Biogenic minerals are often reported to be harder and tougher than their geological counterparts. However, quantitative comparison of their mechanical properties, particularly fracture toughness, is still limited. Here we provide a systematic comparison of geological and biogenic calcite (mollusk shell Atrina rigida prisms and Placuna placenta laths) through nanoindentation under both dry and 90% relative humidity conditions. Berkovich nanoindentation is used to reveal the mechanical anisotropy of geological calcite when loaded on different crystallographic planes, i.e., reduced modulus Er{104} ≥ Er{108} > Er{001} and hardness H{001} ≥ H{104} ≥ H{108}, and biogenic calcite has comparable modulus but increased hardness than geological calcite. Based on conical nanoindentation, we elucidate that plastic deformation is activated in geological calcite at the low-load regime (<20 mN), involving r{104} and f{012} dislocation slips as well as e{018} twinning, while cleavage fracture dominates under higher loads by cracking along {104} planes. In comparison, biogenic calcite tends to undergo fracture, while the intercrystalline organic interfaces contribute to damage confinement. In addition, increased humidity does not show a significant influence on the properties of geological calcite and the single-crystal A. rigida prisms, however, the laminate composite of P. placenta laths (layer thickness, ∼250-300 nm) exhibits increased toughness and decreased hardness and modulus. We believe the results of this study can provide a benchmark for future investigations on biominerals and bio-inspired materials.
Keywords: Calcite; Fracture behaviors; Mechanical properties; Nanoindentation.
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