Materials researchers at Saarland University in Germany can tailor multi-purpose metal foams that are hard as rock but light as air. They are modelled on the structure of bones.
Bones are very strong and can withstand extreme loads almost as well as steel. Despite this, they are incredibly light. The secret lies in a hard shell and a lattice-like structure that leaves room for countless cavities inside the bone. This construction saves material and makes the bone light. Metal foams are able to mimic this principle. Although the foams commonly used today are light, they are complex and expensive to produce. The lattices in them are also too soft and flexible for many applications. "This is the reason why metal foams have so far not had any real market impact," says materials scientist Stefan Diebels, Professor of Applied Mechanics at Saarland University.
Can even withstand explosions
Diebels' research team has now found a way to substantially reinforce the structure of the metal foams, creating a light, extremely stable and versatile material. Aluminium and plastic foams, similar to a simple kitchen sponge, serve as the basic 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," explains Diebels. They are so strong that they are being used as mobile barriers to provide protection from the shock waves caused by explosions. Even when exposed to underwater detonations, the foams simply ‘swallow’ the resulting sound and pressure waves, hence protecting vulnerable marine life from the effects of these powerful shock waves. The researchers use a galvanic bath for the coating. The challenge was to apply the wafer-thin coating deep and evenly inside the foam, as the metal foam acts as a Faraday cage. With the help of a special anode cage, however, materials researcher Anne Jung succeeded in coating the foam evenly with nanocrystals.
Versatile in use
Following nature's example, many products could become lighter and more stable. In this way, load-bearing parts in cars and aeroplanes could be made from 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," explains Jung. There are numerous areas of application for these foams, such as in catalysis, as the material is porous and thus allows liquids and gases to flow through it, or for shock absorption or as a heat shield, as the foams exhibit excellent heat resistance. The foam material can also be used for electromagnetic shielding or in architectural applications, where it finds use as sound-absorbing cladding or as a building design element.