CAMBRIDGE, Mass-- The shells of a sea creature, the mollusk Placuna placenta, are not only exceptionally tough, but also clear enough to read through. Now, researchers at 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. The results are published this week in the journal Nature Materials, in a paper co-authored by MIT graduate student Ling Li and professor Christine Ortiz.
Ortiz, the Morris Cohen Professor of Materials Science and Engineering (and MIT's dean for graduate education), has long analyzed the complex structures and properties of biological materials as possible models for new, even better synthetic analogs.
Engineered ceramic-based armor, while designed to resist penetration, often lacks the ability to withstand multiple blows, due to large-scale deformation and fracture that can compromise its structural integrity, Ortiz says. In transparent armor systems, such deformation can also obscure visibility.
Creatures that have evolved natural exoskeletons many of them ceramic-based have developed ingenious designs that can withstand multiple penetrating attacks from predators. The shells of a few species, such as Placuna placenta, are also optically clear.
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
|Contact: Andrew Carleen|
Massachusetts Institute of Technology