Cellular metallic materials have emerged as a new promising class of materials due to their lightweight porous structures and advanced multi-functional properties. Originally limited to random metallic foams, modern lithography techniques have enabled the design of hierarchical cellular structures with tailored mechanical properties. In addition, the mechanical properties of micro-cellular metals can be further improved by taking advantage of the properties of metals, which are size-sensitive at this length scale. It is therefore expected that the combination of architectured cellular designs with size effects in metals would result in materials with unprecedented mechanical properties.
The present thesis aims at investigating and understanding the influence of size effects on the mechanical properties of three-dimensional micro-architectured metal structures synthesized using advanced lithography techniques and electrochemical deposition of metals. Electrodeposited copper films with engineered microstructures and nickel-based hierarchical micro-cellular structures were manufactured and tested under uniaxial loading conditions. The results of the tests were used to evaluate the contribution of the size effects and/or architecture to the change in mechanical properties and deformation behavior.
The influence of the intrinsic size effect was first demonstrated in electrodeposited pure copper films. Using nanoscale twins to engineer the grain boundaries, it was possible to produce copper films with high strength and significantly tune the mechanical properties by adjusting the twin orientation.
The combination of size effects was then investigated for two types of micro-architecture. The first type of metal micro-architecture was obtained by plating nanocrystalline Ni inside a self-assembled colloidal crystal of microspheres in order to produce a regular inverse opal. The second type was fabricated by metallizing a 3D polymer template produced by 3D laser lithography with an amorphous NiB coating.
Results show that there is a complex interaction between the characteristic external dimension and cellular structure. For regular Ni inverse opal, there is a complex interaction between the ligament size and the grain size which defines the overall strength of cellular metals. In the case of hybrid NiB/polymer structures, the brittle-to-ductile transition in the amorphous NiB coating and the architecture effect are coupled. By varying the NiB thickness and the architecture geometry, it is possible to control the deformation mechanism from global buckling to brittle failure and to tune the energy absorption characteristics of the micro-architectures.
Through these case studies, it was demonstrated that the combination of grain boundary engineering, sub-micron geometrical features and overall architecture can yield 3D metallic micro-architectures, which are both light and strong and have tailored mechanical properties. The mechanical properties of some of these materials exceed the properties of either of the parent materials. Therefore, the constitutive mechanical laws for porous metals should be modified to account for these three effects all together. The results confirmed the potential of our experimental approach and provide a practical way to extend the material property-space.
EPFL2018
