The secret behind a mollusc’s bioceramic armour

  •  A Scanning Electron Microscope (SEM) image of the region surrounding an indentation the researchers made in a piece of shell from Placuna placenta. The image shows the localisation of damage to the area immediately surrounding the stress. Image: Ling Li and James C Weaver
  •  A Scanning Electron Microscope (SEM) image of the region surrounding an indentation the researchers made in a piece of shell from Placuna placenta. The image shows the localisation of damage to the area immediately surrounding the stress. Image: Ling Li and James C Weaver
  •  Another SEM image shows the effects of multiple indentations, again limiting the damage to the immediate surroundings. Image: Ling Li and James C Weaver
Date:11 July 2014 Tags:, , , , , , , ,

Researchers have unearthed the secret behind the Placuna placenta mollusc’s defensive armour – one that is not only exceptionally tough (even though it’s 99 per cent calcite, a weak, brittle mineral), but also optically clear.

The MIT researchers studied the shells of these sea creatures and found that the shells’ unique properties emerge from a specialised nanostructure that allows optical clarity, as well as efficient energy dissipation and the ability to localise deformation.

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, said MIT’s Professor Christine Ortiz. In transparent armour 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 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 to examine the resulting damage.

The material initially isolates damage through an atomic-level process called “twinning” within the individual ceramic building blocks: a crystal breaks up into a pair of mirror-image regions that share a common boundary, rather like a butterfly’s wings. This twinning process occurs all around the stressed region, helping to form a kind of boundary that keeps the damage from spreading outward.

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

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 said.

The findings are published in Nature Materials.

Source: David L Chandler | MIT