A highly elastic composite metal foam could one day be a favorable material for biomedical implants and car bumpers. No, this isn’t a close-up of the surface of a golf ball. It’s a new type of material, one that is porous and elastic, lighter than solid aluminum yet stronger than steel, one that its creators are calling an “ultra high-strength metal matrix composite foam.” A bit of a misnomer, really, considering the foam is made up entirely of stainless steel.

But one particular feature of this metal foam, its modulus of elasticity, is what makes it so lucrative for the biomedical, aerospace and transportation industries. This measure is essentially the relationship between a force applied to an object and the amount of deformation the object experiences as a result of that force; elasticity refers to the object’s ability to bounce back to its original state, undamaged, once the force is removed. Diamonds, for example, have a high modulus of elasticity, while something like a foam stress ball would have a much, much lower one.

The foam is created with a modulus of elasticity very similar to that of natural bone in humans, making it an ideal material for biomedical implants, compared with the modulus of titanium implants, which can be anywhere from 3 to 10 times higher than bone. This is important, the researchers say, because an implant has to function much like a natural bone inside the body, especially when it comes to handling loads. If an implant is too strong, it takes over load-bearing responsibilities and the surrounding bone begins to weaken, which in turn loosens the implant and requires another replacement surgery. The material is also very light due to its porosity, and its rough surface could help natural bone adhere to the implant, further increasing its strength and stability inside the body.

Another advantage of the metal foam, notes the research team from North Carolina State University, is its relative ease to produce. They combine tiny, hollow stainless steel beads with powdered steel in a mold, then put the mold into a hot press, where high pressure and temperature ultimately force the materials to weld themselves together. The end result of this process, a standard one called powder metallurgy, is the composite foam consisting of tiny, uniform pockets of air reinforced by a solid metal matrix. Although the foam is 1/3 the density of solid steel, it can absorb up to 80 times as much energy over the same volume. This is also an improvement over other types of metal foam, where variations in cell size and thickness of the cell walls cause uneven deformation under stress.

Besides biomedical implants, the researchers imagine that the foam could be useful in aerospace or transportation. Putting two cylinders of the foam on a car bumper, for example, would make an impact at nearly 30 mph feel like an impact at only 5 mph.

Their research will be published in the March 2010 issue of Materials Science and Engineering A: “Evaluation of modulus of elasticity of composite metal foams by experimental and numerical techniques”: L. Vendra, Afsaneh Rabiei