Novel Nanocoated Lightweight Metal Foams Become Bone Hard and Explosion-Proof

Metallic foams created by materials scientists Stefan Diebels and Anne Jung at Saarland University are strong enough not only for use in impact protection units in cars but are also able to absorb the shock waves generated by a detonation. Their super lightweight and very robust metal foams can be tailored for a variety of applications. The inspiration behind the new foam system was from nature: bones. Using a patented coating process, the Saarbrücken team is able to create extremely stable, porous metallic foams that can be used, for instance, in lightweight construction projects. The primary lattice substrate is either polymer or aluminum foam, not unlike a kitchen sponge.

Taking inspiration from bones: Materials scientists Stefan Diebels (l.) and Anne Jung can customize their lightweight and strong metal foams for a wide range of applications. (Image credit: Oliver Dietze)

The research team and the start-up company that their work has hatched (Mac Panther Materials GmbH, Bremen, Germany) will be at Hannover Messe where they will be demonstrating their process between April 1st and 5th at the Saarland Research and Innovation Stand (Hall 2, Stand B46).

Bones are one of nature’s many imaginative developments. They are stable and robust and can handle loads nearly as well as steel. But regardless of their strength, they are quite light to be effortlessly moved by humans and animals. The secret lies in the blend of a hard exterior shell that encloses a porous lattice-like network of bone tissue in the interior of the bone. This structure saves on material and minimizes weight. Metal foams are capable of imitating these naturally occurring bone structures. The synthetic foams are porous, open-cell structures that are made from metals and that resemble a sponge. The metal foams presently available are definitely lightweight, but the manufacture process is both complex and expensive. Plus, the steadiness of the sponge-like foam structure is still very weak and not tough enough for numerous applications. This is indeed true of aluminum foam, which is the most widely used type produced at present. “This is the reason why metal foams have so far not had any real market impact,” explains materials scientist Stefan Diebels, Professor of Applied Mechanics at Saarland University.

His study team has discovered a way to considerably reinforce the lattice structure of the metal foams, creating a lightweight, very stable, and flexible material. Diebels and materials scientist Dr. Anne Jung have formulated a patented procedure for coating the individual struts that constitute the open-cell interior lattice. Consequently, the foam’s exterior is stronger and more stable and the structure is currently able to endure extreme loads. However, the treated foam stays astonishingly light. The team began by using aluminum foams but is currently using low-cost polyurethane foams whose strength comes completely from the thin metal coating applied to the lattice structure. “The resulting metal foams have a low density, a large surface area but a small volume. In relation to their weight, these foams are extremely strong and rigid,” says Stefan Diebels. Actually, they are so sturdy that they are being employed as mobile barriers to provide protection from the shock waves generated by explosions. Even when open to underwater detonations, the foams just ‘swallow’ the sound and pressure waves that are generated, thus shielding sensitive marine organisms from the effects of these strong shock waves.

“Most of the applications we focus on are generally less spectacular, such as the use of our foams in lightweight construction,” explains Dr. Anne Jung, a senior research scientist in Diebels’ group. Dr. Jung, in fact, finished two doctoral theses. She was awarded the German Thesis Award from the Körber Foundation for ‘the most important dissertation of the year with significant relevance for society’ for her first doctoral theses on the topic of metal foams. Numerous products can be created lighter and more stable by taking a leaf out of nature’s design creativity. For instance, load-bearing structures in airplanes and cars could be made using the metal foam.

They can be installed as reinforcing struts in the bodywork, while also providing impact protection. The struts can take up large amounts of energy and are able to absorb the force of a collision when parts of the porous core fracture under impact.

Dr. Anne Jung, Senior Materials Scientist, Diebels’ group, Saarland University.

There are several areas of application for these foams, such as in catalysis, as the material is porous and therefore, enables gases and liquids to flow through it, or for absorption of shock or as a heat shield, as the foams display outstanding heat resistance. The foam material can also be used for electromagnetic screening or in architectural applications, where it can be used as a building design element or as sound-absorbing cladding.

The coating is done in an electroplating bath. The most challenging phase of the electroplating process was accomplishing an even coating of the ultrathin layer throughout the complete interior of the foam structure. “The problem”, explains Anne Jung, “is that the metallic foam acts as a Faraday cage.” As the foam’s interior is enclosed by electrically conducting material, electric current and thus the coating is diverted to the foam body’s exterior and does not pass through the interior of the foam—it is like what occurs when lightning hits a car. The innovation happened when Anne Jung planned to use a special anode cage, which permits her to apply an even, nanocrystalline coating through the whole lattice network. “The patented method also functions on the industrial scale with foams with very large surface areas,” adds Jung.

The Saarbrücken team has authored several important scientific papers in the field and is at present considered as one of the world’s foremost research groups in the micromechanical characterization of these porous metal lattices. Using a range of experiments, tension and compression testing, optical microscopy, simulations, and X-ray computed tomography, the researchers have studied the pore geometry, structure, and curvature of the struts and have demonstrated how altering the thickness of the nanocoating can bestow different properties to the foam materials. By varying the composition of the coating, its pore size or thickness, the researchers are able to tailor the foams to match a variety of application requirements. For instance, using nickel to nanocoat the open-cell lattice structure creates especially strong foams, with copper the foam material displays high thermal conductivity, with silver they possess good antibacterial properties, and with gold the foam is very decorative. The Saarbrücken research group, which includes doctoral researchers and students, are carrying on the work of enhancing the production process as well as the material itself.

Background

So as to simplify the commercial and industrial application of their research findings, the Saarbrücken scientists are a part of a technology transfer pilot project along with Saarland University’s Knowledge and Technology Transfer Office (KWT) and the external start-up partners Dr. Andreas Kleine and Michael Kleine, and have founded the company Mac Panther Materials GmbH with headquarters in Bremen. Both Professor Diebels and Dr. Jung have a stake in the new company as does Saarland University’s Knowledge and Technology Transfer company WuT.

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