Fracture toughness of nanoporous gold with hierarchical microstructure

Abstract

Nanoporous gold (np-Au) is a material with sponge-like structure, composed of continuous ligament and pore in nanoscale, which gives low density and high surface to volume ratio. From these advantages, there have been extensive researches to apply np-Au for catalyst, actuator, and sensor. However, brittle behavior of np-Au unlike ductile bulk gold remains as an issue to overcome. Previous researches have shown that brittleness of np-Au occurs by stress concentration on pore surface and catastrophic crack propagation through grain boundary, which had been formed at Au-Ag precursor alloy state. Here, we focus on the effect of the ligament/pore size, grain boundary density, and grain shape on fracture toughness of np-Au. We fabricate well-annealed, cold rolled, and hot rolled Au-Ag precursor alloy for microstructure variation. Well-annealed precursor alloy by melting Au and Ag has microscale grain. Cold rolled precursor alloy is nanocrystalline by recrystallization and hot rolled precursor alloy has anisotropic grain along the rolling direction. By free corrosion dealloying in nitric acid, Ag is selectively etched from alloy and np-Au is formed. Only well-annealed precursor alloy is annealed additionally for coarsening of np-Au, which increases ligament/pore size. Microstructure of precursor alloy and crack propagation are observed by SEM (scanning electron microscope) and EBSD (electron back-scattered diffraction). Stress intensity factor, KΙc is measured with three-point bending to observe the brittle behavior of np-Au, and we discuss effect of ligament/pore size, grain size and grain shape on fracture toughness of np-Au.