Microstructures and mechanical behavior of mollusk shells

Biological hard tissues are load-bearing structural materials, composed of a mineral (ceramic) component and an organic (protein) component. The microstructures of three composite designs commonly found in mollusk shells, the nacreous, foliated, and crossed-lamellar structures are discussed. Although all of these can be viewed as laminar composites, differences exist in the morphology, crystal structure, defect microstructure, and spatial organization of the mineral, CaCo{sub 3}. Electron microscopy investigations of these materials reveal a variety of crystallographic defects, the details of which have been used to refine existing models for biomineralization in these tissues. Flexural strengths in four-point bending are on the order of 100 Mpa. Examples of graceful failure'', i.e. stable crack growth in bulk samples, in the crossed-lamellar and foliated structures are reported. Fractography of such specimens reveals that a number of familiar toughening mechanisms operate together to produce this desirable structural flaw tolerance. Even more unusual is the fact that mollusk shells are harder than the native minerals from which they are made The damage zones and the anisotropy of hardness suggest an explanation for the anomalous hardness based on microstructural design rather than on intrinsic hardening of the mineral phase. 27 refs., 15 figs., 2 tabs.