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 that displays an exceptional combination of strength and fracture resistant behavior, despite being constituted of weak mineral building blocks. NCCR Bio-Inspired Materials Principal Investigator Prof. André Studart and his team have been studying the structure–property correlations observed 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 aligned alumina microplatelets inter-connected by titania mineral bridges.

According to Studart and his colleagues, these bridges play an important role on 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, it 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 ability to carry mechanical load. Because the model synthetic material is structured from ceramic constituents at the same length scale as biological nacre, it sheds, according to the study’s authors, 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 fibre composite materials,” explains Dr. Kunal Masania.

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 be in the use of sacrificial surface layers against abrasion in turbine blades, by 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.