Researchers have uncovered the secrets behind a marine creature’s defensive armour, one that is exceptionally tough and but clear enough to read through.
The mollusk Placuna placenta, known as the window pane shell, is 99% calcite, a weak, brittle mineral but somehow the shellfish makes it very strong.
The shells’ unique properties emerge from a specialised nanostructure which allows optical clarity as well as efficient energy dissipation and the ability to localise deformation, the researchers found.
The results are published 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 analysed the complex structures and properties of biological materials as possible models for new, even better synthetic materials.
Engineered ceramic-based armour, 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 armour systems, such deformation can also obscure visibility.
To test exactly how the shells — which combine calcite with about 1% organic material — respond to penetration, the researchers subjected samples to indentation tests, using a sharp diamond tip in an experimental setup.
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, Li says. This produces a material 10 times more efficient in dissipating energy than the pure mineral, Li adds.
The properties of this natural armour make it a promising template for the development of bio-inspired synthetic materials for both commercial and military applications such as eye and face protection for soldiers, windows and windshields, and blast shields, Ortiz says.
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