Time-dependent deformation of nanoporous gold investigated by spherical nanoindentation

Abstract

Nanoporous gold (np-Au) is attracting increasing attention due to its low density, high specific surface area and high electric conductivity to offer potential benefits for many applications, such as catalyst, sensor and actuator. Unlike bulk polycrystalline Au, np-Au shows brittle behavior even though individual ligaments are composed of Au with the high ductility, making it difficult to use for many applications like MEMS, so the mechanical properties of np-Au have been intensively studied. But so far, there are few studies on creep behavior of np-Au. Time-dependent deformation behavior, such as creep, can be accelerated further at the nanoscale, and studies using nanoindentation tests have been actively conducted to evaluate them. Recent studies have shown that high grain-boundary density in precursor alloys lower the flexural strength of np-Au, while nanoindentation hardness is independent of the within-ligament microstructure. Here we fabricate np-Au samples with different microstructure, grain boundary density and initial dislocation density, and measure its time-dependent deformation properties using spherical nanoindentation We prepare annealed, prestrained, and high-energy ball-milled Au-Ag precursor alloy. Np-Au samples were made by free corrosion dealloying process which selectively etched Ag from Au-Ag alloy. Since the microstructures of precursor alloys such as crystallographic orientation and grain size are preserved during dealloying, we obtain nanocrystalline np-Au with grain size 300 nm from ball-milled precursor alloy. Nanoindentation tests were performed to investigate the effect of such microstructural variation on the creep behavior of np-Au. We used a spherical tip for nanoindentation and the results were analyzed by Garofalo’s equation to calculate creep stress exponent, which is dependent on the creep mechanism. We discuss the characteristics of the creep deformation mechanism of np-Au with open-cell structure, high initial dislocation density and grain boundary density.