Evolution of geometrically-self similarity in coarsened nanoporous gold

Abstract

Nanoporous gold (np-Au) made by dealloying is composed of a bicontinuous network of ligaments (solid) and pores. This material has attracted attention in a variety of applications, such as catalysis, sensors, and actuators, due to its low weight and high specific surface area. Several studies of the mechanical properties of np-Au have shown that the Gibson-Ashby scaling equation for open foam materials cannot be applied directly to np-Au. Accurate scaling laws for np-Au are challenging to derive because of complex issues such as ligament size effect, tension-compression asymmetry, and geometric structure. The change in yield strength with ligament coarsening relies on ligament-size-dependent mechanical behavior (the smaller is the stronger) on the assumption that microstructures of np-Au are self-similar regardless of whether ligaments are coarsened. Few researchers have looked at the relationship between network structure and mechanical properties as well as microstructure of np-Au in terms of morphology, and topology. Thus, it is important to identify the microstructural change of np-Au as coarsening and effect of microstructure on mechanical properties. This study validates change in 3D microstructure of np-Au as coarsening and looks at the relationship between microstructure and mechanical behavior. We fabricated several np-Au samples with various ligament sizes from 60 nm to 1 um, using thermal coarsening at different temperatures and studied the 3D np-Au structures by FIB/SEM tomography so as to look at whether or not np-Au structures are self-similar during structure coarsening. Furthermore, we investigated correlation of microstructure with mechanical behavior by nano-indentation testing and finite element method (FEM) compression simulation. We show that the number of well-connected ligaments that can serve as load-bearing in supporting an applied force affects np-Au mechanical behavior.