Abstract
Nanoporous gold (NPG) is a bicontinuous network of nanoscale gold ligaments and pores, the dimensions of which can be adjusted to exploit strengthening at small ligament sizes. Nevertheless, its mechanics are limited by the lack of ductility under tensile loading. Recently, the impregnation of the open porous structure with an epoxy phase created a metal-polymer composite that combined strengthening at small structural length-scales with enhanced tensile ductility. The present work aims to clarify the effect of factors influencing the mechanical behaviour of composite materials, particularly the properties of the epoxy phase and interface – properties that are still unexplored in NPG-epoxy composites. To this end, a series of novel approaches were developed to investigate the independent contribution of each factor, focusing on the development of representative model samples, micromechanical testing, and finite element simulation.
A novel method to understand the effect of the composite microstructure in the epoxy was made possible by isolating the matrix phase. Taking inspiration from mining techniques, an approach based on gold etching in iodine-iodide solutions successfully leached out the Au from NPG-polymer composites. The remaining bi-continuous open porous polymer (NPP) was investigated using nanoindentation to explore possible size effects as those found in nanoporous gold. The interface was investigated using a model system where a single planar boundary between gold and epoxy was achieved. Using both nanoindentation approaches and novel microshear experiments, the interphase regime and interfacial strength were measured. Moreover, the experimental results inspired a modelling approach of the interfaces for finite element simulations, which was employed to study the interfacial behaviour as a function of the balance between the properties of constituent phases and interfaces. The interfacial model was also employed to simulate the effect of the interfacial strength on the mechanical response and delamination evolution of NPG-epoxy composites.
The findings from the investigations on the isolated factors improved the interpretation of the composite mechanical response. No evidence of a structural length-scale effect in the polymer was found, and the response of the isolated polymer phase could be well predicted by an isostrain law applied to the properties measured on the bulk polymer. Furthermore, no evidence supporting the presence of an interlayer was found between the gold and epoxy phases. The measurement of the interfacial strength showed that the interface in NPG-epoxy composite is the weakest link in the composite's deformation, which was also observed in the simulations. The isolated effect of the interfacial properties was attenuated in the composite by the interconnected structure, which in compression allows the load transfer even after interface failure
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