La technologie de micro indentation est un des moyens de caractérisation (à partir de petits échantillons) qui s'est imposé ces derniers temps dans différents domaines (pharmaceutique, génie civil, industrie pétrolière etc.). Il répond à un certain nombre d'exigences en matière de solution au problème d'échantillonnage. Cette thèse est consacrée à la caractérisation des propriétés mécanique des géomatériaux, et spécialement pour les roches pétrolières comme l'argilite, le grès, la craie ... qui ont été utilisées pour les différentes études expérimentales menées au cours de la thèse. Après avoir présenté la méthode de dépouillement du test d'indentation pour un milieu isotrope, nous avons développé une méthode semi-analytique basée sur la fonction de Green pour caractériser le milieu isotrope transverse en déterminant les cinq paramètres élastique de ce milieu. L'influence des différentes sollicitations (mécaniques, thermiques, hydriques) sur les propriétés mécaniques des roches a été étudiée en utilisant la technologie de micro indentation avec la méthode de dépouillement isotrope transverse. Nous avons essayé de caractériser les paramètres de rupture (C et f) à l'aide du test d'indentation et d'un test de micro compression simple (MCS) effectué par la même machine d'indentation. Par l'essai d'indentation et une méthode d'analyse inverse, nous avons identifié les paramètres d'une loi de comportement élastoplastique (Drucker Prager). En l'absence d'une solution directe du problème d'indentation en régime plastique, nous avons eu recours à une modélisation numérique par un code de calcule élément finis (ABAQUS) pour déterminer la courbe d'indentation calculée. Cette détermination s'est révélée tout à fait probante et a été de plus validée par une simulation d'essais de compression triaxiale sur le même matériau. / The technology of micro indentation is one of the techniques ofmateriaJ characterization (by using small specimens) in various fields (mechanical engineering, civil engineering, oil industry, and pharmaceutical industry). Its main advantage lies in a certain number of practical requirements as regards the solution to the problem of small specimens. The present study is devoted the characterization of the mechanical properties of geomaterials, especially rocks involved in petroleum engineering. After having presented the methodology of the indentation test for isotropic rocks, we developed a semi-analytical method based on the use of Green function to characterize transverse isotropic rocks (five elastic parameters of these rocks). The influence of the various loadings (mechanical, thermal, hydrous) on the rock mechanics properties was studied by using the technology of micro indentation and the methodology proposed for isotropic transverse were used. Moreover, we characterize the failure parameters (C and f) by a combined approach of the indentation test and a test of micro compression (MCS) carried out the indentation device. Finally, we use inverse analysis in order to identify the parameters of a Drucker Prager mode!. ln the absence of a direct solution of the problem of indentation (in plastic regime), we had recourse to a numerical modelling by a finite element code (ABAQUS) to determine the calculated curve of indentation. This determination appeared completely convincing and moreover was validated by a simulation of triaxial compression tests on the same material

Determination of mechanical properties of very thin films is rather difficult task as all of currently using testing techniques have some weakness. This master’s thesis deals with microcompressive test and bulge test. Finite element simulations of the two methods were carried out in order to better understanding of experimental record. Microcompression combines the sample preparation with the use of focused ion beam (FIB) with a compression test carried out using nanoindenter. Cylindrical specimens (pillars) were prepared from Al film deposited on Si substrate using FIB. Experimentally measured data on pillars needs correction to obtain undistorted material properties of Al thin film. A necessary correction using FE modeling is suggested in the thesis. Second part of the work is focused on modeling of bulge test. Pressure is applied on freestanding SiNx film while deflection of the film is measured. Stress state in the film is biaxial making determination of mechanical properties of the film more complicated. The goal is to present how to model the whole problem. In addition, preparation of the specimens was simulated to estimate residual stress in the film. The paper contributes to a better characterization of very thin surface layers and determination of their mechanical properties.

The mechanical properties of crystalline metals are strongly affected when the sample size is limited to the micron or sub-micron scale. At these scales, the mechanical properties are enhanced far beyond classical predictions. Besides, the surface to volume ratio significantly increases. Therefore surfaces and interfaces play a big role in the mechanical properties of these micro-samples. The effect of different interfaces on the mechanical properties of micro-samples is not yet well understood. The aim of this project is to characterize, understand, and predict the effect of confinement on deformation mechanisms at micro-scale. In this study, micro-pillars were fabricated by Focused Ion Beam (FIB). Micro-pillars were homogeneously coated with thin films by magnetron sputtering and cathodic arc deposition. The mechanical properties of carbon-coated-, chromium coated-, naked-, annealed- and non-annealed micro-pillars were measured. Afterwards, the results of micro-compression tests and Automated Crystal Orientation Mapping on Transmission electron microscopy (ACOM TEM) were compared and led to some surprising new findings.Dislocations are blocked by amorphous- and even crystalline coating in the deformed samples. Parallel slip systems were detected in the chromium layer and the copper micro-pillar. Even though the chromium layer has parallel slip systems, dislocation pile-up at the interface was found after deformation. The most significant finding in this study concerns the back stress of the dislocation pile-up, which affects the dislocation sources and causes an increase of the flow stress to generate new dislocations from these sources. Thermal annealing increases the strength and flow stress of FIB fabricated micro samples. The annealing treatment restores the lattice that was damaged by the FIB fabrication process. A higher stress is required to initiate the dislocation nucleation in a pristine lattice. Techniques of fabrication and investigation were developed to study the role of confinement and interfaces on the mechanical properties of materials at micro scale. Mechanisms of deformation were unraveled and a better understanding of the key parameters was reached. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

Dans l’industrie, les traitements mécaniques de surface métalliques permettent d’améliorer les conditions de service des pièces mécaniques. Les effets de contact de ces types de procédés engendrent une forte déformation plastique du matériau et par conséquent une transformation microstructurale en sous-surface. Cette transformation se manifeste dans le raffinement progressif de la microstructure dans une couche de quelques dizaines de micromètres. Celle-ci est souvent dénommé "surface tribologiquement transformée" (en anglais : Tribologically Transformed Surface - TTS). Une telle transformation microstructurale conduit à une augmentation des propriétés mécaniques en extrême surface et rend le matériau plus résistant aux conditions de frottement, usure et fatigue.Dans le cadre de cette étude, deux procédures de transformation microstructurale ont été employées sur un matériau modèle : le fer-α. Pour la première technique (grenaillage), la surface est impactée de façon répétitive avec des billes métalliques projetées à grande vitesse. Concernant la deuxième méthode (micro-percussion), la surface est impactée répétitivement à un endroit précis avec un indenteur conique rigide.L’objet de ce projet se centre sur trois aspects principaux : (i) déterminer les gradients mécaniques et microstructuraux induits sur les deux types de surfaces transformées (grenaillage et micro-percussion), (ii) établir un lien quantitatif entre les mesures faites par deux types d’essais micromécaniques (nano-indentation et micro-compression de piliers) et (iii) mettre en évidence les effets microstructuraux impliqués (taille de grain, densité de dislocations, etc...) dans l’augmentation des propriétés mécaniques par hyper-déformation de surfaces. / The mechanical surface treatments confer better local mechanical properties against wear or fatigue service conditions. In the case of impact-based treatments, the material is exposed to repeated mechanical loadings, producing a severe plastic deformation in the near-surface. It leads to a local and progressive refinement of the microstructure into the affected zone, commonly known as Tribologically Transformed Surface (TTS). For this project, two mechanical surface treatments are used in a model material (pure α-iron): (i) shot-peening and (ii) micro-percussion.The resulting surfaces are characterized by a mechanical property gradient in-depth as a consequence of the microstructural transformation over a few tens of microns. Nowadays, it is well-known that this rise of local mechanical properties could improve the service lifetime of materials. However, a simple micro-hardness test is not quite enough to quantify precisely the engendered variation of mechanical properties and understand the influence of several microstructural effects. For this purpose, two micro-mechanical tests are considered: (i) nano-indentation and (ii) in situ micro-pillar compression.The main issue of this work is to characterize the mechanically-induced transformed surfaces and correlate the mechanical properties gradients with the local microstructural evolutions. Indeed, three main goals are considered: (i) quantify the mechanical and microstructural gradients induced by the surface treatments (shot-peening and micro-percussion), (ii) correlate the results obtained by the means of both mechanical tests (nano-indentation and micro-pillar compression) and finally (iii) investigate the influence of several microstructural effects related with the graded strengthening of hyper-deformed surfaces.

Three testing methods are proposed to characterize properties of fuel cell materials that affect the mechanical durability of proton exchange membranes (PEMs). The first two methods involved measuring the in-plane biaxial strength of PEMs and the biaxial hygrothermal stresses that occur in PEMs during hygrothermal cycles. The third method investigated the nonuniform thickness and compressibility of gas diffusion media which can lead to concentrated compressive stresses in the PEM in the through-plane direction. Fatigue and creep to leak tests using multi-cell pressure-loaded blister fixtures were conducted to obtain the lifetimes of PEMs before reaching a threshold value of gas leakage. These tests are believed to be more relevant than quasi-static uniaxial tensile to rupture tests because of the introduction of biaxial cyclic and sustained loading and the use of gas leakage as the failure criterion. They also have advantages over relative humidity cycling test because of the controllable mechanical loading. Nafion® NRE-211 membrane was tested at three different temperatures and the time-temperature superposition principle was used to construct a stress-lifetime master curve. Tested at 90°C, extruded Ion Power® N111-IP membrane was found to have longer lifetime than Gore™-Select® 57 and Nafion NRE-211 membranes under the same blister pressure profiles. Bimaterial specimens fabricated by bonding a piece of PEM to a substrate material were used to measure the hygral stresses, compressive and tensile, in the PEM during relative humidity cycles. The substrate material and its thickness were carefully chosen so that stresses in the PEM could be obtained directly from the curvature of the bimaterial specimen without knowing the constitutive properties of the PEM. Three commercial PEMs were tested at 80°C by cycling the relative humidity between 90% and 0% and by drying the membrane to 0%RH after submersion in liquid water. Stress histories for all three membranes show strong time-dependencies and Nafion® NRE-211 exhibited the largest tensile stress upon drying. Besides in-plane stresses, hard spots in gas diffusion media (GDM) can locally overcompress PEMs in the out-of-plane direction and cause electrical shorting. In this study, GDM samples sealed with an impermeable Kapton® film on the surface were compressed with uniform air pressure and the nonuniform displacement field was measured with a three-dimensional digital image correlation technique. Hard spots as a result of the nonuniform thickness and compressibility of the GDM were found and their severities as stress risers are evident. Locally, a nominal platen compression (similar to bipolar plate land compression) of 0.68 MPa can lead to compressive stress as large as 2.30 MPa in various hard spots that are in the order of 100s µm to 1 mm in size. / Ph. D.

L'objectif de cette thèse est l'étude de la déformation, à l'échelle microscopique, de la phase MAX Ti2AlN, synthétisée par métallurgie des poudres. Ce travail se divise en trois parties : une première dans laquelle l'accent a été mis sur l'hystérèse mécanique des phases MAX via des essais cyclés, en nanoindentation sphérique et compression ex-situ de micro-piliers, sur des grains d'orientations différentes déterminées par l'EBSD. Dans la deuxième nous nous sommes intéressés à la déformation de micropiliers via des essais de compression cyclés in-situ couplés à la micro-diffraction Laue. L'objectif a été d'analyser les taches diffraction au cours de la déformation du pilier afin de mettre en évidence les mécanismes de déformation élémentaires mis en jeu et d'observer les structures finales via des images MEB post-mortem des piliers. Enfin, une dernière dans laquelle l'objectif a été l'étude des mécanismes de déformation en température à l'échelle microscopique via des essais de nano-indentation allant jusqu'à 800°C. La caractérisation des lignes de glissement en surface et des configurations microstructurales sous l'empreinte a été réalisée par AFM et MET respectivement. Toutes les données recueillies par ces divers essais aux petites échelles, ont permis d'affiner notre compréhension des mécanismes de déformation du monocristal de phase MAX, notamment vis à vis des modèles usuellement proposés dans la littérature. / The thesis's goal is to study the deformation, at microscopic scale, of the MAX phase Ti2AlN synthesized by powder metallurgy. This work is divided into three parts: in the first part, the interest has been put on the hysteretic behavior of the MAX phases via cyclic mechanical solicitations, during spherical indentation tests and ex-situ compression of micro-pillars, on differently orientated grains beforehand determined by EBSD. In the second part, we were interested into the micro-pillar's deformation via insitu cyclic compression tests coupled with Laue micro-diffraction. The goal was to analyse the evolution diffraction lines during the pillar's deformation in order to highlight the elementary deformation mechanisms and to observe the finale structures via the post-mortem SEM imaging of the pillars. Finally, a last part was devoted to study the deformation mechanisms in temperature at microscopic scale via nano-indentation tests up to 800°C. The characterization of the slip lines on the surface has been revealed by AFM and that of t he microstructural configurations (dislocations) under the indent has been done by TEM. All data collected by these various tests at the small scales have refined our understanding of the deformation mechanisms of crystal MAX phase, particularly with respect to the models usually proposed in the literature.