Time dependent plasticity and strain localization in nanocrystalline metallic thin films
(2018)
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- Abstract
- In the constant race towards technological miniaturization, a variety of technologies involve metallic films with thickness much lower than a micron. Mechanical integrity of such films is crucial for the design of reliable devices for instance in flexible electronics, MEMS and deformable coating applications. Understanding ductility and rate dependent plasticity is a major issue due to the nanocrystalline structure of these materials giving rise to unique properties. The lab-on-chip nanomechanical testing method was developed at UCLouvain in order to uncover the mechanisms dictating the propensity of nanocrystalline thin films to creep. During the last decade, the technique has been successfully applied to Al, Pd and Cu thin films. The present work is devoted to the theoretical and numerical modelling of the physics underlying the viscoplasticity of metallic thin films. For the first time since its patenting, the lab-on-chip technique has been fully modelled through finite element simulations. It has been demonstrated that the geometrical design and the etching process can significantly influence the magnitude of the stress and strain and guidelines for future developments have been proposed. Then, the mechanical behaviour of nanocrystalline Pd films has been modelled, relying on a continuum model and assuming only dislocation-based mechanisms as suggested from TEM characterization. The dislocation-based model has allowed the proper reproduction of all the experimental trends. Finally, plastic localization in thin films has been addressed theoretically. As dislocation and/or grain-boundaries based deformation mechanisms can be activated when the film thickness decreases, a phenomenological formulation has been proposed to tackle the complexity of the grain size and thickness effects. Based on an extended imperfection model, the key features that can enhance ductility have been identified.
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Lemoine, G. (2018). Time dependent plasticity and strain localization in nanocrystalline metallic thin films. https://hdl.handle.net/2078.5/126390