Crystal Orientation, Nanostructure And Fracture Behavior Of Aragonite Tablets In Pinctada Maxima Nacre

Nacre is a natural organic-inorganic composite synthetized by mollusks. Over hundreds of million years of natural selection, nacre evolved superior mechanical properties. Compared with geological aragonite, nacre attains a 1000 times higher toughness and 3000 times higher fracture work, respectively. All of nacre’s robust mechanical performances benefit from its elaborate-designed hierarchical architecture. In this consideration, investigating its hierarchical architecture and unrevealing its structure-function relationship can shed additional light on the design and fabrication of high-performance artificial composites.In the present study, we select nacre from Pinctada maxima as a research object and utilize X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and nanoindentation instrument to probe its composition, structure and mechanical properties. Results show that the nacre of Pinctada maxima exhibits a brick-and-mortar structure, in which aragonite tablets are sandwiched with interlamellar organic layers. The hardness values for the top-surface and cross-section are 3.19±0.06 GPa and 3.30+0.07 GPa, respectively, and the corresponding elastic modulus values are 53.70+0.23 GPa and 82.64±2.50 GPa. In contrast to the cross-section, top-surface displays a higher value of H/E, thus possesses a higher wear resistance.Then, we conduct X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy (TEM) observations to investigate the orientation of the aragonite tablets in Pinctada maxima nacre. The systematic nanolath morphology on the (001) surface of aragonite tablets is observed after acidic etching and mechanical polishing. The nanolaths are along the [100] crystallographic orientation of aragonite crystal. Interestingly, most of the tablets share the same nanolath orientation (referred to as the main orientation), which is roughly parallel to the growth line of the nacreous shell, while the others tilted by～30°and 60°with respect to the main one. The preferential orientation of (100), (110) and (130) planes in block nacre should be attribute to the oriented alignment of the numerous tablets.In addition, nanostructure inside the individual aragonite tablet of Pinctada maxima nacre is investigated using transmission electron microscopy techniques. Results indicate that individual tablet possesses a single-crystal nature. The intracrystalline organics are trapped within a continuous crystalline scaffold in an islet-like and sheet-like form, respectively. Moreover, the aragonite tablets are beam-sensitive. In situ irradiation experiments demonstrate that exposure to the electron beam at high magnification for seconds can transform the tablet into polycrystals, leading to the destruction of its intrinsic nanostructure.At last, the fracture behavior of individual aragonite tablet from Pinctada maxima is investigated. The results suggest that intracrystalline organics regulate the fracture mode of the tablet to a large extent. By deflection of cracks, blunting of crack tips, formation and reorientation of nanoparticles and deformation of intracrystalline organics, the tablets spare no effort to achieve energy dissipation. The enhancement of the tablets’energy dissipation efficiency promotes the damage localization of the nacre, thus benefits to maintain the structural and functional integrity of the shell.