Overview

By definition, composite materials are formed from two or more materials that have quite different properties. The resultant material has a heterogeneous microstructure with extraordinary performance that displays a combination of the best characteristics of the component materials. Composites are widely used because their overall properties can be engineered through microstructural design to become superior to those of the individual monolithic counterparts.

Nature has provided some of the best-performing composites such as seashell nacre, bones, macadamia nutshells, wood and bamboo. These natural composites have superior mechanical efficiency in strength, hardness and toughness compared to many man-made composite materials. These biological composites display graded structures at several levels of hierarchy with length scales that range from micro- to nanometres. For instance, seashells have two to three orders of lamellar structure whilst bone has seven orders of hierarchy. Although these materials have complicated hierarchical structures, the most basic level of hierarchy occurs at the nanoscale where nanometresized hard inclusions are embedded within the soft protein matrix. In seashells and bone, this involves the fine dispersion of staggered aragonite bricks and mineralized fibrils, respectively. This nanostructure is expected to play an important role in the overall mechanical properties of biological materials. A bricks and mortar lamellar structure exists in nacre where the thickness of the aragonite bricks is several hundred nanometres. A well-defined organization of microstructure exists in seashells in the form of interlaced bricks (mineral platelets) glued together by protein layers. The length scale of the mineral platelets is normally in...