Nacre toughening due to cooperative plastic deformation of stacks of co-oriented aragonite platelets

Nacre's structure-property relationships have been a source of inspiration for designing advanced functional materials with both high strength and toughness. These outstanding mechanical properties have been mostly attributed to the interplay between aragonite platelets and organic matrices in the typical brick-and-mortar structure. 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 mu m, 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. The hierarchical structure of nacre is known to contribute to its high strength and toughness, providing inspiration for many biomimetic materials. Here, co-oriented 20 mu m stacks of aragonite platelets are shown to contribute to the toughness of nacre, defining a new characteristic length scale.