Typically, a bullet would completely shread a piece of foam, but now the tables have turned. Various types of foam have been developed and used as fire retardants, crack sealants and insulate all mater of objects. However, a team of researchers from NC State has created a metal foam; one they say is capable of stopping bullets and much more.
Metal foams are not any new type of material. In fact, many metal foams are used in applications such as architecture and automotive and for everything from energy absorption to heat exchanges. However, composite metal foams (CMFs) are relatively new, with Dr. Afsaneh Rabiei filing the definitive patent in 2013, based on her work over the previous decade. Dr. Rabiei and team developed their CMFs over a period of several years, which resulted in a CMF so strong, they were able to show it as an effective use in body armor, stopping 7.62 × 51 mm M80 and 7.62 × 63 mm M2 armor piercing rounds traveling at 847 meters per second (m/s).
The composite material is made of uniform, hollow metallic spheres in a solid metal matrix, with tests on low-carbon and stainless steel spheres in either a steel or aluminum matrix. Those hollow spheres is where the foam gains its strength and foam-like characteristic allowing the structure to collapse much like a sponge. The team developed two different methods to create their CMFs, one based on casting a low-melting point material, in this case aluminum, around the hollow steel spheres with a higher melting point. The other uses a sintering technique that bakes a metal powder around the small metal spheres.
Those methods allow for a super-strong composite material that was shown to be capable of, not only shielding one from a bullet, but shielding against X-rays, gamma rays and even neutron radiation, making the applications for space exploration quite interesting. A deeper look at the patent shows that the applications for the foam are almost endless, with claims of the possibility, aerospace vehicles and nuclear fuel containment, medical applications such as machine shielding or bone replacement, as well as fire shielding for buildings.