Fabrication of hierarchically structured nanoporous gold by hot rolling process

Abstract

Nanoporous gold (np-Au) is a material with sponge-like structure, composed of continuous liga-ment and pore in nanoscale. Due to low density and high surface area-to-volume ratio by structural property, there have been many researches to apply np-Au to catalyst, actuator, and sensor. However, brittle behavior of np-Au unlike ductile gold in bulk scale remains as an issue to be overcome. Previous researches have shown that the brittleness of np-Au appears by stress concentration on pore surface and catastrophic crack propagation through grain boundary, which had been formed under Au-Ag precursor alloy state. Here, we focus on crack propagation through the grain boundary and investigated dependency of fracture toughness on grain boundary structure. We fabricated well-annealed, cold-rolled, and hot-rolled Au-Ag precursor alloy for microstructure variation. Well-annealed precursor alloy has grain size in micro scale, cold rolled precursor alloy has grain size in nano scale, and hot rolled precursor alloy has anisotropic grain in micro scale. By free corrosion dealloying in nitric acid, Ag is selectively etched from precursor alloy and np-Au is formed with grain boundary structure of well-annealed, coldrolled, and hot-rolled precursor alloy. Microstructure of precursor alloy and np-Au is observed by SEM (scanning electron microscope) and EBSD (electron back-scattered diffraction). Dependency of fracture toughness, KⅠc on grain boundary structure is investigated under tension with nano-UTM and effect of grain boundary density and grain boundary structure on fracture toughness and crack propagation path is discussed.