Mother of pearl inspires better composites

NCCR Bio-Inspired Materials researchers at the Swiss Federal Institute of Technology in Zurich (ETHZ) are investigating improved composites, using nacre found in mollusk shells as an example of lightweight materials with exceptional strength.

Nacre, commonly called mother of pearl, is a biological composite found in mollusks such as abalone. It displays an exceptional combination of strength and fracture-resistant behavior, despite being constituted of weak mineral building blocks. Viewed under a scanning-electron microscope, the material looks like a miniature brick wall with mortar joints. The bricks are actually nano-scale calcium carbonate plates that are stacked upon one another and connected by mineral bridges, while the spaces in between are filled with an organic ”mortar”.

NCCR Bio-Inspired Materials Principal Investigator Professor André Studart and his team have been investigating the structure-property correlations in nacre with the goal of establishing guidelines for lightweight composite design. To achieve this, they fabricated tunable nacre-like brick-and-mortar composites from magnetically aligned alumina microplatelets that are interconnected by titanium dioxide mineral bridges, and sealing off the empty spaces with a polymer.

According to Studart and his colleagues, these bridges play an important role in the fracture resistance of nacre-inspired materials, and the researchers established a model that isolates and quantifies the influence of mineral bridge density on the composite’s fracture properties. In other words, the model allows them to evaluate how many of these bridges are necessary to maximize the fracture strength of the composite.

The results of this research, published in the influential journal Proceedings of the National Academy of Sciences, allowed the scientists from the ETHZ Complex Materials group to demonstrate that the number of mineral bridges per platelet can be tuned to enhance the composite’s ability to carry a mechanical load. Because the model synthetic material is structured from ceramic constituents at the same length scale as biological nacre, it sheds light on the fundamental role of mineral bridges in natural brick-and-mortar structures while also demonstrating outstanding mechanical properties that compete with state-of-the-art composite materials.

“We are able to create composites with an unparalleled combination of strength and fracture resistance, achieving a new record within nacre-like composites, while demonstrating the same stiffness as high-performance carbon fiber composite materials,” explains Studart.

According to the researchers, these design principles can be directly and easily integrated into composites. The proposed technique could be used to tailor regions of a composite for complex mechanical load introduction, such as bolted joints. Other possible applications could include the use of sacrificial surface layers against abrasion in turbine blades, mounting nacre-like architectures to the leading edge of a foil for aircraft for impact resistance, or even on high friction surfaces where wear and abrasion are of paramount importance.

Reference: Grossman, M.; Bouville, F.; Masania, K.; Studart, A. R. Quantifying the role of mineral bridges on the fracture resistance of nacre-like composites, PNAS, 2018, 115, 12698-12703.