The effects of post-fabrication annealing on the mechanical properties of freestanding nanoporous gold structures

The elastic modulus, residual stress and porosity of nanoporous gold are reported for freestanding microfabricated beams and blanket films on substrates. After fabrication, the nanoporous samples were annealed at various temperatures (up to 400 °C) to induce changes in porosity and mechanical properties. The porosity of the samples was characterized by digital processing of scanning electron microscopy images; mechanical properties were measured using combinations of freestanding beam deflection, wafer curvature and nanoindentation. The relative density of all sample geometries increased as the annealing temperature was increased. However, the evolution of the average pore size (with annealing) depended on the geometry, and hence on the boundary conditions, of the specimen. Differences in porosity evolution were reflected in the mechanical property measurements: while the elastic modulus and residual stress generally increased with increasing annealing temperature (due to densification), pore coalescence in films on substrates leads to the most dramatic changes.