Bones are strong and stable and can cope with loads almost as well as steel. But despite their strength, they are light enough to be easily moved by humans and animals. The secret lies in the combination of a hard, exterior shell that encases a porous lattice-like network of bone tissue in the interior of the bone. This structure saves on material and reduces weight.
Metal foams are able to mimic these naturally occurring bone structures. The synthetic foams are porous, open-cell structures manufactured from metals that have the appearance of a sponge. The metal foams currently available are lightweight but the production process is both complicated and expensive. And the stability of the sponge-like foam structure is still too weak and not resilient enough for many applications.
Researchers have significantly strengthened the lattice structure of the metal foams, producing a lightweight, extremely stable and versatile material. The individual struts that make up the open-cell interior lattice are coated and as a result, the exterior of the foam is stronger and more stable. The structure is now able to withstand extreme loads but the treated foam remains light.
Inexpensive polyurethane foams, whose strength comes entirely from the thin metal coating applied to the lattice structure, were used. The resulting metal foams have a low density, a large surface area, and a small volume. In relation to their weight, the foams are extremely strong and rigid, and can be used as mobile barriers to provide protection from the shock waves caused by explosions. Even when exposed to underwater detonations, the foam swallows up the resulting sound and pressure waves, thus protecting sensitive marine organisms from the effects of these powerful shock waves.
Applications for the foams include catalysis, as the material is porous and thus allows liquids and gases to flow through it; for shock absorption; or as a heat shield, as the foams exhibit excellent heat resistance. The foam material can also be used for electromagnetic screening or in architectural applications, where it finds use as sound-absorbing cladding or as a building design element.
The coating is applied in an electroplating bath. The challenge of the electroplating process was achieving a uniform coating of the ultrathin layer throughout the entire interior of the foam structure. The metallic foam acts as a Faraday cage. As the interior of the foam is surrounded by electrically conducting material, electric current is diverted to the exterior of the foam body and does not travel through the interior of the foam — similar to what happens when lightning strikes a car.
Varying the thickness of the nanocoating can impart different properties to the foam materials. By varying the composition of the coating, its thickness, or the pore size, researchers can customize foams to meet different application needs; for example, nanocoating the open-cell lattice structure with nickel produces particularly strong foams. With copper, the foam material exhibits high thermal conductivity; with silver, it provides good antibacterial properties; and with gold, the foam is highly decorative.