Evolution of Structure, Composition, and Stress in Nanoporous Gold Thin Films with Grain-Boundary Cracks

Abstract

Nanoporous gold (np-Au) thin films were fabricated from Au-Ag alloy films sputtered onto substrates. At several stages of dealloying, the evolution of the microstructure and Ag content were analyzed and stress in the np-Au thin films was measured. A nanoporous structure evolved almost immediately throughout the film thickness, and the ligament width coarsened during further dealloying, with a time dependence of t 1/8 . The initial alloy films, which contained 25 at. pct Au, became stress free after extended dealloying and during thermal cycling up to 200°C. Preferential dissolution caused cracking at grain boundaries, which accommodated a portion of the volume contraction from dealloying, but the films nonetheless remained attached to their substrates.

Grain Boundary Parting Limit during Dealloying

“…[3] Grain boundaries often corrode preferentially and serve as sites for crack nucleation. [5] For example in Ag-Au, grain boundaries preferentially dealloy and could act as local stress concentrators. [5,6] Inhibiting grain boundary corrosion could provide a potential route to mechanically stabilize dealloyed materials.…”

“…[5] For example in Ag-Au, grain boundaries preferentially dealloy and could act as local stress concentrators. [5,6] Inhibiting grain boundary corrosion could provide a potential route to mechanically stabilize dealloyed materials. Despite the ubiquity of grain boundaries in practical bulk dealloyed materials, few studies have focused on investigating their influence on the dealloying reaction.…”

“…Grain boundaries in nanograined Ag 3 Au preferentially etch during dealloying, as anticipated from the results of prior studies of coarse grain materials (Supplementary Figure S3). [5] The average grain boundary composition in Ag 3 Au is 17% Au (Supplementary Figure S4), indicating Ag segregation, which kinetically promotes the localized corrosion. The results are summarized in the Supplementary Data, since they are relatively consistent with prior work on large grained Ag-Au alloys and are included here primarily for comparison.…”

“…The results are summarized in the Supplementary Data, since they are relatively consistent with prior work on large grained Ag-Au alloys and are included here primarily for comparison. [5] Figure 1 shows Cu 6 Au 4 , Cu 3 Au, and Cu 9 Au dealloyed to 20% of their theoretical columbic capacity (see Supplementary Figure S5 for fully delloyed), along with Ag 3 Au for comparison. The early stages of dealloying in Cu-Au are spatially heterogeneous.…”

“…The local coarsening rate of ligaments follow approximately a t 1/8 dependence ( Figure 2) that is in agreement with results in the literature. [5] The longer temporal response of ligament coarsening, after initial dealloying, explains its insensitivity to dealloying potential and relative sensitivity to temperature, [16] indicating that the response relates more to diffusional relaxation of strain gradients than the nature of the dealloying potential driving force itself.…”