Monolithic nanocrystalline Au fabricated by the compaction of nanoscale foam

Abstract

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.

Deformation Behavior of Nanoporous Metals

“…Finally, the grain structure of the ligaments or the lack thereof is still a subject of debate and is beyond the scope of this discussion [9]. However, even if we assume that the ligaments develop a nanocrystalline grain structure during dealloying, this would still not account for the high strength found experimentally since the yield strength of nanocrystalline Au [51,52] is only 4 times higher than that of coarse-grained material.…”

Scaling equation for yield strength of nanoporous open-cell foams

“…The fact that the increase of strength with decreasing ligament size is observed for a fixed initial foam density reveals that the defect cannot be due to densification effects. In a previous publication we have presented that our particular foam processing method produce ligaments that have a nanocrystalline grain structure [41].…”

“…However, even if we assume the ligaments are nanocrystalline, it still does not account for the high strength found experimentally since the yield strength of a nanocrystalline Au sample [41,42] is only 4 times higher than that of coarse grained Au.…”

Microscopic failure behavior of nanoporous gold

“…15 However, transmission electron micrographs reveal that the np-Au samples investigated in the present study exhibit a nano-crystalline grain structure. 8,16 Thus the intergranular facets in Figure 1a cannot reflect the grain structure of np-Au, but seem to be a remnant of the coarse-grain microstructure of the Ag-Au starting alloy (see discussion below).…”