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Uncovering the Secrets of a Mollusk’s Bioceramic Armor

Posted on: 03/29/2014
The shells of the mollusk Placuna placenta, are not only exceptionally tough, but also clear enough to read through. Researchers at the Massachusetts Institute of Technology (MIT) have analyzed these shells to determine exactly why they are so resistant to penetration and damage—even though they are 99 percent calcite, a weak, brittle mineral.

The shells’ unique properties emerge from a specialized nanostructure that allows optical clarity, as well as efficient energy dissipation and the ability to localize deformation, the researchers found.

To test exactly how the shells—which combine calcite with about 1 percent organic material— respond to penetration, the researchers subjected samples to indentation tests, using a sharp diamond tip in an experimental setup that could measure loads precisely. They then used high-resolution analysis methods, such as electron microscopy and diffraction, to examine the resulting damage.

The material initially isolates damage through an atomic-level process called “twinning” within the individual ceramic building blocks. Part of the crystal shifts its position in a predictable way, leaving two regions with the same orientation as before, but with one portion shifted relative to the other. This twinning process occurs all around the stressed region, helping to form a kind of boundary that keeps the damage from spreading outward.

The MIT researchers found that twinning then activates “a series of additional energy-dissipation mechanisms … which preserve the mechanical and optical integrity of the surrounding material.” This produces a material that is 10 times more efficient in dissipating energy than the pure mineral.

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