Nanoporous-gold-based composites: toward tensile ductility

We report on mechanical tests on interpenetrating-phase nanocomposite materials made by vacuum impregnation of nanoscale metal networks with a polymer. The metal component is nanoporous gold made by dealloying, whereas two epoxy resins and polyurethane are explored as the polymer component. The composites are strong and deformable in compression. Although previous observations invariably indicate tensile brittleness for nanoporous gold, composite samples made from cm-sized nanoporous samples enable macroscopic tensile and four-point bending tests that show ductility. This implies that the high strength of individual metal objects such as nanowires can now be incorporated into a strong and ductile material from which macroscopic things can be formed. In fact, a rule-of-mixture-type analysis of the stresses carried by the metal phase suggests quantitative agreement with data reported from separate experiments on small-scale gold nanostructures. Composites of nanoporous gold and polymers that exhibit both high strength and good ductility have been developed by researchers in Germany. Although nanoporous gold shows excellent deformability in compression, macroscopic samples undergo brittle failure in tension and bending. Now, Ke Wang at Hamburg University of Technology and co-workers have hit upon a new strategy for overcoming the poor ductility of nanocomposite gold — forming a composite by impregnating it with a polymer. They fabricated centimeter-sized samples of nanoporous gold vacuum-impregnated with two epoxy resins and polyurethane and demonstrate their ductility in macroscopic tensile and four-point bending tests. The plastic deformation behaviour was quantitatively consistent with a simple rule-of-mixture relationship. This approach permits the high strength of metal nanostructures to be incorporated in strong, ductile materials that can be used to form macroscopic objects. Nanoporous metals made by dealloying can take the form of macroscopic bodies that exhibit a uniform and highly interconnected network of nanoscale ‘ligaments’. Our study used this material as the reinforcement phase in novel interpenetrating-phase nanocomposites. Tensile tests on our cm-sized composite samples for the first time demonstrate tensile ductility in a nanoporous-metal-based material. Whereas the strength, σ, of pure nanoporous metal scales with the phase faction, ϕ, as σ ∝ ϕ3/2, the composite has a linear scaling relation σ ∝ ϕ that favors strengthening at small solid fraction. We find this strengthening also in quantitative agreement with the data behind the well-known ‘smaller is stronger’ of metal nanostructures. Thus, our material’s design strategy exploits the high strength of individual metal nano-objects such as nanowires for making a strong and ductile material from which macroscopic things can be formed.