Coated metal foam withstands explosions

As impact protection in cars or as protection against shock waves from explosions, the areas of application of the rock-hard yet lightweight metal foam, which has been developed by materials researchers Stefan Diebels and Anne Jung from Saarland University, are many and varied. The model from nature is the structure of bone: It consists of a hard shell and a scaffold of fine beads that leave room for countless cavities inside. This construction saves material and makes the bone light. Metal foams imitate this model: The porous materials are made from metals and look like a sponge. Although the foams commonly used today are lightweight, they are complex and expensive to produce. The beads are also too soft and pliable for many applications, such as the most commonly foamed aluminum today. "This is why metal foams have not yet been able to establish themselves on the market," says materials scientist Stefan Diebels, Professor of Technical Mechanics at Saarland University.

Diebel's research team has found a way to reinforce the framework of the metal foams. The result is a lightweight, extremely stable and versatile material. Each of the beads is coated using a patented process in such a way that the metal foam, stabilized from the inside out, can withstand extreme loads. At the same time, it is still lightweight. They use aluminum foams and now even inexpensive plastic foams made of polyurethane as a framework, which are made strong by the coating alone. "The resulting metal foams have a low density and large surface area with a small volume. In relation to their weight, they are extremely rigid and strong," explains Diebels. They are so strong that they act as mobile protective walls to absorb shock waves during explosions. They can also simply "swallow" sound and pressure waves during underwater explosions - and thus protect sensitive marine life from the consequences. "But above all, we are also thinking about less spectacular applications such as lightweight construction," explains lecturer Anne Jung. For the first of her two doctoral theses on metal foams, she received the Körber Foundation's German Study Award for "the most important dissertation of the year with particular social significance."

Following nature's example, many products can be made lighter and more stable. For example, load-bearing parts in cars and airplanes can be made from the metal foam: "They can be installed as rigid struts in the car body and at the same time take on the function of impact protection. They absorb a lot of energy and absorb the force of an impact if some of the pore layers break," says Anne Jung. The foams can be used in a variety of ways: as a catalyst, as the material can be flowed through, as vibration damping or as a heat shield, as it is very heat-resistant. It can also be used as electromagnetic shielding or even in architecture, for example for sound-absorbing cladding or as a design element. The researchers use an electroplating bath for the coating. The trickiest part was applying the wafer-thin coating deeply and, above all, evenly inside the foam. The problem: "The metal foam acts like a Faraday cage," says Anne Jung. Its interior is surrounded on all sides by conductive material, so electricity is conducted around the outside, just like the coating, and does not run through it - like lightning around a car. The materials researcher achieved the breakthrough with a special anode cage and can now coat the foam evenly and completely nanocrystalline. "The patented process also works industrially for large-area foams," she says. With its scientific publications, the Saarbrücken group is now one of the world leaders in the micromechanical characterization of metal beads. Using experiments, simulations, tensile and compression tests, light microscopy and X-ray computed tomography, they have investigated the structure, geometry of the pores and the curvature of the webs and have shown, among other things, how nanocoatings of different thicknesses give the metal foam different material properties. By varying the coating, its thickness or the pore size, they can adapt the material to different requirements. For example, coating with nickel makes the foams stable, with copper they are good heat conductors, with silver they are antibacterial and with gold the foam looks good as decoration. The researchers are working on further optimizing the process and the material. Students and doctoral candidates are also involved in this research.

According to documents from Saarland University.