Crystal structure, microstructure, and mechanical properties of heat-treated oyster shells

We investigated the crystal structure and mechanical properties of oyster shells subjected to heat treatment under increasing temperature conditions. The shell contained folia and chalky layers. The folia layer comprised two CaCO₃ phases: 72.3% calcite and 27.7% aragonite. The lattice parameters of the calcite and aragonite present in the folia layer did not correspond to those of the synthesized sample. The anisotropic lattice expansion was observed in calcite and aragonite in the folia layer during heat-treatment. The chalky layer has also the anisotropic lattice expansion, but the expansion was disappeared at 573 K. The microhardness (HV value) of the folia layer decreased rapidly from 122 to 11 HV at temperatures 573-673 K owing to the phase transformation from aragonite to calcite in this temperature range. The microhardness of the chalky layer at RT was 125 HV, which decreased to 15 HV at 373 K. Crack propagation with increasing temperature was investigated using a micro-Vickers apparatus. In the folia layer, cracks were produced inside the prism, and they propagated along the lamellar structure. The cracks initiated and propagated along the organic biopolymer interlayers in a zigzag manner. No cracks were observed in the chalky layers of the heat-treated samples. The toughness of the chalky layer was superior to that of the folia layer. From our results, we can conclude that oyster shells comprise two types of materials with different mechanical properties.