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Micromechanical and three-dimensional microstructural characterization of nanoporous gold-epoxy composites
Citation Link: https://doi.org/10.15480/882.1354
Publikationstyp
Doctoral Thesis
Date Issued
2017
Sprache
English
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg-Harburg
Place of Title Granting Institution
Hamburg
Examination Date
2017-01-19
TORE-DOI
Nanoporous gold (npg), a bicontinuous network of nanoscale gold ligaments and pores, displays tunable mechanical behavior through the variation of internal length-scales. However, it is severely limited by its lack of ductility in tension. By infiltrating the porous structure with epoxy, a composite material with enhanced tensile ductility and ow stress is achieved. The present work aims at a three-fold investigation of micromechanical behavior, mechanisms of deformation and failure of the npg-epoxy composite, focusing on 3D microstructural characterization, finite element simulation and micromechanical testing.
In order to understand the composite microstructure, high resolution 3D reconstructions of the npg ligament network were achieved with focused ion beam (FIB) based tomography, taking advantage of epoxy infiltration on FIB machining of porous media. It is assumed that the infiltration of epoxy does not significantly alter the geometry of the npg network. Samples of varying structural length-scales were used, with mean ligament diameters in the range of tens to hundreds of nanometers, as achieved through isothermal annealing of npg. Quantitative analyses of the 3D reconstructions were carried out in terms of metric properties (e.g., relative density, ligament diameter distribution and specific surface area), topological properties (e.g., connectivity density), and directional properties (e.g., directional tortuosity). Importantly, representative volumes (RVs) were identified, which reflects the global structural properties of the material. It was found that npg coarsens in a nearly self-similar manner. This allows the identification of structural parameters, which can be used to describe the mechanical behavior of the global npg structure over varying length scales. FEM simulations applied to meshed RVs of the 3D reconstructions strongly suggest that the effective relative density of the load bearing ligament structure is the critical structural parameter in determining the mechanical behavior of npg structural geometry, rather than the solid relative density alone, as is often assumed.
After infiltration with epoxy, the densification during deformation of npg-epoxy composites is strongly suppressed. This leads to a strongly enhanced strength compared to pure npg. However, the dependence of the yield strength on the mean ligament size is much weaker in the composite structure as compared to pure npg. The size effects in pure npg have been commonly attributed to dislocation activities within the gold ligaments, whereas in the composite material the influence of the interface on the motion of the dislocations and the epoxy chains must be considered. The interfacial behavior leads to a strengthening effect in the composites but weakens the size effects. The comparisons of the elastic moduli from the analytical models predictions, the experiments and the FEM simulations demonstrated the influences of the connectivity, the effective relative density of the load bearing ligament structure, the directional tortuosity of the ligaments and the interface on the elastic behavior of the npg-epoxy composites. Interfacial failure was experimentally observed under compression and tension. The tension-compression asymmetry investigation revealed that the npg-epoxy composite is stronger in compression than in tension. These various observations point to the important roles that the hard phase, i.e., the interconnected ligament network, the soft phase, i.e., the continuous epoxy, and the interface between them play on the mechanical behavior of npg-epoxy composites.
In order to understand the composite microstructure, high resolution 3D reconstructions of the npg ligament network were achieved with focused ion beam (FIB) based tomography, taking advantage of epoxy infiltration on FIB machining of porous media. It is assumed that the infiltration of epoxy does not significantly alter the geometry of the npg network. Samples of varying structural length-scales were used, with mean ligament diameters in the range of tens to hundreds of nanometers, as achieved through isothermal annealing of npg. Quantitative analyses of the 3D reconstructions were carried out in terms of metric properties (e.g., relative density, ligament diameter distribution and specific surface area), topological properties (e.g., connectivity density), and directional properties (e.g., directional tortuosity). Importantly, representative volumes (RVs) were identified, which reflects the global structural properties of the material. It was found that npg coarsens in a nearly self-similar manner. This allows the identification of structural parameters, which can be used to describe the mechanical behavior of the global npg structure over varying length scales. FEM simulations applied to meshed RVs of the 3D reconstructions strongly suggest that the effective relative density of the load bearing ligament structure is the critical structural parameter in determining the mechanical behavior of npg structural geometry, rather than the solid relative density alone, as is often assumed.
After infiltration with epoxy, the densification during deformation of npg-epoxy composites is strongly suppressed. This leads to a strongly enhanced strength compared to pure npg. However, the dependence of the yield strength on the mean ligament size is much weaker in the composite structure as compared to pure npg. The size effects in pure npg have been commonly attributed to dislocation activities within the gold ligaments, whereas in the composite material the influence of the interface on the motion of the dislocations and the epoxy chains must be considered. The interfacial behavior leads to a strengthening effect in the composites but weakens the size effects. The comparisons of the elastic moduli from the analytical models predictions, the experiments and the FEM simulations demonstrated the influences of the connectivity, the effective relative density of the load bearing ligament structure, the directional tortuosity of the ligaments and the interface on the elastic behavior of the npg-epoxy composites. Interfacial failure was experimentally observed under compression and tension. The tension-compression asymmetry investigation revealed that the npg-epoxy composite is stronger in compression than in tension. These various observations point to the important roles that the hard phase, i.e., the interconnected ligament network, the soft phase, i.e., the continuous epoxy, and the interface between them play on the mechanical behavior of npg-epoxy composites.
Subjects
Nanoporous gold
Composites
Focused ion beam tomography
3D microstructure analysis
Nanomechanics
FEM simulation
DDC Class
620: Ingenieurwissenschaften
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