Nanomechanical behaviour of open-cell nanoporous metals: homogeneous versus thickness-dependent porosity

Abstract

Two dierent nanoporous materials, porous copper prepared by dealloying and porous nickel prepared by electrodeposition, have been studied by means of nanoindentation experiments at dierent maximum applied loads. While nanoporous Cu is homogeneous along its cross-section, the electrodeposited Ni lms show a graded porosity, with smaller pores and thicker pore walls close to the lm's surface. The mechanical properties of the two materials have been extracted using a methodology based on scaling laws and subsequent interpretation has been performed using nite element simulations. Two dierent deformation mechanisms are observed for nanoporous Cu and nanoporous Ni, respectively. Dealloyed porous copper behaves as an homogeneous material without evident eect of densication and with mechanical properties that are independent of the applied load. Given this homogeneity, it is possible to t the entire loading - unloading curve for dierent maximum applied loads with a single set of mechanical properties. Conversely, electrodeposited porous nickel shows a decrease in the reduced Young's modulus, an increase in yield stress and a constant hardness when the maximum applied load during nanoindentation is increased. While the decrease in the reduced Young's modulus can be explained in the context of thickness inhomogeneity of the electrodeposited porous nickel (i.e., increase of porosity with depth), this cannot explain, and actually would go against, the observed increase in the yield stress, which is instead associated to the decrease in the ligament size.