Finally, the mineral bridges between the platelets increase toughness, shear strength, and stiffness 20, 22.
Moreover, nano-asperities found on platelet interfaces and platelet pull-out are important mechanisms that increase the shear strength and tensile toughness of nacre 17, 18, 19, 20, 21. Although only a tiny portion (<5%) of the material, the organic matrix serves greatly to toughen the nacre by dissipating energy as cracks open through bridging between the platelets and unfolding its protein structure 8, 15, 16.
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Crack deflection at both micro- and mesoscales, crack blunting, and microcrack formation all contribute to enhanced toughness by creating tortuous cracks of a limited extent and preventing catastrophic failures 6, 7, 8, 14. Nacre’s work of fracture is ~3000 times that of monolithic aragonite 7, and its high toughness has been attributed to various chemo-mechanical mechanisms generally related to its hierarchical structure (Fig. Each platelet behaves as a single crystal 10, and only small misorientation was found owing to nanoparticles and twinning domains 11, 12, 13. In many shells, the outer layer is composed of calcite prisms and the inner layer-nacre-has a “brick-and-mortar” structure, where ~500 nm thick aragonite (CaCO 3) platelets (>95% of mass) are interspersed with an organic matrix, mainly composed of elastic biopolymers, such as chitin and proteins 9. The shell is a composite, primarily made of two different polymorphs of calcium carbonate: calcite and aragonite. Among biological materials, mollusk shells have been extensively studied for their outstanding combination of stiffness, strength, and toughness 6, 7, 8, 9.
Most of the structural materials found in nature are characterized by multi-scale hierarchy, which confers them multiple functions and mechanical benefits 5. Inspired by nature, scientists and engineers have mimicked some of these materials to design advanced functional materials 3, 4. Organisms have evolved over geologic time to fine-tune their structural materials with multi-functionality and remarkable mechanical properties 1, 2.
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