Dealloyed nanoporous metal has been reported as a high
strength nanoporous metal by many groups. For example, Biener
and coworkers [140] demonstrated that the plastic deformation
of np-Au under compressive stress by depth-sensing nanoindentation,
which reflected the intrinsic ductile behavior of the gold ligaments.
For a polycrystalline np-Au with a grain size of 10–60 nm, a
mean hardness of 145 (±11) MPa and a Young’s modulus of 11.1
(±0.9) GPa were obtained from analyzing the load–displacement
curves. Volkert and coworkers [141] reported that np-Au with a
ligament/pore size of 15 nm was even stronger than polycrystalline
solid gold. By the combination of nanoindentation, column
microcompression, and molecular dynamics simulations, it is
found that the yield strength of np-Au is ligament size-depended
[142]. The strength increases with the decrease of ligament size
and despite being a highly porous material, the ligaments in np-
Au can approach the theoretical yield strength of gold. Besides
the ligament size, the mechanical properties of nanoporous metals
are also related to other parameters such as the preparation and
testing conditions. Nanoporous metals become quite brittle if it
contains many cracks which are formed during the dealloying process
[143]. For example, np-Au with a ligament size as small as
4 nm fractured in the elastic region and at very low stress (below
10 MPa) when it contains many cracks. The mechanical properties
of nanoporous metals also depend on the modification of ligament
surface. It has been demonstrated that coating metal oxides (Al2O3)
on np-Au can significantly enhance its hardness and stiffness [91].