Custom-made Synthetic Mother-Of-Pearl
Tougher, harder, more fracture resistant – researchers at the ETH Zurich are developing a material inspired by mother-of-pearl, with specifically adjustable physical properties.
Natural mother-of-pearl, like the kind produced by mussels, is one of the hardest, strongest and stiffest natural materials. Under the electron microscope, mother-of-pearl looks like a miniature brick wall with joints filled with mortar. The bricks are composed of tiny calcium carbonate platelets, which are stacked on top of each other and connected by bridges. The mortar is an organic substance.
Researchers from the Eidgenössische Technische Hochschule (ETH) in Zurich have been investigating and imitating this structure. The materials scientists use a special process they developed themselves to produce such materials, which are similar to mother-of-pearl.
Instead of using calcium carbonate platelets, they use commercially available aluminium oxide platelets, a few dozen micrometres in size, and an epoxy resin that acts as the joint cement. The researchers align the magnetised platelets, which are distributed in an aqueous solution, in the required direction using a rotating magnetic field. Under high pressure and temperatures of around 1000 °C they solidify the material with the addition of a resin. This results in a composite material with a similar microstructure to natural mother-of-pearl.
Metal oxide bridges strengthen the material
In order to make this synthetic mother-of-pearl even stronger and harder, the aluminium oxide platelets are coated in titanium oxide. From around 800 °C, drops of titanium oxide form on the surface of the platelets and mature into mineral connecting bridges, strengthening the entire microstructure. "These bridges also influence significantly the strength of the material," explains 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 specific pressure and temperature settings to produce synthetic 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, and which are comparable in terms of stiffness to carbon-fibre composites. With this, the team has established a new world record in combining stiffness, strength and crack resistance in this class of bio-inspired materials. With the newly developed technology, it is now possible to produce mother-of-pearl-like materials with custom-made properties for the respective applications. Possible applications include aircraft manufacturing, aerospace and construction.