Microstructural Characterizations And Mechanical Behavior Of Clinocardium Californiense Shell

The biological mineralized materials, such as bones, teeth, shells, have superior mechanical properties to traditional synthetic materials, and they often possess an assembly of complex structures for adapting to environmental and functional needs although the basic unit of main inorganic phase is the most common materials. Shells, as a kind of natural biological materials, have draw attentions by investigators and designers because of its unique structures and excellent mechanical properties. In the present work, a kind of natural biological material called Clinocardium californiense shell was selected as the target material, and its microstructure, crystal compositions, Vickers hardness, Young's modulus, indentation toughness and the quantitative relationship between indentation-induced cracks and indentation toughness were studied by using optical microscopy (OM), scanning electrical microscopy (SEM) and micro-hardness tester for the in-depth investigation on the structure-related mechanical behavior of this shell.The C. californiense shell can be divided into three parts, i.e., the dorsal side, body part and marginal side based on the variation of microstructures along the longitudinal cross-section. Specifically, all areas exhibit a cross-lamellar structure on the dorsal side, a hierarchical structure comprising three layers including inner (with a cross-lamellar structure), middle (with a complex cross-lamellar structure) and outer (with a prismatic structure) layers was observed on the body part, while on the marginal side, the orientation of aragonite sheets shows an obvious deflection in addition to the variation of the arrangement of sheets. Thus, this shell shows an obvious anisotropic characteristic.XRD analysis of the different parts of C. californiense shell demonstrates that the main phase composition of all three parts are crystalline CaCO3 (Aragonite), although there are some differences in diffraction peaks for different parts.The distributions of hardness H and Young's modulus E of three parts were investigated, which shows that the structural architecture, the dimensions of different-order lamellae, the orientation between the indenting direction and aragonite sheet all contribute to the unique mechanical properties of this shell. It is also found that a low value of the ratio of hardness to Young's modulus (H/E) corresponds to an improved toughness. The middle layer with the densest and most complicated structure on the body part shows the highest hardness, the highest Young's modulus and the lowest HIE ratio (corresponding to the highest indentation toughness).The fractal analysis of the relationship between the indentation-induced cracks and indentation toughness shows that a larger value of fractal dimension Dc may reflect a higher indentation toughness Kc and more complicated structural architecture. The measured fractal dimension Dc for the indentation-induced cracks can be well adopted as a measure of indentation toughness Kc of C. californiense shell.