Le contrôle de la friction en régime limite s’effectue par le biais de lubrifiants qui, par l’adsorption de molécules sur les surfaces, réduisent les contacts directs entre aspérités. Dans ce contexte, l’objectif de cette thèse est de comprendre et de coupler les mécanismes, à la fois, d’adsorption et d’auto-organisation de différents acides gras sur des surfaces, et les mécanismes de friction interfaciale sous des conditions stationnaires et transitoires. L’effet de l’architecture moléculaire, modifiée par la présence et la conformation d’une insaturation dans la chaîne aliphatique des acides gras, a également été analysé. La caractérisation in-situ, à l’échelle moléculaire, a été réalisée avec le tribomètre moléculaire ATLAS développé au LTDS. Cet appareil permet des déplacements quasi-statiques et dynamiques, d’une sphère mise en regard d’un plan, suivant trois axes. Trois solutions d’acides gras, en faible concentration dans du dodécane, ont été analysées. Les déplacements et les forces, normaux et tangentiels, sont mesurés à l’aide de capteurs capacitifs d’une résolution respective de 0.015 nm et 10 nN. Des sollicitations dynamiques superposées permettent de caractériser simultanément la rhéologie de l’interface confinée en termes d’amortissement et de raideur, dans les deux directions. Les résultats montrent que les acides gras s’adsorbent par interaction physique sur les surfaces pour former des films visco-élastiques d’une épaisseur d’environ 15 Å sur chaque surface. Le taux de couverture et la cinétique d’adsorption de ces couches dépendent de l’architecture moléculaire des acides gras. Cette dernière gouverne également la friction interfaciale, qui a été qualifiée de supraglissement, et la rhéologie des monocouches auto-assemblées. Les différentes organisations de films conduisent ainsi à différentes évolutions de la friction en fonction de la vitesse de glissement et de la pression de contact. La réponse en friction transitoire et l’accommodation lors du glissement vers un nouvel état stationnaire ont de plus été décrites par des distances caractéristiques, de plusieurs nanomètres, reflétant le renouvellement statistique des spots de contact, et par des temps de relaxation, de l’ordre de la seconde, décrivant le réarrangement moléculaire au sein de l’interface. Enfin, une modélisation théorique de la friction limite a été proposée afin de comprendre l’origine moléculaire de la friction entre monocouches d’acides gras mettant ainsi en évidence le couplage fort de deux échelles spatiales et temporelles. / Friction in boundary lubrication can be controlled by the adsorption of molecules on surfaces that reduce direct contacts between asperities. In this context, the aim of this thesis is to understand and to couple the mechanisms of adsorption and self-organization of different fatty acids on surfaces, with the mechanisms of interfacial friction under steady-state regime and transient conditions. The effect of the molecular architecture, modified by the presence and conformation of one unsaturation in the aliphatic chain of fatty acids, was also analyzed. In-situ characterization, at the molecular level, was performed with the molecular tribometer ATLAS, developed at LTDS. This apparatus allows quasi-static and dynamic displacements, of a sphere in front of a plane in three directions. Three fatty acids solutions, in low concentration in dodecane, were analyzed. The displacements and the forces, normal and tangential, are measured using capacitive sensors with a resolution of 0.015 nm and 10 nN respectively. Dynamic superimposed measurements allow a simultaneous rheological characterization of the confined interface in terms of damping and stiffness in two directions. The results show that the fatty acids adsorb on the surfaces by weak interactions and form viscoelastic films with a thickness of about 15 Å on each surface. The coverage rate and adsorption kinetics of these layers depend on the molecular architecture of the fatty acids. This architecture also governs interfacial friction, which has been described as superlubricity, and the rheology of self-assembled monolayers. Various film organizations have thus led to different evolutions of the friction as a function of the sliding speed and the contact pressure. The transient friction response and its accommodation during slip to a new stationary state have also been described by characteristic distances of several nanometers, reflecting the statistic average renewal of the contact spots, and by relaxation times of the order of the second, describing the molecular rearrangement within the interface. Finally, theoretical modeling of boundary friction has been proposed to understand the molecular origin of the friction between monolayers of fatty acids and highlight the strong coupling between both spatial and temporal scales.

No description available.

No description available.

A l’échelle micro-nanométrique, la fiabilité et la durée de vie des microsystèmes (MEMS),généralement réalisés en silicium, sont fortement affectées par les effets de frottement, d’adhesion,d’usure... L’objectif de ce travail est d’étudier les mécanismes de frottement et d’usure sur deswafers de silicium. Le comportement micro-nanotribologique de monocouches auto-assemblées(SAMs) d’alkyltrichlorosilane, déposées sur des wafers de silicium de différentes orientationscristallographiques – i.e, Si(100), Si(111) et Si(110), a été étudié à l’aide d’un nanotribomètre. Lesparamètres modifiés au cours de l’étude sont les suivants : la longueur de la chaîne, les paramètrestribologiques (charge normale, vitesse de glissement, distance de glissement, taux d’humidité relativeet température du substrat) et les propriétés de surface du silicium (orientation cristallographique,topographie). Les résultats expérimentaux montrent que le comportement nanotribologique desmonocouches greffées sur des substrat polis est influencé par l’homogénéité du film et la fractiond’aire qu’il couvre. Ces deux paramètres étant contrôlés par le temps d’immersion et l’orientationcristallographique du substrat. La topographie du silicium a également été modifiée de manièreà créer des motifs périodiques (microstructure par DRIE). Le comportement tribologique de cessurfaces revêtues d’OTS est contrôlé par les variations des propriétés physico-chimiques dessurfaces et la fragilité de la microstructure. Pour s’affranchir des problèmes de fragilité, des motifsstructurés à l’échelle nanométrique sont réalisés par nano-impression. / At micro and nanoscale, fiability and durability of micromechanical devices (MEMS), usuallymanufactured of silicon, are strongly affected by the friction effects, adhesion, wear... The aim ofthis work is to study the mechanisms of friction and wear of silicon wafers. Micro/nanotribologicalstudy of self-assembled monolayers (SAMs) derived from n-alkyltrichlorosilanes deposited on siliconwafers displaying various crystallographic orientations – i.e, Si (100), Si (111) and Si (110) – hasbeen conducted using a nanotribometer (ball-on-disc). The parameters that have been varied are: the alkyl chain length, the tribological parameters (normal load, sliding velocity, sliding distance,relative humidity level and substrat’s temperature) and surface characteristics of the silicon substrates(crystallographic orientation, roughness). On smooth silicon substrats, experimental results show thatthe tribological behaviour of SAMs is control by the film’s homogeneity and the surface coverageof the monolayer in connection with the time immersion and the crystallographic orientation of thesubstrate. The topography of silicon was also modified by changing the microstructure by DRIE inorder to create periodic patterns. The tribological behavior of OTS SAM grafted on microstructuredsurfaces was controlled by the changes in physico-chemical properties and the fragility of thepatterns. To overcome the problems of fragility of these surfaces, patterns at the nanoscale areachieved by nanoimprint.

Interfacial features of polymers are a complex, fascinating topic, and industrially very important. There is clearly a need to understand interactions between polymer layers as they can be used for controlling surface properties, colloidal stability and lubrication. The aim of my Ph.D study was to investigate fundamental phenomena of polymers at interfaces, covering adsorption, interactions between polymer layers and surfactants, surface forces and friction between adsorbed layers. A branched brush layer with high water content was formed on silica surfaces by a diblock copolymer, (METAC)m-b-(PEO45MEMA)n, via physisorption. The adsorption properties were determined using several complementary methods. Interactions between pre-adsorbed branched brush layers and the anionic surfactant SDS were investigated as well. Surface forces and friction between polymer layers in aqueous media were investigated by employing the Atomic Force Microscopy (AFM) colloidal probe technique. Friction forces between the surfaces coated by (METAC)m-b-(PEO45MEMA)n in water are characterized by a low friction coefficient. Further, the layers remain intact under high load and shear, and no destruction of the layer was noted even under the highest pressure employed, about 50 MPa. Interactions between polymer layers formed by a temperature responsive diblock copolymer, PIPOZ60-b-PAMPTMA17 (phase transition temperature of 46.1 °C), was investigated in the temperature interval 25-50 °C by using the AFM colloidal probe technique. Friction between the layers increases with increasing temperature (25-45 °C), while at 50 °C friction was found to be slightly lower than that at 45 °C. We suggest that this is due to decreased energy dissipation caused by PIPOZ chains crystallizing in water above the phase transition temperature. The structure of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers was determined by X-ray reflectometry. Surface forces and friction between DPPC bilayer-coated silica surfaces were measured utilizing the AFM colloidal probe technique. Our study showed that DPPC bilayers are able to provide low friction forces both in the gel (below ≈ 41°C) and in the liquid crystalline state (above ≈ 41°C). However, the load bearing capacity is lower in the gel state. This is attributed to a higher rigidity and lower self-healing capacity of the DPPC bilayer in the gel state. Friction forces in single asperity contact acting between a micro-patterned silicon surface and an AFM tip was measured in air. We found that both nanoscale surface heterogeneities and the µm-sized depressions affect friction forces, and considerable reproducible variations were found along a particular scan line. Nevertheless, Amontons’ first rule described average friction forces reasonably well. Amontons’ third rule and Euler’s rule were found to be less applicable to our system. / <p>QC 20141209</p>

No description available.

In case of metalworking operations, the purpose of lubrication is served by a complex mixture of two or more phases, these mixtures are known as metalworking fluids (MWFs). For many decades oil-in-water emulsions have been used as metalworking fluids. The particular advantage of using oil-in-water emulsion in metalworking operations is that it combines the cooling property of water and the lubrication property of the oil. To explain the lubrication mechanism for oil-in-water emulsions as metalworking fluids a variety of models and theories has been proposed. To understand the lubrication mechanism, the role of each ingredient in the tribological process needs to be studied. In the present study a model for lubrication which determines force and proximity regimes of droplets based on the droplet size distribution is proposed. Dynamic light scattering (DLS) is used to characterize the emulsions. The small droplets are found to be the ones which enhance lubricity. DLVO (Derjaguin-Landau-Verwey-Overbeek) theory is used to validate the results. The concentration and type of surfactant is found to be the performance controlling parameter. A further analysis of the three interfacial energetics; oil/water, oil/substrate, water/substrate, is studied in the presence and absence of surfactants with the help of a Goniometer, Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM). Such energetics reflects the rate at which the excess surfactant molecules accumulate at the water/oil interface and desorb into the phases. The tribological response is recorded using AFM and the nanotribometer (NTR). Frictional response of the chemisorbed self-assembled monolayer of surfactant (sodium oleate) on the steel substrate reflects that a tribofilm helps in lubricating the contact under boundary lubrication by creating a low shear strength material. Water being the continuous phase in oil/water emulsion a thin water layer adjacent to steel substrate is always present. This thin layer on the solid substrate acts as a barrier to the lubricating oil droplets to reach the metal surface. The focus of the present work is also to investigate conditions which permit the disjoining of the water film to allow the oil to lubricate the metal substrate. AFM is used to study the interaction force between an oil droplet and the steel substrate through water. An oil encapsulated SiO2 colloidal probe used to simulate the oil droplet. The charge regulatory status of the substrates and interfaces are found to be critical in mapping the force characteristics when DLVO interaction is considered. The condition for activation of non-DLVO (hydration, hydrophobic, capillary) forces are also identified and found to be dependent on the physical states of surfaces. Disjoining of the thin film can be controlled by selecting surfactants based on interfacial energetics and attractive force characteristic can be achieved to facilitate lubrication.

Polymer brushes provide the responsive smart surfaces which can be used for fabrication of various devices. In this thesis work, adhesion, friction, and wear of polystyrene (PS) - poly(2-vinyl pyridine) (P2VP) and polystyrene - poly(acrylic acid) (PAA) binary brushes and corresponding monobrushes were investigated in dried state under controlled environment. Spin-coated films were also investigated for comparison. The aim was to explore possibilities to control/tune adhesion, friction, and wear between inorganic or polymeric surfaces by use of polymer brushes. Atomic force microscopy (AFM) with sharp silicon nitride tip and colloidal probes was employed to investigate the nanoscale adhesion and friction forces between different inorganic and polymeric surfaces. Adhesion and friction on the polymer brushes were comparable to that on the spin-coated films. Adhesion and friction force values were correlated, and were in accordance with the wettability of the brush surfaces for most of the samples. Switching in the adhesion and friction forces was observed for the PS+P2VP and PS+PAA binary brushes on treatment with selective solvents. Maximum switching in adhesion force and friction coefficient was by a factor of 2.7 and 5.4, respectively. Furthermore, switching of friction for mixed brush surface was observed during macroscale friction measurements using nanoindenter. Friction coefficients at macroscale were higher than those at the nanoscale. Moreover, adhesion and friction forces between the surfaces were significantly influenced by the humidity, grafting density of polymer brushes, chemical composition of top of the binary brush surface, and tip scan velocity. Nanowear studies were carried out with AFM using sharp silicon nitride tip while macrowear studies were carried out using nanoindenter. Nanowear on the surfaces was affected by molecular entanglements, adhesion and friction forces as well as shape and status of the tip. It was observed that the typical wear mode for PS brushes (treated with toluene) was ripple formation. In case of P2VP brushes (treated with ethanol) and PAA brushes (treated with pH 10 water), wear occurred via removal of the polymeric material. Wear mechanism observed for the monobrushes was similar to that observed for the spin-coated thick films of the same polymeric material. However, extent of the wear on the brush surfaces significantly differed from that on the spin-coated films. In case of PS+P2VP and PS+PAA binary brush samples, change in the wear mode was observed on treatment with the different selective solvents. On treatment with toluene (PS on the top), both of these binary brushes showed the wear by formation of the ripples. On the other hand, when these binary brushes were treated with selective solvent for P2VP or PAA, wear occurred mainly via removal of the polymeric material. The amount of wear increased with the number of scans for all the polymer brush samples. Moreover, wear on the polymer brush surfaces was also increased on increase in the applied load and decrease in the scan speed. Wear behavior on macroscale was averaged due to contact between surfaces at large number of asperities. Our results show that adhesion, friction, and wear of polymer surfaces can be controlled/tuned by the use of binary polymer brushes.

A remote-sensing acoustic method for implementing position control feedback in Scanning Probe Microscopy (SPM) is presented. The capabilities of this feedback control using the new Whispering Gallery Acoustic Sensing (WGAS) method is demonstrated in a Shear-force Scanning Probe Microscope that uses a sharp probe attached to a piezoelectric Quartz Tuning Fork (QTF) firmly mounted on the microscope's frame. As the QTF is electrically driven its mechanical response reaches the SPM frame which then acts as a resonant cavity producing acoustic modes measured with an acoustic sensor strategically placed on the SPM head. The novelty of the WGAS resides in using an SPM frame with a perimeter closely matching the intervening acoustic wavelength to act as a resonant cavity. The whispering gallery cavity constitutes an acoustic amplifier for the mechanical motion of the QTF probe. The observed monotonic behavior of the whispering gallery acoustic signal as a function of the probe sample distance is exploited here for tip-sample distance control with nanometer sensitivity, thus allowing topographic characterization as the probe is scanned across the sample's surface. This thesis includes a description of a Labview based programming for the Field Programmable Gate Array (FPGA) card used in the automated control of the WGAS feedback microscope, a solution for improving the effective resolution of the Digital to Analog Converter (DAC) and initial results towards theoretically modeling the WGAS working principle.

Ionic liquids (ILs) present new approaches for controlling interactions at the solid-liquid interface. ILs are defined as liquids consisting of bulky and asymmetric ions, with a melting point below 373 K. Owing to their amphiphilic character they are powerful solvents but also possess other interesting properties. For example, ILs can self-assemble and are attracted to surfaces due to their charged nature. As a result, they are capable of forming nanostructures both in bulk and at interfaces. This thesis describes how the solid-IL interface responds to external influences such as elevated temperatures, the addition of salt and polarisation. An improved understanding of how these factors govern the surface composition can provide tools for tuning systems to specific applications such as friction. Normal and friction forces are measured for ethylammonium nitrate (EAN) immersed between a mica surface and a silica probe, at different temperatures or salt concentrations. The results demonstrate that an increase in temperature or low concentrations of added salt only induce small changes in the interfacial structure and that the boundary layer properties remain intact. In contrast, at sufficiently large salt concentrations the smaller lithium ion prevails and the surface composition changes. The interfacial layer of a similar IL is also investigated upon the addition of salt and the results reveal that lithium ions affect the surface composition differently depending on the ion structure of the IL. This demonstrates that the surface selectivity strongly depends on the ion chemistry. Remarkably, a repulsive double layer force manifests itself for EAN at 393 K, which is not observed for lower temperatures. This indicates a temperature dependent change in EAN’s microscopic association behaviour and has general implications for how ILs are perceived. A new method is developed based on a quartz crystal microbalance to investigate how the surface compositions of ILs respond to polarisation. The approach demonstrates that interfacial layers of both a neat IL and an IL dissolved in oil can be controlled using potentials of different magnitudes and signs. Furthermore, the method enables two independent approaches for monitoring the charges during polarisation which can be used to quantify the surface composition. The technique also provides information on ion kinetics and surface selectivity. This work contributes to the fundamental understanding of the solid-IL interface and demonstrates that the surface composition of ILs can be controlled and monitored using different approaches. / Jonvätskor möjliggör nya tillvägagångssätt för att kontrollera interaktioner vid gränsskiktet mellan fasta ytor och vätskor. Jonvätskor definieras som vätskor som består av stora och asymmetriska joner med en smältpunkt under 373 K. På grund av sin amfifila karaktär är de starka lösningsmedel men har också andra intressanta egenskaper. Jonvätskor kan till exempel självorganisera sig och attraheras till ytor på grund av sin laddning. En följd av detta är att de bildar nanostrukturer både i bulk och på ytor. Denna avhandling beskriver hur gränsskiktet mellan fasta ytor och jonvätskor svarar på yttre påverkan såsom en ökning i temperatur, tillsättning av ett salt samt polarisering. En ökad förståelse för hur dessa faktorer styr ytkompositionen av jonvätskor kan bidra med verktyg för att kontrollera system till specifika applikationer såsom friktion. Normala- och friktionskrafter mäts för etylammonium nitrat (EAN) mellan en glimmeryta och en kolloidprob vid olika temperaturer eller saltkoncentrationer. Resultaten visar att en ökning av temperatur eller låga koncentrationer av tillsatt salt bara marginellt framkallar ändringar i strukturen på gränsytan och att det adsorberade lagret förblir intakt. När saltkoncentrationen emellertid var tillräckligt hög får den mindre litiumjonen överhanden och ytsammansättningen ändras. Ytlagret av en liknande jonvätska undersöks också vid tillsättning av salt och resultaten avslöjar att litiumjoner påverkar ytsammansättningen annorlunda beroende på jonstrukturen av jonvätskan. Detta visar att ytselektiviteten starkt beror på jonkemin. En repulsiv dubbellagerkraft yttrar sig anmärkningsvärt för EAN vid 393 K vilket inte observeras vid lägre temperaturer. Detta indikerar en ändring i EANs mikroskopiska sammansättningsbeteende och har generella återverkningar för hur jonvätskor uppfattas. En ny metod har utvecklats baserad på en kvartskristall mikrovåg för att undersöka hur ytsammansättningen av jonvätskor reagerar på polarisering. Denna metod visar att det adsorberade lagret av både en ren jonvätska och en jonvätska löst i olja kan kontrolleras genom att applicera spänningar med olika tecken och storlekar. Dessutom möjliggör metoden två oberoende tillvägagångssätt för att övervaka laddningarna under polarisering vilket kan användas för att kvantifiera ytsammansättningen. Tekniken ger också information om jonkinetik och ytselektivitet. Detta arbete bidrar till den grundläggande förståelsen av gränsskiktet mellan fasta ytor och jonvätskor och visar att ytsammansättningen av jonvätskor kan kontrolleras och övervakas med olika tillvägagångssätt. / <p>QC 20160518</p>