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No description available.

No description available.

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The mechanical properties of materials are usually evaluated by performing a tensile or hardness test on the sample. Tensile tests are often time consuming, destructive and need specially prepared specimens. On the other hand, there is no direct theoretical correlation between the hardness number and the mechanical properties of a material although phenomenological relationships do exist. The advantages of indentation techniques are that they are non-destructive, quick, and can be applied to small material samples and localized in fashion. Mechanical properties are typically determined from spherical indentation load-depth curves. This process is again a time consuming one and not suitable for situations where a quick assessment is required such as in the sheet metal rolling industry. In the present study, a novel method of measuring mechanical properties of the material by multiple spherical indentations is developed. A series of indentations are made on the substrate with a spherical indenter with different loads. The diameter of the indentation is related to the load applied to determine the mechanical properties of the material, namely the yield strength and the work hardening parameters. To determine the diameter of the indentation quickly, a fiber optic sensing technique is developed. An incident light beam from a semiconductor laser is coupled back into an optical fiber upon reflection from the metal surface. By measuring the diffused light power reflected from the metal surface, the diameter of the indentation is measured. The spherical indentation technique is difficult for real time mechanical property measurement of sheet metal in a processing line. Problems arise as the strip is traveling at 2,000 to 4,000 ft/min (10,000 to 20,000 mm/sec) in the processing line. As a first step in developing a process that could be implemented in a real time processing line, a preliminary study has been conducted for the prediction of yield strength by laser shock processing.

Les barrières thermiques avec zircone EB-PVD pour les turbines aéronautiques sont soumises à des conditions extrêmes qui conduisent à l'écaillage du dépôt. La prévention de leur endommagement est donc nécessaire pour assurer l'intégrité des pièces. Afin de comprendre et de reproduire leur évolution dans une turbine, les barrières thermiques actuelles sont soumises à des essais longs de cyclage thermique. L'essai LASAT est un essai d'adhérence rapide à mettre en oeuvre qui se place en complément du cyclage thermique. L'impulsion laser appliquée sur la face nue de l'AM1 génère une onde de choc de compression qui se propage jusqu'à la surface libre de la zircone. La réflexion forme une onde de traction qui effectue le trajet inverse et peut rompre les interfaces qu'elle traverse. Le décohésion génère une tache blanche dans la zircone directement visible à l'oeil. Ce phénomène optique est élucidé en relation avec la microstructure de la zircone et la présence d'une fissure à l'interface. Pour connaitre le potentiel de l'essai, une large gamme d'échantillons avec différentes orientations du superalliage, quatre préparations de sous-couche, cinq microstructures de zircone et deux vieillissements thermiques ont été utilisés.Leur caractérisation a permis de les classer et de comparer leurs évolutions et leurs endommagements par cyclage thermique ou par LASAT. Le dimensionnement des fissures interfaciales par des méthodes non destructives a été réalisé par piézospectroscopie en exploitant les cartographies associées au signal defluorescence, par profilométrie et à partir de la tache blanche. Une approche simple et innovante exploitant et optimisant le comportement optique de la zircone est mise en place. Les tailles des fissures relevées ont mis en évidence le rôle des ondes 2D et permis la réalisation de l'essai LASAT-2D. Ici, ce n'est plus l'apparition de la fissure qui est recherchée, mais sa taille qui peut directement informer de l'adhérence à partir d'un seul choc laser. La modélisation numérique a confirmé le rôle de ces ondes 2D et leur potentield'utilisation par des abaques LASAT-2D. Ces courbes permettent de distinguer différentes préparations de barrières thermiques brutes d'élaboration ou vieillies. Un protocole complet est ainsi fourni pour le contrôle, la mesure et le suivi de la tenue mécanique de barrières thermiques sur des éprouvettes usuelles industrielles. Dans des essais complémentaires, le LASAT-2D a été appliqué en "face avant", avec le choc coté zircone, sur des éprouvettes et des pièces industrielles. Les mêmes tendances que pour le LASAT-2D développé dans cette thèse sont observées. Ceci autorise la perspective de l'application de cet essai et de cette méthodologie sur des formes complexes et fermées, telles les aubes de turbine. / EB-PVD thermal barrier coatings used in aircraft turbines are subjected to extreme conditions that lead to their spallation. To ensure the integrity of the parts, it's necessary to prevent coating damages. In order to understand and reproduce their evolution in a turbine, current thermal barrier coatings are subjected to long thermal cycling. The LASAT is a rapid adhesion test that could complement the thermal cycling. The laser pulse applied to the AM1 face generates a compressive shock wave which propagates towards the free surface of the zirconia. The reflection of this wave generates a tensile shock wave which can damage the interface and create a white spot in zirconia top coat. To determine the potential of the test, a wide range of samples with different superalloy orientations, bondcoat preparations, zirconia microstructures and thermal aging were used. Characterizations were carrying out to classify and compare their evolution and their damage by thermal cycling or LASAT. The size of interfacial cracks by non-destructive tests was achieved by piezospectroscopie maps associated with the fluorescent temporal signal, by profilometry and from the white spot. A simple and innovative approach by optimizing the optical behavior of zirconia is developed. These results highlight the role of 2D waves and allow the realization of the testLASAT-2D. Here, it is not the appearance of the crack that is looking for, but its size, which can directly inform about the mechanical adhesion from a single laser shock. Numerical modeling has confirmed the influence of 2D shock waves and their potential using LASAT-2D charts. These curves are used to distinguishdifferent preparations of thermal barrier coatings as produced and after thermal aging. A complete protocol is thus provided for monitoring, measuring and determine the interface strength of thermal barrier coatingson industrial specimens. On further testing, LASAT-2D was applied directly on the coating of specimens and industrial parts. The same results as for the 2D-LASAT developed in this thesis are observed. This allows the prospect of the application of this test and the methodology on complex and closed shapes, suchas turbine blades.

<p>Laser shock imprinting (LSI) is a novel fabrication technique capable of manufacturing various membrane materials. This top-down imprinting process can fabricate membranes in high precision, high throughput, and large scalability. It reveals a variety of applications ranging from electronics to photonics, which is beneficial from its reliable and precise modulation of micro/nanostructures. </p> <p> In this thesis, we firstly proposed and developed a cost-effective LSI process to manufacture hierarchical micro/nanostructured power generators. By combining the conventional soft lithography technique, LSI is well compatible with it to fabricate metal membranes towards soft templates. It is a significant progress from the originally-developed silicon wafer template layout because it effectively reduces the process cost by replacing sophisticatedly developed silicon wafers with low-cost photocurable polymers. In addition, the use of polymer expands the boundary limit of geometrical complexity from simple patterns to hierarchical structures, as a result, we successfully conducted LSI technology to fabricate biomimic leaf structures into metallic membranes with the help of soft SU-8 templates. These fabricated metallic membraned are used as water-driven triboelectric nanogenerators. In addition to the introduction of polymer template, we further developed a successive laser shock imprinting (SLSI) process to fabricate hierarchical nanostructures in a higher resolution. Typically, grating templates are collected via recycling blank discs and used as soft templates. Then multiple times of LSI process are conducted to manufacture membranes into complex nanostructures. The use of blank disc further reduces cost and increase process resolution. The highlight of this part of work is to feature the introduction of metallic thin films on disc template, which plays a significant role during this high strain rate imprinting process. Then, the imprinting mechanism was investigated through the finite element method to validate the experimental findings. Lastly, this soft template LSI process was applied to fabricate low dimensional materials such as nanowires (1D) and nanomembranes (2D), potentially introducing homogeneous and inhomogeneous strain field. Kelvin probe force microscopy was used to directly probe strain-induced changes. This soft-template LSI process reveals a new route of precisely fabricating low dimensional membranes into nanoelectronics systems. </p>

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<p>Since the successful exfoliation of graphene, two-dimensional (2D) materials have attracted a lot of scientific interest due to their electronic, chemical, and mechanical properties. Due their reduced dimensionality, these 2D materials exhibit superior mechanical and optoelectronic properties when compared to their bulk counterparts. Within the family of 2D materials, the ultrathin transition metal dichalcogenides (TMDs) such as Tungsten diselenide and Molybdenum disulphide have gained significant attention due to their chemical versatility and tunability. Furthermore, it is possible to leverage the distinct characteristic properties of these 2D materials, which are held together by van der Waals forces, by stacking different 2D layers on top of each other resulting in van der Waals (vdW) heterostructures. Due to the absence of feasible methods to effectively deform the crystal structures of these 2D materials and vdW heterostructures, their mechanical properties have not been thoroughly understood. The atomistic simulations can effectively capture the material behavior at the nanoscale level and help us not only not only understand the mechanical properties of these materials but also aid in the development of tailored processes to tune the material properties for the design of novel metamaterials. Using atomistic simulations, we develop the process - property relationships which can guide the direction of experimentation efforts, thereby making the process of discovering and designing new metamaterials efficient. </p><p>In this work, we have used laser shock nanostraining technique which is a scalable approach to modulate the optomechanical properties of 2D materials and vdW materials for practical semiconductor industry applications. The deformation mechanisms of 2D materials such as graphene, boron nitride (BN) and TMDs such as WSe₂ and MoS₂ are examined by employing a laser shocking process. We report studies on crystal structure deformation of multilayered WSe₂ and monolayer graphene at ultra-high strain rate using laser shock . The laser shocking process generates high pressure at GPa level, causing asymmetric 3D straining in graphene and a novel kinked-like locking structure in multilayered WSe₂. The deformation processes and related mechanical behaviors in laser shocked 2D materials are examined using atomistic simulations. Moiré heterostructures can be obtained by introducing a twist angle between these 2D layers, which can result into vdW materials with different properties, thereby adding an additional degree of freedom in the process-property design approach. We were able to successfully create a tunable stain profile in 2D materials and vdW heterostructures to modulate the local properties such as friction, and bandgap by controlling the level of laser shock, twist angle between the 2D layers and by applying appropriate laser shock pressure . We thus extend this knowledge to further explore the pathways of strain modulation using a combination of laser shocking process, moiré engineering, and strain engineering in 2D materials consisting of graphene, BN, and MoS₂ and to develop the process - property relationships in vdW materials. </p><p>In summary, this research presents a systematic understanding of the effect of laser shocking process on the van der Waals materials and demonstrates the modulation of mechanical and opto-electronic property using laser nanostraining approach. This understanding provides us with opportunities for deterministic design of 2D materials with controllable properties for semiconductor and nanoelectronics applications.</p>