Au cours de cette thèse ont été développées de nouvelles techniques de mesure des propriétés mécaniques par nanoindentation, adaptées à des conditions expérimentales difficiles : les très faibles enfoncements, les surfaces rugueuses, et la température. Une simulation numérique par éléments finis d'un échantillon de silice chauffé à 80°C, indenté par un diamant Berkovich dont la température initiale est de 25°C, dans de l'air à 60 °C, a mis en évidence la nécessité de chauffer l'indenteur. De plus, les essais expérimentaux à haute température effectués sur ce même échantillon ont permis de montrer que le signal de déplacement de l'indenteur est fortement perturbé par de faibles variations de température (<0,1°C), rendant le calcul des propriétés mécaniques imprécis avec la technique de mesure classiquement utilisée en nano-indentation. Une nouvelle technique, reposant sur la mesure de l'amplitude de la seconde harmonique du signal de déplacement, a été développée pendant cette thèse. Elle permet la détermination des propriétés mécaniques indépendamment de la mesure de l'enfoncement. Elle est donc adaptée pour des tests à haute température. Elle été expérimentée sur des matériaux homogènes (silice, PMMA), sur un matériau (monocristal de calcite) possédant une dureté plus élevée en surface (Indentation Size Effect), et sur des couches minces de PMMA déposées sur un substrat de silicium, à température ambiante. Les résultats ont montré que les propriétés mécaniques sont mesurées de façon plus précise aux faibles enfoncements. Les essais expérimentaux sur la calcite ont mis en évidence que l'Indentation Size Effect observé sur cet échantillon est mieux détecté avec la technique de la seconde harmonique. Elle permet aussi de calculer la pénétration de l'indenteur a posteriori. Une seconde technique de mesure des propriétés mécaniques, reposant sur le calcul de la dérivée de la hauteur de contact en fonction de la pénétration, a été développée. Elle permet une mesure des propriétés mécaniques des matériaux par nano-indentation plus précise que la méthode classique à température ambiante aux faibles pénétrations. Les mesures sont aussi améliorées sur les échantillons rugueux et pour les essais réalisés à haute température.

Inconel 718 is a high nickel content superalloy possessing high strength at elevated temperatures and resistance to oxidation and corrosion. The non-traditional manufacturing process of wire-electrical discharge machining (EDM) possesses many advantages over traditional machining during the manufacture of Inconel 718 parts. However, certain detrimental effects are also present. The top layer of the machined surface is melted and resolidified to form what is known as the recast layer. This layer demonstrates microstructural differences from the bulk workpiece, resulting in altered material properties. An experimental investigation was conducted to determine the main machining parameters which contribute to recast layer formation in wire-EDM of Inconel 718. It was found that average recast layer thickness increased with energy per spark, peak discharge current, current pulse duration, and open-voltage time and decreased with sparking frequency and table feed rate. Over the range of parameters tested, the recast layer was observed to be between 5 and 10 μm in average thickness, although highly variable in nature. Surface roughness of the cut parts showed an increase with energy per spark. Electron Probe Microanalysis (EPMA) revealed the recast layer to be alloyed with elements from the wire electrode. X-ray diffraction testing showed the residual tensile stresses evident near the cut surface to decrease with energy per spark. Additionally, nano-indentation hardness testing indicated that the recast layer is reduced in hardness and elastic modulus compared to the bulk material. Vibratory tumbling was found to be a moderately effective post-processing tool for recast layer removal when using pre-formed ceramic abrasive media or fine grained aluminum oxide.

Recent studies demonstrate that mechanical deformation of small volume systems can be significantly different from those of the bulk. One such interesting length scale dependent property is the increase in the yield stress with decrease in diameter of micrometer rods, particularly when the diameter is below a micrometer. Intermittent flow may also result when the diameter of the rods is decreased below a certain value. The second such property is the intermittent plastic deformation during nano-indentation experiments. Here again, the instability manifests due to smallness of the sample size, in the form of force fluctuations or displacement bursts. The third such length scale dependent property manifests as ’smaller is stronger’ property in indentation experiments on thin films, commonly called as the indentation size effect (ISE). More specifically, the ISE refers to the increase in the hardness with decreasing indentation depth, particularly below a fraction of a micrometer depth of indentation. The purpose of this thesis is to extend nonlinear dynamical approach to plastic deformation originally introduced by Anantha krishna and coworkers in early 1980’s to nano and micro-indentation process. More specifically, we address three distinct problems : (a) intermittent force/load fluctuations during displacement controlled mode of nano-indentation, (b) displacement bursts during load controlled mode of nano-indentation and (c) devising an alternate framework for the indentation size effect. In this thesis, we demonstrate that our approach predicts not just all the generic features of nano-and micro-indentation and the ISE, the predicted numbers also match with experiments. Nano-indentation experiments are usually carried-out either in a displacement controlled (DC) mode or load controlled (LC) mode. The indenter tip radius typically ranges from few tens of nanometer to few hundreds of nanometers-meters. Therefore, the indented volume is so small that the probability of finding a dislocation is close to zero. This implies that dislocations must be nucleated for further plastic deformation to proceed. This is responsible for triggering intermittent flow as indentation proceeds. While several load drops are seen beyond the elastic limit in the DC controlled experiments, several displacement jumps are seen in the LC experiments. In both cases, the stress corresponding to load maximum on the elastic branch is close to the theoretical yield stress of an ideal crystal, a feature attributed to the absence of dislocations in the indented volume. Hardness is defined as the ratio of the load to the imprint area after unloading and is conventionally measured by unloading the indenter from desired loads to measure the residual plastic imprint area. Then, the hardness so calculated is found to increase with decreasing indentation depth. However, such size dependent effects cannot be explained on the basis of conventional continuum plasticity theories since all mechanical properties are independent of length scales. Early theories suggest that strong strain gradients exist under the indenter that require geometrically necessary dislocations (GNDs) to relax the strain gradients. In an effort to explain the the size effect, these theories introduce a length scale corresponding to the strain gradients. One other feature predicted by subsequent models of the ISE is the linear relation between the square of the hardness and the inverse of the indentation depth. Early investigations on the ISE did recognize that GNDs were required to accommodate strain gradients and that the hardness H is determined by the sum of the statistically stored dislocation (SSD) and GND densities. Following these steps, Nix and Gao derived an expression for the hardness as a function of the indentation depth z. The relevant variables are the SSD and GND densities. An expression for the GND density was obtained by assuming that the GNDs are contained within a hemispherical volume of mean contact radius. The authors derive an expression for the hardness H as a function of indentation depth z given by [ HH 0 ]2 = 1+ zz ∗ . The intercept H0 represents the hardness arising only from SSDs and corresponds to the hardness in the limit of large sample size. The slope z ∗ can be identified as the length scale below which the ISE becomes significant. The authors showed that this linear relation was in excellent agreement with the published results of McElhaney et al for cold rolled polycrystalline copper and single crystals of copper, and single crystals of silver by Ma and Clarke. Subsequent investigations showed that the linear relationship between H2 verses 1/z breaks down at small indentation depths. Much insight into nano-indentation process has come from three distinct types of studies. First, early studies using bubble raft indentation and later studies using colloidal crystals (soft matter equivalent of the crystalline phase) allowed visualization of dislocation nucleation mechanism. Second, more recently, in-situ transmission electron microscope studies of nano-indentation experiments have been useful in understanding the dislocation nucleation mechanism in real materials. Third, considerable theoretical understanding has come largely from various types of simulation studies such as molecular dynamics (MD) simulations, dis¬location dynamics simulations and multiscale modeling simulations (using MD together with dislocation dynamics simulations). A major advantage of simulation methods is their ability to include a range of dislocation mechanisms participating in the evolution of dislocation microstructure starting from the nucleation of a dislocation, its multiplication, formation of locks, junctions etc. However, this advantage is offset by the serious limitations set by short time scales inherent to the above mentioned simulations and the limited size of simulated volumes that can be implemented. Thus, simulation approaches cannot impose experimental parameters such as the indentation rates or radius of the indenter and thickness of the sample, for example in MD simulations. Indeed, the imposed deformation rates are often several orders of magnitude higher than the experimental rates. Consequently, the predicted values of force, indentation depth etc., differ considerably from those reported by experiments. For these reasons, the relevance of these simulations to real materials has been questioned. While several simulations, particularly MD simulation predict several force drops, there are no simulations that predict displacement jumps seen in LC mode experiments. The inability of simulation methods to adopt experimental parameters and the mismatch of the predicted numbers with experiments is main motivation for devising an alternate framework to simulations that can adopt experimental parameters and predict numbers that are comparable to experiments. The basic premise of our approach is that describing time evolution of the relevant variables should be adequate to capture most generic features of nano and micro-indentation phenomenon. In the particular case under study, this point of view is based on the following observation. While one knows that dislocations are the basic defects responsible for plastic deformation occurring inside the sample, the load-indentation depth curve does not include any information about the spatial location of dislocation activity inside the sample. In fact, the measured load and displacement are sample averaged response of the dislocation activity in the sample. This suggests that it should be adequate to use sample averaged dislocation densities to obtain load-indentation depth curve. Keeping this in mind, we devise a method for calculating the contribution from plastic deformation arising from dislocation activity in the entire sample. This is done by setting up rate equations for the relevant sample averaged dislocation densities. The first problem we consider is the force/load fluctuations in displacement controlled nano-indentation. We devise a novel approach that combines the power of nonlinear dynamics with the evolution equations for the mobile and forest dislocation densities. Since the force serrations result from plastic deformation occurring inside the sample, we devise a method for calculating this contribution by setting-up a system of coupled nonlinear time evolution equations for the mobile and forest dislocation densities. The approach follows closely the steps used in the Anantha krishna (AK) model for the Portevin-Le Chatelier (PLC) effect. The model includes nucleation, multiplication and propagation of dislocation loops in the time evolution equation for the mobile dislocation density. We also include other well known dislocation transformation mechanisms to forest dislocation. Several of these dislocation mechanisms are drawn from the AK model for the PLC effect. To illustrate the ability of the model to predict force fluctuations that match experiments, we use the work of Kiely at that employs a spherical indenter. The ability of the approach is illustrated by adopting experimental parameters such as the indentation rate, the radius the indenter etc. The model predicts all the generic features of nano-indentation such as the Hertzian elastic branch followed by several force drops of decreasing magnitudes, and residual plas¬ticity after unloading. The stress corresponding to the elastic force maximum is close to the yield stress of an ideal solid. The predicted values for all the quantities are close to those reported by experiments. Our model allows us to address the indentation-size effect including the ambiguity in defining the hardness in the force drop dominated regime. At large indentation depths where the load drops disappear, the hardness shows decreasing trend, though marginal. The second problem we consider is the load controlled mode of indentation where sev¬eral displacement jumps of decreasing magnitudes are seen. Even though, the LC mode is routinely used in nano-indentation experiments, there are no models or simulations that predict the generic features of force-displacement curves, in particular, the existence of sev¬eral displacement jumps of decreasing magnitudes. The basic reason for this is the inability of these methods to impose constant load rate during displacement jumps. We then show that an extension of the model for the DC mode predicts all the generic features when the model is appropriately coupled to an equation defining the load rate. Following the model for DC mode, we retain the system of coupled nonlinear time evolution equations for mobile and forest dislocation densities that includes nucleation, multiplication, and propagation threshold mechanisms for mobile dislocations, and other dislocation transformation mechanisms. The commonly used Berkovich indenter is considered. The equations are then coupled to the force rate equation. We demonstrate that the model predicts all the generic features of the LC mode nano-indentation such as the existence of an initial elastic branch followed by several displacement jumps of decreasing magnitudes, and residual plasticity after unloading for a range of model parameter values. In this range, the predicted values of the load, displacement jumps etc., are similar to those found in experiments. Further, optimized set of parameter values can be easily determined that provide a good fit to the load-indentation depth curve of Gouldstone et al for single crystals of Aluminum. The stress corresponding to the maximum force on the Berkovich elastic branch is close to the theoretical yield stress. We also elucidate the ambiguity in defining hardness at nanometer scales where the displacement jumps dominate. The approach also provides insights into several open questions. The third problem we consider is the indentation size effect. The conventional definition of hardness is that it is the ratio of the load to the residual imprint area. The latter is determined by the residual plastic indentation depth through area-depth relation. Yet, the residual plastic indentation depth that is a measure of dislocation mobility, never enters into most hardness models. Rather, the conventional hardness models are based on the Taylor relation for the flow stress that characterizes the resistance to dislocation motion. This is a complimentary property to mobility. Our idea is to provide an alternate way of explaining the indentation size effect by devising a framework that directly calculates the residual plastic indentation depth by integrating the Orowan expression for the plastic strain rate. Following our general approach to plasticity problems, we set-up a system of coupled nonlinear time evolution equations for the mobile, forest (or the SSD) and GND densities. The model includes dislocation multiplication and other well known dislocation transformation mechanisms among the three types of dislocations. The main contributing factor for the evolution of the GND density is determined by the mean strain gradient and the number of sites in the contact area that can activate dislocation loops of a certain size. The equations are then coupled to the load rate equation. The ability of the approach is illustrated by adopting experimental parameters such as the indentation rates, the geometrical quantities defining the Berkovich indenter including the nominal tip radius and other parameters. The hardness is obtained by calculating the residual plastic indentation depth after unloading by integrating the Orowan expression for the plastic strain rate. We demonstrate that the model predicts all features of the indentation size effect, namely, the increase in the hardness with decreasing indentation depth and the linear relation between the square of the hardness and inverse of the indentation depth, for all but 200nm, for a range of parameter values. The model also predicts deviation from the linear relation of H2 as a function of 1/z for smaller depths consistent with experiments. We also show that it is straightforward to obtain optimized parameter values that give a good fit to polycrystalline cold-worked copper and single crystals of silver. Our approach provides an alternate way of understanding the hardness and indentation size effect on the basis of the Orowan equation for plastic flow. This approach must be contrasted with most models of hardness that use the SSD and GND densities as parameters. The thesis is organized as follows. The first Chapter is devoted to background material that covers physical aspects of different kinds of plastic deformation relevant for the thesis. These include the conventional yield phenomenon and the intermittent plastic deformation in bulk materials in alloys exhibiting the Portevin-Le Chatelier (PLC) effect. We then provide background material on nano-and micro-indentation, both experimental aspects and the current status of the DC controlled and LC controlled modes of nano-indentation. Results of simulation methods are briefly summarized. The chapter also provides a survey of hardness models and the indentation size effect. A critical survey of experiments on dislocation microsructure that contradict / support certain predictions of the NixGao model. The current status of numerical simulations are also given. The second Chapter is devoted to introducing the basic steps in modeling plastic deformation using nonlinear dynamical approach. In particular, we describe how the time evolution equations are constructed based on known dislocation mechanisms such as nucleation, multiplication formations of junctions etc. We then consider a model for the continuous yield phenomenon that involves only the mobile and forest densities coupled to constant strain rate condition. This problem is considered in some detail to illustrate how the approach can be used for modeling nano-indentation and indentation size effect. The third Chapter deals with a model for displacement controlled nano-indentation. The fourth Chapter is devoted to adopting these equation to the load controlled mode of nano¬indentation. The fifth Chapter is devoted to modeling the indentation size effect based on calculating residual plastic indentation depth after unloading by using the Orowan’s expression for the plastic strain rate. We conclude the thesis with a Summary, Discussion and Conclusions.

Properties of samples and their coatings may be affected by the electron beam. This paper deals with the analysis of the microstructure, phase and chemical composition and the determination of mechanical characteristics of inconel steel substrate and CoNiCrAlY coatings deposited via different types of thermal spraying (HVOF, cold spray), in combination with modifications by the electron beam technology. During the study it was found that the deposition did not change the chemical composition. Further it was found that the interaction of the electron beam with the material did not change the chemical composition, but there is a change in the structure and a reduction of porosity and surface roughness, resulting in a change of mechanical properties such as decreasing hardness or increase of the modulus of elasticity.

Fundamental understanding of high strain rate deformation behavior of materials is critical in designing new alloys for wide-ranging applications including military, automobile, spacecraft, and industrial applications. High entropy alloys, consisting of multiple elements in (near) equimolar proportions, represent a new paradigm in structural alloy design providing ample opportunity for achieving excellent performance in high strain rate applications by proper selection of constituent elements and/or thermomechanical processing. This dissertation is focused on fundamental understanding of high strain-rate deformation behavior of several high entropy alloy systems with widely varying microstructures. Ballistic impact testing of face centered cubic Al0.1CoCrFeNi high entropy alloy showed failure by ductile hole growth. The deformed microstructure showed extensive micro-banding and micro-twinning at low velocities while adiabatic shear bands and dynamic recrystallization were seen at higher velocities. The Al0.7CoCrFeNi and AlCoCrFeNi2.1 eutectic high entropy alloys, with BCC and FCC phases in lamellar morphology, showed failure by discing. A network of cracks coupled with small and inhomogeneous plastic deformation led to the brittle mode of failure in these eutectic alloys. Phase-specific mechanical behavior using small-scale techniques revealed higher strength and strain rate sensitivity for the B2 phase compared to the L12 phase. The interphase boundary demonstrated good stability without any cracks at high compressive strain rates. The Al0.3CoCrFeNi high entropy alloy with bimodal microstructure demonstrated an excellent combination of strength and ductility. Ballistic impact testing of Al0.3CoCrFeNi alloy showed failure by ductile hole growth and demonstrated superior performance compared to all the other high entropy alloy systems studied. The failure mechanism was dominated by micro-banding, micro-twining, and adiabatic shear localization. Comparison of all the high entropy alloy systems with currently used state-of-the-art rolled homogenous armor (RHA) steel showed a strong dependence of failure modes on microstructural features.

La technologie de projection plasma sous basse pression a attiré l’attention de nombreux chercheurs comme une nouvelle technique qui permet d’établir un pont entre la projection thermique conventionnelle et le dépôt physique en phase vapeur. Ainsi, cette technologie étend les limites de projection thermique classique et augmente également la vitesse de croissance des dépôts par rapport aux procédés PVD ou CVD classiques. Cette technique peut évaporer totalement ou partiellement les poudres injectées et mener à la réalisation de revêtements à microstructure colonnaire et/ou plus denses, difficiles à réaliser avec des procédés de projection thermique conventionnels. La projection plasma de suspension a été effectuée pour la première fois sous basse pression. L’injection axiale de suspension avec une torche tri-cathodes permet d’augmenter l’échange enthalpique entre le jet de plasma et les poudres après l’évaporation du solvant. La spectroscopie à l’émission optique (OES) a été utilisée pour estimer la température électronique et vérifier l’existence de phase vapeur d’YSZ dans le jet de plasma. Finalement, des revêtements plus denses furent réalisés (comparés à ceux préparés par projection plasma de suspension à pression atmosphérique présentant des particules fondues, agglomérées et de la condensation de vapeur. Des tests de nano-indentation instrumentée ont été effectués sur la surface polie des dépôts réalisés. Les résultats montrent des valeurs de 5,8 GPa pour la dureté et 114,5 GPa pour le module d’élasticité, augmentant de 61% et 31%, respectivement, en comparaison avec les valeurs obtenues par SPS sous atmosphère ambiante. Les essais de projection de poudre YSZ agglomérée ont été réalisés avec une torche F4-VB dans le but de synthétiser une phase vapeur d’YSZ. On observe que les dépôts peuvent se former derrière les échantillons en céramique, sans vis-à-vis du plasma, par condensation de vapeur. En face de cette torche, des revêtements composites ont été obtenus par un mélange de poudres fondues et condensation de vapeur, simultanément. Cependant la quantité de phase vapeur est très faible dans le jet de plasma. Pour comprimer ce jet sous basse pression et afin d’améliorer l’échange d’enthalpie entre le jet de plasma et les poudres injectées, une buse rallongée a été mise en place sur la torche F4-VB. Les revêtements présentent ainsi une microstructure plus dense. Ceci est attribué à la haute vitesse des particules fondues vers le substrat suite à l’utilisation de la buse modifiée. Ce type de revêtement montre une valeur maximale de microdureté Vickers de 1273 Hv100 g. Par ailleurs, la réalisation de dépôts de carbures a été effectuée. Les résultats montrent la possibilité de former des carbures par projection plasma sous basse pression. Les revêtements composites (TiC/Ti) sont déposés par projection plasma réactif sous basse pression en utilisant le méthane comme gaz porteur. La température électronique Te calculée est d’environ 6200 K selon les résultats d’OES, ce qui est supérieur à la température d’ébullition du Ti et de TiC. Le revêtement de Ti pur présente une microstructure dense alors que TiC/Ti présente une microstructure lamellaire. Cependant, la quantité de TiC dans les revêtements est d’environ 20 vol.%. La microdureté Vickers, effectuée sur surface polie, a tendance à diminuer de 846±152 à 773±86 Hv100 g avec l’augmentation de la distance de projection. / As a new technology, the very low pressure plasma spraying has attracted attentions of many researchers, making it possible to establish a bridge between the conventional atmospheric plasma spraying (APS) and the vapor deposition (PVD or CVD). As a result, this new technology enlarges the limitation of APS and increases the deposition rate in comparison with the PVD or CVD. It is possible to evaporate partially the injected material and even evaporate completely and finally realize the columnar or dense coatings from the vapor or the mixture of vapor and liquid. The suspension plasma spraying is performed for the first time at low pressure. Taking consideration of the configuration of the three-cathode torch, the axial injection of the suspension is conducted which can increase the enthalpy change between the plasma jet and the sprayed material. The data of optical emission spectroscopy (OES) could be used to calculate the electron temperature and verify the existence of vapor of YSZ in the plasma jet. Finally, the dense coating was prepared by suspension plasma spraying at low pressure, which is composed of the melted particles, the agglomerated particles and the vapor deposition. The test of nano-indentation is conducted on the polished surface. It shows a value of 5.8 GPa for the microhardness and 114.5 GPa for the elastic modulus, increasing 61% and 31%, respectively, compared with the values obtained by SPS in the ambient atmosphere. In this study, another torch F4-VB is also conducted even of it has a low power in compared with that of O3CP torch. The powder feed rate is reduced to about 1.5 g·min-1 to achieve the vapor of YSZ taking the low power input of the torch into consideration. The columnar structure coating is realized from vapor deposition out of line of sight of projection upon the ceramic tubes. The composite structure coating is deposited by the mixture of melted particles and the vapor deposition simultaneously in front of this torch. But the quantity of vapor of YSZ is low in plasma jet. In order to compress the larger plasma jet and then improve the enthalpy change between the plasma jet and sprayed particles, an extended nozzle is prepared. It shows that the coating has a dense structure, which can be attributed to higher velocity of the melted powders. The coating shows a maximum value of microhardness Vickers up to 1273 Hv100 g. The composite coating of TiC/ Ti is realized by reactive plasma spraying using the methane as the carrier gas. The electron temperature Te is calculated to be 6200 K, which is over the boiling point of TiC and Ti. The coating Ti shows a dense structure and the composite coating TiC/Ti shows a lamellar structure. But the quantity of TiC in the composite coating is very low, about 20 vol.%. The Vickers microhardness is performed on the polished surface. It shows a decreasing tendency from 846 ± 152 to 773 ± 86 Hv100g with the increase of spraying distance. The tribological test is also implemented showing a high value of the coefficient of friction of 0.78 to 0.85, which can lead to a high abrasion. In order to synthesize a larger quantity of TiC in the composite coating, a higher power input torch should be put into action in the future.

The different topics covered in this thesis include photosensitivity in germanosilicate fibers/glasses and application of fiber Bragg grating sensors in simultaneous strain and temperature discrimination. Fiber Bragg Gratings are wavelength dispersive refractive index structures manufactured through ultra-violet (UV) exposure of optical fibers. Their applications range from wavelength division multiplexing filters, dispersion compensators and fiber laser resonators for telecommunication applications to different types of point or distributive sensors for a variety of applications. One aim of this thesis has been to understand the mechanism of photosensitivity in germanosilicate fibers/preforms. Studies undertaken in this part of the thesis include thermal dynamics of Fiber Bragg Gratings and nano-indentation on ultra-violet irradiated germanosilicate glass preforms. An interesting, periodic appearance of a new peak has been observed in the reflected spectrum of Bragg grating inscribed in a germanosilicate fiber during thermal treatment. The new peak occurs on the longer wavelength side of the spectrum during heating and on the shorter wavelength side during cooling, following an identical reverse dynamics. A commercial grating with 99.9% reflectivity also shows a similar decay dynamics. The observed temperature induced distortion in refractive index modulation profile has been understood in the light of compaction-densification model. It is proposed that during the fabrication process of a grating, the modulation in the thermal expansion coefficient brought about by the interference fringes results in a non-uniform expansion throughout the grating length which in turn results in the distortion of the refractive index profile with increase/decrease in temperature. Since the reflection spectrum of a grating can be approximated as the Fourier transform of the refractive index profile, any distortion in the index profile results in the observed anomalous behaviour in the reflection spectrum. Nano-indentation studies have been performed to measure the changes in mechanical properties of a glass preform subjected to different levels of ultra-violet exposure. The results reveal that short term exposure leads to an appreciable increase in the Young’s modulus suggesting the densification of the glass, confirming the compaction-densification model. However, on prolonged exposure, the Young’s modulus decreases, which provides the first direct evidence of dilation in the glass leading into the Type IIA regime. The present results rule out the hypothesis that continued exposure leads to an irreversible compaction and prove that index modulation regimes are intrinsic to the glass matrix. In the second part of the thesis, three different schemes have been proposed for the use of Fiber Bragg Gratings as strain-temperature discriminating sensors: (a) The first method is based on the measurement of the different characteristic wavelength shifts of two types of gratings. Strain and temperature sensitivities of a Type I Bragg grating (G1) in germania doped silica fiber, fabricated under normal conditions, and zero strain, are compared with that of a Bragg grating inscribed under pre-strained condition (G2). Experimental results show that both, strain and temperature sensitivities of G1 and that of G2 are different. Based on this study, we have proposed an approach which enables simultaneous discrimination of axial strain and temperature. (b) In the second method, a single sensing element has been used to encode strain and temperature into an additional parameter other than the wavelength shift. The thermal out-diffusion of germanium from the core of a photosensitive fiber under elevated temperature is exploited to form a Fabry-Perot filter with a single Fiber Bragg Grating. The filter is fabricated using the standard phase-mask technique and one-time exposure. Energy Dispersive X-Ray analysis is used to measure the out-diffusion. The filter is used as a sensor for simultaneous measurement and discrimination of strain and temperature. The proposed technique, where a single grating is used to discriminate the parameters, provides a large advantage over other existing methods. (c) In the third method, a compact design based on cross-wire arrangement of Fiber Bragg Gratings having identical Bragg resonance and different reflectivity is proposed for simultaneously sensing strain (uniaxial) and temperature. Two gratings are assembled orthogonal to each other on an aluminium base. The cross-wire design allows the two sensors to experience the same temperature but different strain. The gratings are identified by their respective reflectivity and, strain and temperature are resolved from the shift in Bragg wavelength. The proposed design exploits the fact that strain is a vector and temperature is a scalar parameter. This sensor has wide industrial application in discriminating strain from temperature effects.

Afin d’améliorer la durée de vie des micro-relais MEMS ohmiques, plusieurs traitements de surface de films minces d’or sont réalisés pour augmenter leur dureté tout en conservant une résistance électrique de contact faible.Les revêtements ultrafins de ruthénium (20 à 100 nm) déposés sur l’or augmentent la dureté des surfaces de contact d’un facteur 15. L’implantation ionique de bore ou d’azote (3,5 ppm à 10 % atomique) à une profondeur de 100 nm dans le film mince d’or permet d’atteindre un gain en dureté de 75%. Le contrôle (AFM, EBSD et DRX) de la microstructure induite met en évidence le durcissement par solution solide par insertion. Mais au-delà de 1% atomique, les atomes d’azote quittent le réseau cristallin de l’or pour former des précipités de nitrure d’or.L’analyse AFM (rugosité et diamètre) des empreintes résiduelles (quelques μm²) réalisées par nano-indentation sphérique, imitant le cyclage et le fluage des surfaces de contact de ces MEMS, démontre l’apport de ces traitements de surface. De plus, leurs résistances électriques de contact, mesurées par nano-indentation instrumentée reproduisant un micro-contact identique à un dispositif réel, sont similaires à celle de l’or pur.La modélisation discrète mécanique du contact rugueux est ajustée à la mesure de la déformation mécanique de nano-rugosités en comparant les relevés topographiques réalisés par AFM avant et après nano-indentation sphérique. La comparaison entre la modélisation et la mesure de la résistance électrique de contact indique que pour les gammes de force utilisées dans les micro-relais MEMS (inférieure au mN), seule une fraction allant de 2% à 9% de la surface de contact réelle est conductrice. / Ohmic MEMS switches made by gold thin films are promising devices but their mechanical contacts are one of the critical concerns for enhancing reliability. For this reason, surface processes are investigated in this work to improve both mechanical and electrical contact resistance (ECR) of MEMS gold contacts. Ruthenium ultra-thin films (20 to 100 nm) deposited on a top of gold layer increase surface hardness by a factor of fifteen. In parallel, surface implantations of both boron (<10% atomic) or nitrogen (<0.1% atomic) into gold reveals a solid solution hardening by insertion, thus increasing the hardness of initial film by about 75% and 25%, respectively. Notably, above 0.1% atomic of nitrogen, atoms precipitate from the tetra or octahedral sites of gold inducing a decrease of hardness.Static and multi load/unload spherical nano-indentation are performed on treated gold thin films to simulate the mechanical actuation of ohmic MEMS switches. Analysis of residual imprints (about few µm²) from treated surface exhibits both minimal local deformation and adhesion forces that reduce stiction probability. In-situ measurement of ECR for treated gold by instrumented nano-indentation, reproducing the design of MEMS, is in the same range of pure gold-to-gold configuration.A new mechanical discrete model of rough contact is introduced, confronted and validated to the experimental mechanical surface deformation obtained by comparison of AFM images before and after spherical nano-indentation. An electrical discrete model is added and fitted to the ECR measurements. In ohmic MEMS switch load range (< 1 mN), the conductive area is found to be about 2% to 9% of the real contact area.

Les aciers inoxydables austénitiques métastables sont le siège de différents mécanismes de déformation qui sont à l'origine des propriétés mécaniques qui distinguent ce type d’alliages. Cependant, ces dernières, dépendant de la microstructure locale, sont fortement anisotropes. Par ailleurs, la déformation d'un échantillon massif serait différente de celle obtenue en surface. De ce fait, une étude détaillée trouve tout son intérêt. Le présent travail vise donc à identifier les principaux mécanismes de déformation et de leur évolution progressive, en se basant sur une déformation contrôlée de grains austénitiques individuels par des tests mécaniques de nanoindentation monotoniques et cycliques. Les courbes correspondantes au chargement-déchargement révèlent des informations détaillées sur les propriétés mécaniques sous-jacentes qui pourraient être liées à une étude complète de la structure de déformation en surface et en volume par différentes techniques de caractérisation à une échelle très fine. La déformation en fonction du temps, les phénomènes de transformation de phase réversible sous charge, l'anisotropie cristalline, l'influences de la taille des grains, la transmission de la plasticité et la tenue en fatigue ont été mis en évidence et étudiés / Metastable austenitic stainless steels feature an abundance of different deformation mechanisms, which contribute to the distinguished mechanical properties of these alloys. However, these properties are known to depend on the local microstructure and also are highly anisotropic. Furthermore, deformation is expected to be different for the bulk and the surface of a sample. In this sense, a discrete study is not trivial. The present work aims at investigation of the main deformation mechanisms and their gradual evolution, by employing controlled deformation of individual austenite grains via monotonic and cyclic nanoindentation. The corresponding loading–unloading curves have given extensive information about underlying mechanical properties, which could be related to an exhaustive reconstruction of the deformation substructure, both in surface and bulk, by different small scale characterization techniques. Amongst others, features such as time-dependent deformation, reversible phase transformation under load, crystalline anisotropy and grain size influences, besides plasticity transmission and fatigue behavior have been found and analyzed

L’engouement mondial pour les appareils nomades et la course à la sobriété énergétique font de la diminution de la taille des systèmes microélectroniques (MEMS) un enjeu majeur pour les prochaines années. Les micro batteries au lithium sont aujourd'hui le moyen le plus efficace pour stocker et alimenter des dispositifs avec une très forte densité énergétique. Les incorporer dans des cartes de crédit comportant un écran et des touches intégrés est l’un des défis que relèvent les multinationales comme ST Micro Electronics. Ces micro batteries contiennent cependant du lithium métallique qui peut s'avérer très dangereux quand il est en contact avec de l’eau ou de l’air humide. Ainsi, afin de protéger les composants à une exposition à l’humidité, une encapsulation de l’ensemble de la batterie est nécessaire. L'encapsulation polymère a l’avantage, comparativement à d’autres matériaux, de présenter un faible coût de mise en forme et un faible poids. Cependant, de tels systèmes d'encapsulation sont aujourd'hui insuffisants pour garantir une durée de vie de plusieurs années des composants car en présence d’humidité ou d’une variation de température importante la tenue mécanique des assemblages peut être fragilisée. L'objectif de la thèse est donc de réaliser et d'étudier le comportement mécanique et structural d’assemblage de couches minces de polymères et de métaux en température et en humidité. Deux types de polymères ont été choisis pour ce projet :1. Le chlorure de polyvinylidène (PVDC), un polymère commercial très utilisé pour ses bonnes propriétés barrières à l'eau 2. Un oligomère acrylate reticulable par voie thermique et UV synthétisé au sein de l'IMMM. / The increasing number of mobile devices and the race to energy sobriety make the decrease of the size of microelectronic systems (MEMS) a major challenge. Today, Lithium micro batteries are currently the best solution for high-power-and-energy applications. Incorporate them into credit cards containing a screen or associate them to electronic sensors for the supervision is the challenge which raises international companies such as ST Microelectronics. However, these micro batteries contain some lithium metal which can be dangerous if the metallic lithium is in contact with water or humid air. In addition, the substance can spontaneously ignite in the contact of the humidity. So, in order to avoid the problems of safety, we absolutely have to protect the lithium contained in our micro batteries using an encapsulation layer. Polymeric encapsulation has the advantage, compared with other materials (ceramic, metal), to present a moderate cost of shaping and a low weight. However, such systems of encapsulation are today insufficient to guarantee a satisfactory life cycle of components. Indeed, in the presence of humidity or of a too important temperature variation, the mechanical assemblies can be weakened and engender an irreparable break. The objective of the thesis is therefore to realize and study the mechanical and structural behavior of assembly of thin layers of polymers and metals in temperature and humidity.Two types of polymers were selected for this project:1. Polyvinylidene chloride (PVDC), a commercial polymer widely used for its good barrier properties to water.2. A thermally and UV-crosslinkable acrylate oligomer synthesized in the IMMM.