Monolithic nanocrystalline Au fabricated by the compaction of nanoscale foam

We describe a two-step dealloying/compaction process to produce nanocrystalline Au. First, nanocrystalline/nanoporous Au foam was synthesized by electrochemically driven dealloying. The resulting Au foams exhibited porosities of ∼60% with pore sizes of 40 and 100 nm and a typical grain size of <50 nm. Second, the nanoporous foams were fully compacted to produce nanocrystalline monolithic Au. The compacted Au was characterized by transmission electron microscopy and x-ray diffraction and tested by depth-sensing nanoindentation. The compacted nanocrystalline Au exhibited an average grain size of <50 nm and hardness values ranging from 1.4 to 2.0 GPa, which were up to 4.5 times higher than the hardness values obtained from polycrystalline Au.