A thin film is like a taut sheet: the thinner it is, the harder it is to push through. Scientists have noticed that films made of very different materials (graphene, graphene oxide, polymers) resist projectile penetration surprisingly well under high-speed impact. And the thinner the film, the more energy it absorbs per unit mass. But why this happens has long remained a mystery.
It turns out it's all about how a thin layer suppresses long deformation waves. In a thick material, such waves propagate freely and make it softer. In a thin film, there isn't enough room for them, so the material becomes stiffer. This is similar to how a shorter string sounds higher: it has no room for long waves, so it vibrates faster.
This law works for all studied materials, regardless of their chemical composition and degree of disorder. So the cause is not chemistry but the very geometry of the thin layer. Such understanding will help create lighter and stronger protective coatings — from bulletproof vests to spacecraft shields.
Graphene, a layer of carbon one atom thick, is so strong that, according to calculations, an elephant standing on a pencil would not tear a sheet of graphene if it were perfect.