If you are taking on gunfire and looking for a place to take cover, “behind the foam” is probably not going to be your first thought. Maybe it should at least be your second, because the composite metal foams being created by professor, Afsaneh Rabiei, at North Carolina State University are worth a second look.
Metal foams are particularly interesting because they combine all the things we like about metals without such high density. For instance, ductility, electrical and thermal conductivity, and high-energy impact absorption can be paired with typically nonmetallic properties such as light weight and even buoyancy.
To be clear, metal foams (more accurately “cellular solids”) are not going to float away in a light breeze, and as they get lighter (higher porosity) they get weaker. In an application like armor, however, “light” is a very relative term, and there has been great emphasis on the reduction of mass and increased efficiency (for instance, the Army’s Ultra Light Vehicle (ULV)).
When you pre-fill a metal with holes to create a foam, you might expect that would reduce its effectiveness against new holes from projectiles. The pores in the foam collapse while deforming and absorb more energy than its solid counterpart (per mass), but the metal foam is not alone in taking on the brunt of the force.
According to the scientific article describing this work, the armor plating is a sandwich panel consisting of a boron carbide outer layer (strike face) with a core of steel composite metal foam (metallic powder sintered around hollow metal spheres) and either KevlarTM or 7075 aluminum as an inner layer.
The plates were subjected to some heavy firepower in the form of a 7.62 x 63 millimeter M2 armor piercing projectiles fired from a “Mann” gun. The resulting inner layer deformation was 8 mm., and the National Institute of Justice (NIJ) standard allows up to 44 mm.
As the demand for performance and efficiency continues, it is likely these challenges will be met by combining new materials in new ways as illustrated here.
A video of the projectile impact is below.
More available from NCSU.
Photo from Rabiei Group