The simultaneous presence of liquid and particles induces friction mechanism inside the tribofilm reducing friction due to its nanostructuration

Influence of the Volume Structure on the Tribological Properties of Lamellar Tribofilms

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