Nacre Toughening Due to Cooperative Plastic Deformation

Among biological materials, mollusk shells have been extensively studied for their outstanding combination of stiffness, strength, and toughness. The shell is a composite, primarily made of two different polymorphs of calcium carbonate: calcite and aragonite, with the outer layer being composed of calcite prisms and the inner layer—nacre—having a “brick-and-mortar” structure. The shell’s outstanding mechanical properties have been mostly attributed to the interplay between aragonite platelets and organic matrices. Here, we show that crystallographically co-oriented stacks of aragonite platelets, in both columnar and sheet nacre, define another hierarchical level that contributes to the toughening of nacre. By correlating piezo-Raman and micro-indentation results, we quantify the residual strain energy associated with strain hardening capacity. Our findings suggest that the aragonite stacks, with characteristic dimensions of around 20 µm, effectively store energy through cooperative plastic deformation. The existence of a larger length scale beyond the brick-and-mortar structure offers an opportunity for a more efficient implementation of biomimetic design.

Learn more (opens external site)