Mother of pearl, the rainbow-colored material that lines the insides of shells and other shells, is known as the hardest material in nature. Now, a research team from the University of Michigan has shown exactly how it works in real time.
The combination of mother-of-pearl hardness and resilience, better known as mother-of-pearl, has astonished scientists for more than 80 years. If humans could mimic it, it could lead to a new generation of ultra-strong plastics for structures, surgical implants, and countless other applications.
"We humans can produce harder materials in unnatural environments, such as extreme heat and pressure, and we can not repeat the kind of nanotechnology that molluscs have achieved, and the combination of the two approaches could be a spectacular new generation of Materials lead, and this paper is a step in that direction, "said Robert Hovden, Deputy Professor of UM Materials Science and Engineering.
Researchers have known the basics of pearlescing secrecy for decades – it consists of microscopic "stones" of a mineral called aragonite, coated with a "mortar" of organic matter. This arrangement of bricks and mortar clearly gives strength, but nacre is far stronger than the materials suggest.
Hovden's team, which included Jiseok Gim, a research associate for UM materials science, as well as geochemists from Macquarie University, Australia and other universities, worked together to solve the mystery.
At the Michigan Center for Materials Characterization of UM, the researchers employed tiny piezoelectric micro-depressors to exert force under a ballpoint pen sleeve on the shells of Pinna nobilis, commonly known as a noble pens pod, electron microscope. They watched what happened in real time.
They found that the "stones" are actually multi-sided tablets only a few hundred nanometers in size. Normally, these tablets remain separate, layered and padded with a thin layer of organic "mortar". However, when the trays are loaded, the "mortar" squeezes to the side and the tablets lock and form a substantially solid surface. When the force is removed, the structure recoils without losing strength or elasticity.
This elasticity differs by itself from the most advanced man-made materials. For example, plastics can rebound by impact, but lose some strength each time. Mother-of-pearl has lost none of its elasticity in repeated strokes up to 80% of its yield strength.
When a crack forms, nacre confines the crack to a single layer rather than allowing it to spread, leaving the shell's structure intact.
"It is unbelievable that a mollusk that is not the most intelligent creature makes so many structures on so many scales," said Hovden. "It's about making single molecules of calcium carbonate and arranging them into nanolayers glued together with organic material, right through to the structure of the shell, which combines mother-of-pearl with several other materials."
Hovden believes that people can use the shell methods to produce nanotechnologically produced composite surfaces that are significantly lighter and stronger than those available today.
"Nature gives us these highly optimized structures with millions of years of evolution," he said. "We could never do enough computer simulations to develop them – they are just there for us to discover."
The study was published in Nature Communications .
Materials scientists learn how mother-of-pearl is made
Jiseok Gim et al., Nanoscale Deformation Mechanism, Reveals the Resilience of the Pinna nobilis Shell in Nacre, Nature Communications (2019). DOI: 10.1038 / s41467-019-12743-z
Uncovering the mystery of nature's hardest material (2019, 23 October)
retrieved on October 23, 2019
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