Introducing the latest in bioceramic armor: lightweight and transparent yet tough-as-nails, and totally inspired by the natural exoskeleton of a bivalve mollusk.
The defensive armor of the windowpane oyster (Placuna placenta) is incredibly resistant to penetrating attacks and damage -- a surprising fact given how it’s 99 percent calcite, a weak and brittle mineral. (The other 1 percent is an organic material, and not adamantium or anything.) The outer shell even stays transparent despite suffering multiple blows, which can’t be said for manmade windshields or visors. Some Indian and Philippine cities have used the translucent shell for lampshades and as cheaper alternative to window glass.
Pure calcite shatters easily. But when Ling Li and Christine Ortiz of MIT probed the “penetration resistance” and deformation mechanisms of the marine creature, they learned that the shells are arranged into thin layers that reorganize when stressed. The shells’ unique properties, they discovered, are a result of a specialized nanostructure that allows for: optical clarity, efficient energy dissipation, and the ability to localize deformation.
First, they subjected the shell to indentation tests. They used a sharp diamond tip to apply precisely measured, nano-sized indentations to the shell sample. Then they examined the resulting damage with an electron microscope.
They found that the shell is able to isolate the damage through a process called “twinning.” That’s when a crystal of calcite breaks into thin, crystalline twins with an arrangement that’s mirrored across the shared twin boundary -- like butterfly wings the size of atoms. The twinning occurs all around the area that’s stressed, forming a boundary that keeps the damage from spreading.
Not only does the formation of twins confine the deformation to a small volume, it also activates other energy dissipating events including: nanocracking within and between the calcite crystals, crack deflection, and the stretching of the sheets of organic material between the layers of calcite. These all work to “preserve the mechanical and optical integrity of the surrounding material,” Li says in a news release.
The result is a material that’s 10 times more efficient in dispersing energy than the pure, raw calcite. New Scientist has a video of Li jabbing and smacking the shell and a cube of calcite.
The work was published in Nature Materials this week.
Images: Ling Li (top) & Ling Li and James C. Weaver (middle)
