Learning from nature – the porous structure of a bone and metal foam

Metal foam is a man-made metallic solid with a cellular, sponge-like structure. The pores can either be connected to each other (open-cell metal foam) or separated from one another with gas inclusions (closed-cell metal foam).

Metal foam is typically an ultra-light material which is achieved by high porosity (as a measure of the proportion of the cavity to the total volume). The low density together with high stiffness make metal foams the ideal material for lightweight construction applications. By selecting the porosity, the coating material and its thickness, the weight of a part can be produced precisely according to specific customer needs.

Thanks to its favourable stress-strain property (see stress-strain diagram), metal foam, especially open-cell foam is a superior material for absorbing energy and far more superior to comparable solid metal parts. This can be clearly seen in the area below the curve in the stress-strain diagram as this represents the amount of energy absorbed by the metal foam or the solid metal part. The cellular microstructure allows, on the one hand, for large plastic deformations at virtually constant stress, and on the other hand, this results in an enormous energy absorption capacity with regard to the mass. Metal foams, in addition to their low density, are therefore an ideal energy absorber, e.g. for the automotive or aerospace industries. Another ideal area of application in the military or private sector is protection from bullets or explosions (bombs, mines, IEDs, etc.) and the resulting shock waves and shrapnel.

In addition to applications in lightweight construction and as energy absorbers, there are a number of other possibilities for using the unique properties of metal foams in a ‘smart’ way. Irrespective of whether the metal foam is open celled or closed celled, it can be used for electromagnetic shielding due to its metallic material. Metal foams are furthermore very temperature-resistant and can be used as fireproof material. The interesting futuristic morphology is also suitable for architectural applications which feature a special design.

In contrast to closed-cell foams, gases or liquids can flow through the metal foam of open-cell metal foams, thus taking advantage of the large inner surface. Together with the high thermal conductivity, the chemical resistance and the high melting point of metals, they are ideal for use as a filter, a catalyst or even heat exchangers, for example. In addition, metal foams are excellent vibration and/or sound absorbers. The sound waves that strike the metal foam are reflected many times within the pores and thus strongly weakened.

By the connection of the pores to one another, open-cell metal foams can also be filled with a further medium, regardless whether liquid or solid. The open-cell metal foam can therefore also provide a stable framework structure for fillings which results in numerous other possibilities. The metal foam may, for example, be filled with an insulating material to create an inherent load-bearing, insulating material with metal foam as a support structure. Electrochemical applications are also possible in which, for example, the metal foam serves as matrix material within a battery.

There are very good reasons why Mac Panther Materials concentrates on open-cell metal foams.

Gases or liquids can only flow through open-cell metal foams and thus function as a filter, heat exchanger, catalyst or battery component.

Even in the case of sound absorption, sound waves can penetrate through the open pores better into the metal foam and can be weakened by the multiple reflections.

Energy absorption is not possible with closed-cell metal foams as the gas within the closed pores cannot escape and thus rebounding occurs during a collision. This rebounding is a reversible effect in which no energy is absorbed. Even point loads on open-cell foams do not lead to damage in the immediate vicinity which is important for protection devices, for example.

Furthermore, open-cell metal foams permit finishing of surfaces such as corrosion protection or for improving mechanical properties.

Reproducible properties of metal foams are of great importance to industrial use. For this application, coated, open-cell metal foams are advantageous. Pure metal foams exhibit up to 20% dispersion in mechanical properties due to defects in moulds or a defective pore structure. With coated metal foams or polyurethane foams, generally also referred to as hybrid foams, essentially the coating, which can be manufactured in a reproducible way, determines these properties. They are therefore more suitable for industrial use.