Indentation size effect in nanoporous gold

We find that hardness of nanoporous gold (np-Au) measured by nanoindentation tends to increase with decreasing indentation depth, similar to the indentation size effect (ISE) in solid materials. While ISE in solid materials is attributed to a local increase in density of geometrically necessary dislocations (GNDs), the origin of ISE in np-Au has not been studied systematically. We prepare four np-Au samples with ligament sizes of 26, 73,126, and 630 nm by free corrosion dealloying and post heat treatments. For the normalized hardness (hardness/macroscopic hardness) vs normalized indentation depth (indentation depth/three times ligament size), we find that the ISE trends for three np-Au samples of ligament sizes 26, 73, and 127 nm are almost identical, while an enhanced ISE is shown for np-Au with greatest ligament size, 630 nm. We investigate ISE in np-Au based on nanomechanics model for nanoindentation on np-Au with a sharp indenter, uniaxial compression and pure shear testing for np-Au. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.