Metal foams are light and surprisingly tough. Actually, make that very tough: In this video, a composite metal foam turns an armour-piercing bullet to dust on impact, as if it were a piece of chalk. The experiment was performed by researchers from NC State, led by Afsaneh Rabiei. It saw a M2 armour-piercing projectile — 7mm in diameter — being fired at a lump of metal foam. Rabiei explained to PhysOrg what happened:
We could stop the bullet at a total thickness of less than an inch, while the indentation on the back was less than 8 millimetres. To put that in context, the National Institute of Justice standard allows up to 44 millimetres indentation in the back of an armour.
These metal foams can be made in many different ways. Some are manufactured by bubbling gas through molten metal, while others cast metallic alloy around hollow metal spheres to provide voids.
Either way, metal foams — which have existed in one form or other for decades now — seem to finally be coming of age.
Gif from a video by North Carolina State University