Energy-driven microstructure and mechanical properties of mineral-organic biocomposite - Pteria penguin shell

Abstract

The high performance of biocomposites largely stems from their ability to self-organize. Our research on the Pteria penguin reveals a hierarchical columnar calcitic (CC) microstructure essential to its mechanical properties. Electron backscatter diffraction shows that each column constitutes a near-to-single-crystal composed of nanounits. Growth is stress-assisted, causing one-sided bending and considerable orientation differences between the top and bottom of columns. The arising stresses are efficiently relaxed by using division along low-energy planes. The detected phenomenon can underlie the branching process observed in more advanced microstructures resulting from species’ adaptations over time. The stresses can also be reduced by the frequent formation of unusual twins resembling intra-columnar flow zones. Using a custom program to detect crystal lattice matching in reciprocal space reveals that a low-energy plane initiating a well-developed column is used by the neighbors, forming coherence zones. Long-range hierarchical order is also supported by the organic matrix with hexagonal channels, geometrically compatible with calcite crystals. Consequently, columns with the same orientation or in twin relation, based on a 60◦ rotation around the c-axis, are formed in a direct neighborhood or at a distance. The final biocomposite structure enables effective load transfer and crack suppression, achieving a notable compressive strength of 670 MPa, despite being built from weak components. The developed programs for identifying microstructural cohesion in crystalline materials allow the implementation of the discovered universal rules of hierarchical self-organization in 3D printing technology. This process can be enhanced by artificial intelligence (AI)-accelerated multiscale simulations.