Nanomechanical Properties of a Biocomposite, Mollusk Shell Nacre

Abstract

Mechanical properties of the structural elements of mollusk hard tissues were determined using nanoindentation techniques. Red abalone (Haliotis refuscens) has a dual laminate ceramic/polymer biocomposite, evolved for protection against impact. The nacre (mother-of-pearl) section of the shell has excellent bending strength (180 MPa) and fracture toughness (12 MPa m1/2), orders of magnitude stronger and tougher than monolithic CaCO3, it's primary component. The outer prismatic structure is comprised of columnar calcite (rhombohedral CaCO3) crystallites oriented normal to the shell surface and the inner nacreous structure consists of layers of pseudo-hexagonal shaped aragonite (orthorhombic CaCO3) platelets arranged in a brick and mortar microarchitecture (Fig. 1). The platelets (0.25 μm thick and 5 μm edge length) are surrounded by a thin layer (10-25 nm) of organic phase within and between the individual layers. This sub-um layered structure results in excellent bulk properties, not present in man-made composites of similar phase composition.