Mechanical behavior of nanotwinned nanoporous gold

Abstract

Nanoporous gold (np-Au) has been extensively studied for catalyst, sensor, actuator and other applications due to its open-cell porous structure with high surface-to-volume ratio. Np-Au shows brittle behavior even though individual ligaments are composed of ductile metal, making it difficult to use to MEMS devices and other applications, and hence the mechanical properties of np-Au need further investigation. In previous work, we found 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. Recent studies have shown that a nanotwin (nt) structure in Cu and Ag can significantly enhance both strength and ductility over an ultra-fine or coarse grain structure due to the large density of twin boundaries. The nt structure can be created in electrodeposited and sputtered films by exploiting stacking fault energy, deposition rate and temperature. Here we fabricate nanotwined nanoporous gold (nt np-Au) thin film and measure its mechanical properties using nanoindentation and tensile testing. We fabricate nt Ag-Au thin film by co-sputtering of Ag and Au targets in a magnetron sputtering machine. Varying the annealing time after deposition lets us control grain size and twin density in the nt precursor thin film. After free corrosion dealloying in nitric acid, microstructures of precursor and np-Au are observed by TEM (transmission electron microscope), SEM (scanning electron microscope) and EBSD (electron back-scattered diffraction) and mechanical properties are investigated by nanoindentation and tensile testing to correlate the np-Au morphology and plasticity.