The mollusk shells (like bivalves) are biocomposite materials composed of calcium carbonate (CaCO3) crystals in the form of calcite or aragonite embedded within the organic matrix. They may be organized in different spatial configurations or hierarchical structures encompassing several layers. One of the most common structures, i.e. prismatic columnar, is built of long calcite lamellas elongated in the growth direction. The prisms are separated by thin organic sheaths controlling the nucleation process and determining the orientation as well as the growth of calcium carbonate crystals. This structure demonstrates unusual mechanical behavior due to the fact that it presents high compressive strength, hardness, stiffness and it is lightweight at the same time. Bearing in mind the aforementioned properties, the shells are natural inspiration to create synthetic, ceramic-based composite material with superior strength and low mass at the same time. Therefore, detailed microstructural characterization of shells could give an explanation for their unusual mechanical behavior.
In this work, the microstructure and misorientation distribution function (MDF) of the selected bivalves shells using SEM, X-ray computed tomography and EBSD was evaluated. The microstructure observations revealed that the calcite layer contains many long prisms having about 50 μm in width, but it changes with the distance from shell’s edge. Moreover, the MDF calculations demonstrate that the selected species of mollusks prefer strictly defined set of misorientations which are characterized by low energy boundaries. What is more, the calcite grains are not randomly oriented, but in such a way to form strong bonding.