The hierarchical structure and mechanical performance of a natural nanocomposite material: The turtle shell

Turtle shells protect themselves from predatorial attack, which could provide ideas and pathways for bioinspired synthetic materials. Despite with relatively weak constituents and low biomineralization, turtle shells possess unusually robust mechanical properties due to their well organized and layered structures that are accurately designed from nanoscale to macroscale in nature. Minerals (calcium phosphate and calcium sulfate polycrystal mixture) randomly disperse in the keratin, and forming organic-inorganic nanocomposite platelets. Such platelets are basic building blocks that stack orderly and compactly in the radial direction, and constructing individual platelets into a layered micro-configuration. The outstanding tensile mechanical performance of turtle shells has much relationship with rehydration and the growth orientation of the keratin cells. Compressive mechanical properties, growth texture of keratin, topography and mineral components’ distribution of turtle shell are investigated by AFM experiment effectively. Such excellent mechanical properties of turtle shells, which integrated with nanocomposite ingredients and layered structure, may inspire the biomimetic strategies for advanced multi-functional materials, especially for artificial armor.