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Effect of plasticity on thin film buckling
This research activity has a multi-scale approaches of the buckling of thin films on substrates. It aims to study the influence of plasticity mechanisms on the delamination and buckling of thin crystalline films deposited on substrates. In order to carry out this activity, atomic-scale simulations by molecular dynamics as well as mesoscopic-scale finite element simulations are carried out. These numerical simulations are supplemented, when possible, by an analytical treatment of buckling within the framework of the elastic theory of thin plates in collaboration with G. Parry (SIMAP, Grenoble).
Recently, free air optical observations of ductile gold films deposited on silicon substrates have shown different buckling structures. In order of increasing size, circular blisters, crescent-shaped blisters and finally donut-shaped blisters have been experimentally characterized. In this context, weused finite element simulations to set out the coupled effects of overpressure and film plasticity on the morphology of flamed structures. In agreement with the observations, we assumed that a plastic fold develops on the perimeter of the circular blisters. The application of an overpressure of the order of atmospheric pressure on the surface of the film therefore led us to the generation of the different structures observed experimentally at different sizes. These simulations allowed us to conclude that the plastic folding / pressure coupling could explain the varied morphologies observed at the different sizes of delaminated zones on the surface of gold films. Still using finite elements, a dynamic study of the propagation of straight wrinkles in the presence of an interface step between a film and its substrate was then carried out using a code developed by G. Parry. Thus, we highlighted an effect of deviation of the wrinkle by walking.
We are also interested in the elementary plasticity mechanisms activated in aluminum films subjected to increasing deformation. In 0 K molecular dynamics simulations, it has been shown that the interface energy drives the plasticity of the film during buckling. Different phenomena have been identified as a function of this adhesion energy: emission of interface dislocations and delamination, formation of cracks in the film from interface cracks or formation of twins. In a second step, we studied by molecular dynamics at finite temperature, the plasticity mechanisms activated during the buckling of an Al film when an S (51) (551) grain boundary is placed in this film. We showed that the plasticity initiated at this joint strongly modifies the profile of the wrinkle. Two mechanisms have been identified, the formation of extrinsic faults in the upper part of the film in tension and intrinsic faults in the lower part in compression. Following the activation of these microscopic mechanisms involving perfect dislocations and Schockley partial dislocations, we identified two mesoscopic parameter, a plastic deformation and a bending angle. There integration into a description of the buckling in the frame of the theory of thin plates allowed us to reproduce the profiles of the films after buckling obtained in the simulations.
This activity concerns the theoretical study of the mechanical stability of nanostructures of different geometries with respect to the introduction of dislocations, when stresses are present in these composite structures. Several problems have thus been addressed such as the introduction of a wedge dislocation dipole in semi-epitaxial flat layers buried in a matrix or the formation of prismatic dislocation loops in cylindrical or spherical core / shell structures. The final objective of these studies is to propose stability diagrams with respect to the formation of these defects as a function of parameters such as the stresses and the geometric characteristics of the layers. In the same vein, the fracturing of cylindrical materials subjected to axial stress was studied and the distribution of cracks was characterized. More recently, attention has focused on the effects of disclinations placed in grain boundaries of strong disorientation and sources of significant deformations, on the formation of dislocations from free surfaces. The conditions of these nucleations were specified as a function of the disorientation associated with these disclinations and their position relative to the free surfaces of the materials considered.
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Buckling of hyperelastic materials
This thematic concerns the folding problems of buried layers and multilayers composed of Neo-Hookean materials in the framework of a collaboration with M. Holland and M. Darayi, of the University of Notre Dame, USA. These composite materials, which can undergo large deformations with significant heterogeneities of elastic moduli, are the subject of numerous studies in biology (morphogenicity and growth of tissues, development of tumors, etc.) or in electronics (deformable devices on the skin).
In this context, we were able to characterize in the linear regime the instability thresholds of folding under external deformation of a layer buried in a matrix, then of two layers in elastic interaction in a matrix. The effects of the elastic moduli of different materials, layer thicknesses and the distance between layers were thus characterized on the folding modes (symmetrical, antisymmetric), on the wavelengths and the corresponding critical deformations. It appears through these studies that for the layers harder than their matrix, the symmetrical folding mode is selected.
