Artificial Mother of Pearl: Stronger Lighter Armor

http://prlo.aps.org/files/focus/v20/st1/nacre1_BIG.jpg

One of nature's strongest materials is nacure, colloquially known as "Mother of pearl." This is the surface coating of pearls and the iridescent coating that lines the inside of many mollusks such as oysters, abalone, and mussels. Nacure is composed of 95% aragonite, a hard but brittle calcium carbonate mineral, with the remaining 5% made up of soft organic molecules. While on their own, each material has poor mechanical properties, when layered together by the mollusks, the nacure can be (in energy terms) up to 3,000 times more resistant to fracture.

The secret to the incredible increase in fracture toughness is due to the arrangement of constituent components. The spacing between the aragonite and organic layers varies between nanometers to micrometers.

In order to reproduce this complex design, researchers have been freezing salt water. When seawater freezes, the ice creates several thin scaffolding layers and all the impurities in the freezing water are pushed out in between these layers. Using this process, a suspension of alumina and salt water is made and is then frozen. The alumina creates a scaffold and the voids in between are filled with a common polymer called polymethylmethacrylate (PMMA).

http://images.iop.org/objects/phw/news/10/1/13/0601131.jpg

While the increase in toughness from components to composite is not as drastic as actual nacure, the synthetic  material still boasts an impressive 300 fold increase. The key to material toughness is the ability to dissipate strain energy. This is achieved in the alumina/PMMA composite because the polymer allows the ceramic layers to barely slide across each other. Acting like oil in an automobile engine, the PMMA allows strain to be dissipated and prevents catastrophic failure in the form of fracture.

This is all well and good, but polymers do not generally bear loads very well. If we can eventually replace the PMMA with a fairly soft metal instead, not only will the composite still absorb strain energy, it will also be able to handle greater loads. The creation of these kinds of materials will allow thinner and lighter shells for things like transport vehicles, energy generation, body armor and more.