Natural mother-of-pearl, such as mussels, is one of the
hardest, most stable and stiff natural materials. Researchers have
always been fascinated by it. The structure of mother-of-pearl is
exquisite under the electron microscope; it looks like a miniature brick
wall, the joints of which are filled with mortar. The bricks are
composed of tiny Calcium carbonate plates stacked on top of each other
and interconnected with mineral bridges, and filled with a mortar
composed of an organic substance.
ETH researchers from the Group for Complex Materials led by André R.
Studart have been investigating and imitating this structure. The
materials scientists use a special process developed by them to produce
such mother-of-pearl-like materials.
They use commercially available aluminum oxide plates a few dozen
micrometers in size and an epoxy resin that acts as a joint cement. In a
rotating magnetic field, the researchers align the magnetized plates
dissolved in aqueous solution as desired in one direction, and under
high pressure and temperatures of around 1000 degrees Celsius they
solidify the material with the addition of a resin. This results in a
composite material with similar microstructure to natural
mother-of-pearl.
Bridges strengthen structure
In order to make the artificial mother-of-pearl even more stable and
harder, the team now used such plates coated with titanium oxide.
Titanium oxide begins to melt at around 800 degrees, which is a lower
melting point than aluminium oxide. Titanioum oxide droplets form on the
surface of the platelets and turn into bridges, thus strengthening the
entire structure. "These bridges also influence significantly, the
strength of the material," says Kunal Masania, co-author of a study that
has just been published in the technical journal PNAS.
The density of these titanium bridges can be precisely adjusted by
pressure and temperature, to produce artificial mother-of-pearl with the
desired physical properties such as stiffness, strength and fracture
toughness. With the help of a model and experiments, the researchers
calculated which pressure and temperature conditions promote the
formation of the respective properties that are comparable in stiffness
to carbon-fibre composites. With this, the team have established a new
world record in combining stiffness, strength and toughness in this type
of bio-inspired material.
With the newly developed technology, mother-of-pearl-like materials
can be produced that have tailor-made properties for the respective
application. Possible applications include construction, aircraft and
space.