Mother of pearl recovered after 80% ultimate strain

Mother of pearl recovered after 80% ultimate strain
Transmission electron microscopy of a pearl plate indentation experiment J. Gim et al. / Nature Communications, 2019

Scientists have measured the ability of mother of pearl to resist mechanical stress at the nanoscale. It turned out that this material, which is considered the most mechanically strong natural substance, that is, requiring the greatest effort to fracture, is elastically restored after deformation corresponding to 80 percent of the limit. The internal structure of the material returned to its original state after loads of 1.2 gigapascals (approximately 12 thousand atmospheres).

There are several different mechanical parameters of substances that are responsible for different aspects of resistance to external loads. It is necessary to distinguish between hardness, impact strength and stiffness: the first is responsible for the reaction to slow pressure, the second characterizes the ability to absorb energy upon impact, and the third is associated with resistance to deformation.

Depending on the selected parameter, various substances may be called the “strongest” in a non-strict sense. In particular, the hardest known substance is diamond, but in terms of impact strength, it is not only not a champion, but also refers to fragile materials. The goal of many studies in the field of materials science is to search for both solid and durable substances.

The properties of the material may depend on its structure, which greatly complicates the analysis and increases the number of possible options many times over. It is known that mother-of-pearl, the substance lining the inner surface of shells of some mollusks, has the highest impact strength among natural materials.

Scientists already know that this property is determined precisely by the microstructure: mother of pearl consists of microscopic plates of the inorganic mineral aragonite (crystals of calcium carbonate CaCO 3 ), between which is an organic matrix of biopolymers such as chitin. As a result, nacre is approximately 40 times more resistant to cracking than monolithic crystals of calcium carbonate.

The small-scale structure of mother of pearl is unattainable for modern technology. Mankind has learned to create materials with higher mechanical properties, but this requires special conditions, such as high temperature and pressure, while mollusks synthesize a protective shell without such tricks. Together, these circumstances make mother of pearl an extremely interesting substance from the point of view of materials science.

Scientists from the US, Australia, France and Germany, under the direction of Robert Hovden (Robert Hovden) from the University of Michigan studied the mechanical properties, resistance to deformation and ultimate strength clamshell noble pinny ( Pinna nobilis ). The researchers cut out pieces of mother-of-pearl about 100 nanometers in size, pressed on them with the tip of an intender, while illuminating the sample with a beam of electrons, which made it possible to measure the stresses arising in the thickness.

It turned out that the application of sufficient force compresses the aragonite plates until the occurrence of surface contact, in which they actually form a continuous substance. In this state, the voltage begins to redistribute over a significantly larger volume.

Nevertheless, this change turned out to be reversible: if the external compression did not exceed 1.2 gigapascals, then after its removal the plates were separated and returned to their original position, preserving the mechanical properties.

The researchers’ results also allowed us to confirm that when a crack occurs in the material, it propagates along only one plate, leaving the others intact, which allows the mother of pearl to maintain properties under repeated high loads. The authors believe that their work will allow the development of new nanocomposites that will be lighter and stronger than modern ones, and also do not require extreme conditions for production.