Micro/nanomechanical characterization of a natural nanocomposite material - the shell of Pectinidae

Micro/nanomechanical characterization of the shell of a scallop, a member of the Pectinidae family, has been carried out. Hardness and elastic modulus were measured by nanoindentation using a nanoindenter. Micro/nanoscale cracks were generated by microindentation using a microindenter. The shell's crossed lamellar structure and indentation cracks were imaged using an optical microscope, an atomic force microscope and a scanning electron microscope. It was found from nanoindentation tests that the shell exhibits a hardness of about 5 GPa and elastic modulus of about 87 GPa. Nanoindentation resulted in pile-up around the indent. In the middle and bottom layers primary cracks propagate along the first-order lamellar boundaries and numerous secondary cracks branch off along the second-order lamellar boundaries. The additional energy required for crack propagation results from the secondary cracks along the second-order lamellar boundaries. Cracks formed in the top layer of the shell do not show the crack diversion mechanism due to the lack of first-order lamellar organization. Fracture mechanisms were discussed in conjunction with architecture, hardness, elastic modulus, and energy-dissipation during cracking.