Sapphire
Nacre – Model system
Natural composites can achieve mechanical properties far beyond those of their constituent materials, due to their underlying sophisticated microstructures. A typical example is nacre (also known as mother-of-pearl), which constitutes the inner layer of mollusk shells. Nacre is a brick-and-mortar like composite consisting to 90-95% (volume fraction) of hard aragonite tablets that are bonded by a soft organic material. We find that the deformation induced by nanoindentation spreads widely into nacre rather than concentrating in a small region underneath the indentation. Delocalization of the strain energy in the structure suppresses nucleation/propagation of cracks and, therefore, increases the fracture toughness of the composite. Furthermore, a cross-sectioned TEM sample in thin membrane form (~100 nm in thickness) extends the elastic limit far beyond its bulk counterpart, displaying a potential for rigid, tough but resilient substrate applications.
Conch shell – The toughest material
As a natural biocomposite, Strombus gigas, commonly known as the giant pink queen conch shell, exhibits outstanding mechanical properties, especially a high fracture toughness. It is known that the basic building block of conch shell contains a high density of growth twins with average thickness of several nanometres, but their effects on the mechanical properties of the shell remain mysterious. We revealed a toughening mechanism governed by nanoscale twins in the conch shell. A combination of in-situ fracture experiments inside a transmission electron microscope, large-scale atomistic simulations and finite element modelling show that the twin boundaries can effectively block crack propagation by inducing phase transformation and delocalization of deformation around the crack tip. This mechanism leads to an increase in fracture energy of the basic building block by one order of magnitude, and contributes significantly to that of the overall structure via structural hierarchy.
Limpet Teeth – The Strongest Natural Materials with Auxecity
Materials with negative Poisson’s ratio, commonly referred to as auxetic materials, have attracted considerable attention due to their superior mechanical properties such as enhanced shear resistance, indentation resistance, and fracture toughness. While several structural mechanisms giving rise to auxecity have been suggested in the past (e.g., nonaffine deformation, noncentral force interaction, and chiral structure), a microstructure that unites auxeticity with both high strength and high stiffness is rare. Combining in-situ nanomechanical testing with microstructure-based micromechanical modeling, we show that limpet teeth possess a unique microstructure uniting auxecity with ultrahigh strength (~3.6 GPa). Specifically, the microstructure in the leading part of limpet teeth consists of an amorphous hydrated silica (SiO2•nH2O) matrix embedded with bundles of single crystal iron oxide hydroxide (a-FeOOH, goethite) nanorods arranged in a pseudo-cholesteric pattern. During deformation, this microstructure allows local coordinated displacement and rotation of the nanorods, enabling auxetic behavior while maintaining one of the highest strengths among natural materials. Key microstructural features leading to these properties include the pseudo-cholesteric arrangement of nanorods at appropriate volume fractions and aspect ratios, the anisotropic elasticity of nanorods with a goethite crystal structure that tends to align its compliant axis to the loading direction, as well as a nanometer-thick interfacial phase facilitating strong cohesion and effective stress transfer between the nanorods and matrix. These findings rationalize the superior ability of limpet teeth to resist contact-induced damage, wear, and fracture, and lay a foundation for designing biomimetic auxetic structures combined with extreme strength and high stiffness.
Chiton teeth – the hardest material
Cryptochioton Stelleri, one of the biggest chiton species, has Conveyer-belt like radular consists of many teeth. To scape the algae from the rocks, which is how they feed themselves, its teeth has been developed to mitigate the stress and damage. Among the biomaterials, C. Stelleri teeth are reported as a hardest biomaterial found in nature. Using our State-of-the-Art Transmission Electron Microscope, We found the graded microstructure from surface to bulk region. Since the hardness of the material is governed by the surface microstructure, We believe that graded surface microstructure will play important role on hardness of Chiton Teeth. By combining in-situ TEM deformation experiment, we are digging into the secret of nature at atomic scale to get inspiration from nature.
