Isolating the role of geometrical structure on the mechanical properties of nanoporous metals

Nanoporous (NP) metals are three-dimensional (3D) structures with characteristic length-scale of its constituents (ligaments, junctions, and pores) in the range from a few to hundreds of nanometers. Such materials are of great interest for many applications, including catalysis, biological material analogues, and the next generation interconnect materials in electronics packaging. Investigations targeting understanding of the mechanical properties of such materials have tried to separate effects of the geometrical arrangement of the 3D network from those due to the nanostructure (abundance of surfaces, presence of grains, and other defects). Traditionally, this has been achieved by assuming that the network is geometrically similar to that of a macroscopic, low-density metal foam. The goal of this work is to attack the problem using a comprehensive approach that involves isolating the prominent geometry and size scale effects and examining their specific contributions individually for a range of relative densities. Specifically, 3D printed models printed and tested in compression at the macroscale replicate the geometrical arrangement of the nanoporous 3D network independently of any size effects. The relative modulus and relative compressive yield strength for the 3D printed structures with same arrangement as the nanoporous solid exhibit different scalings with density compared to stochastic macroscale foams. The deformation mechanism in stochastic macroscale foams is isolated in the ligaments and switches from bending to compression dominated behavior as the relative density increases. In contrast, due to the presence of enlarged junctions, the deformation mode for the 3D printed nanoporous structures remains bending dominated even at high relative densities. Nanoscale experiments and molecular dynamics (MD) simulations provide a glimpse into the relative modulus and strength scalings and reveal a more nuanced dependence, with unexpected enhancement in both the relative modulus and relative strength. Obtaining a clear understanding of the contribution of geometrical structure on the properties of nanoporous metals will significantly advance our understanding of how to tailor NP metal microstructure such as grains, interfaces, and surfaces to enhance the physical properties of the material. Thus, the findings reported here could inform future studies to maximize the versatility and potential of nanoporous metal structures.Ph.D

Fabrication of elastomer/metal bilayers using strain engineering

Bilayers are constructed from two materials with distinct properties and one physical dimension an order of magnitude smaller than the others. Due to the property mismatch, dimensional changes in each material will be different when the bilayer is exposed to different environments, resulting in out-of-plane deformations to minimize potential energy. The resulting curvature change can be tuned by altering the material elastic properties and relative thicknesses. In addition, instabilities or defects may initiate during the fabrication process, impacting the eventual curvature of the bilayer. Existing analyses predict that surface instabilities, such as wrinkles and folds, form in high-property-mismatch bilayers and become more prominent with increased compressive strains locked at the interface. Under compressive stress, cracks are assumed to form only when the interfacial strength is weak. The aim of this work is to describe synthesis parameters and explain observed phenomena on bilayers formed when thin metal-alloy films are sputter deposited on compliant substrates. The deposition parameters create residual compressive strains and strong adhesion in the bilayers, resulting in instabilities that deviate from theory. The experiments revealed cracks on surfaces with strong interfacial adhesion, and nested wrinkles, with smaller wavelengths than predicted. A numerical model, adopted to explain the observations, revealed that nested wrinkles, with wavelengths comparable to experimental wavelengths, form under certain mismatch-strain profiles. The model suggests that cracks can initiate from the peak of wrinkles when the critical fracture strength of the coating is exceeded. The work impacts design of bilayers with extreme property mismatches since the fracture toughness of the film may be low enough to initiate cracks and possibly compromise formation of any desired instabilities.M.S