Investigation On Stability-mechanical Property Relationship Of Hierarchical Structure Of Shell Nacre

Nature has evolved multiple highly structural and functional materials through bottom-up nanofabrication approach. Among them, the mollusc shells are very representative biomineral structure. Their dominant component is approximately 95 weight% calcium carbonate, either calcite or aragonite. The crystallization of calcium carbonate is strictly controlled by organic macromolecules (proteins, glycoproteins, and polysaccharides) secreted by the mantle. Nacre (mother of pearl) is the main stacked microstructure found in the interior of many mollusc shells. Due to the inorganic-organic composition, highly ordered stacking manners and inner sub-micro/nanostructure, nacre can sustain significant inelastic deformation and exhibits amazing toughness, comparing with the man-made ceramic composites. Therefore researches on nacre have aroused widespread interest from areas of mechanics, material science and other interdisciplines. However, main work has been centered on the stacked microstructure and correlative mechanical behavior. Though at the smaller length scales some important works have been reported about the nanocomposite structure within nacre, many secrets still need to be explored. For example, how is the nanostructure and mechanical properties of individual aragonite platelets in nacre? What role can organic matrix play in the toughening and strengthening mechanisms of nacre at different length scales, especially at nanometer scale? In this work the nacre from shells of Haliotis discus hannai was studied with chemical and thermal analyzing techniques, nanoindentation and bend test, coupled with advanced micro/nanostructure characterizing methods and finite element modeling. The hierarchical structure of nacre and its stability and mechanical behavior have been systematically investigated and the following results are achieved:(1) Basing on the possible intrinsic difference on chemical and mechanical stabilities of nacre at different length scales, the hierarchical structure and mechanical properties of shell nacre was studied from a new perspective of chemical stability and chemical-mechanical coupling. Firstly, combining chemical deproteinization and demineralization methods with micro/nanoscale characterizing techniques, it is found that nacre of Haliotis discus hannai is a stacked microstructure with aragonite platelets and interplatelet organic matrix thin layers. A single platelet is further a nanocomposite structure of nanoparticles and intraplatelet organic matrix framework. Size and uniformity of the nanoparticles and the distribution of the intraplatelet organic matrix have great distinction for different platelets. Then both interplatelet and intraplatelet organic matrix can be decomposed by sodium hydroxide solution, but the nanocomposite structure of individual aragonite platelets has the higher chemical stability than the stacked microstructure of nacre. Lastly, through macroscopic bend tests and nanoindentation experiments carried upon the sodium hydroxide-etched stacked microstructure and nanocomposite platelet respectively, it is found the modulus and strength of the former are reduced more significantly. Therefore individual aragonite platelets have the higher chemical-mechanical stability than the stacked microstructure of nacre.(2) Basing on the idea that the hierarchical structure of nacre possibly exhibits quite distinct thermal stability and mechanical properties at different length scales, mechanical tests coupling with thermal loads was designed elaborately. It is found that the thermal stability of the stacked microstructure of nacre is much worse than its inner primary element—nanocomposite structure of aragonite platelet. Macroscopic bend tests show that at 250℃Young's modulus and flexural strength of the microstructure of nacre are almost decreased to zero and its layer-by-layer stacked structure is destroyed completely. However the modulus and hardness of the aragonite platelet present great decrease until at 400℃and its nanocomposite structure is collapsed. Therefore a single aragonite platelet has much higher thermal-mechanical stability, comparing with the stacked microstructure of nacre.(3) Basing on the above-mentioned investigation into thermal and chemical stabilities and mechanical properties of the hierarchical structure of nacre, it is proposed that organic matrixes at different length scales have intrinsic difference about their stabilities and toughening mechanisms to nacre. Combing mechanical tests at macro/nano scales with thermogravimetric analysis, it is confirmed that interplatelet organic matrix generally made up of insoluble and nonextractable proteins and intraplatelet organic matrix made up of soluble polyanionic proteins have toughening and strengthening effects on the micro/ nanostructure within nacre, respectively. Then interplatelet and intraplatelet organic matrixes are quite distinct on chemical and thermal stabilities. Interplatelet organic matrix is about 80 weight% of all organic matrixes and chiefly decomposed before 400℃, while the other intraplatelet organic matrix is mainly decomposed after 400℃. Especially, the thermal decomposition of only 20% of interplatelet organic matrix is enough to destroy the stacked microstructure of nacre thoroughly. is proven to be an effective way to characterize the hierarchical structure like nacre at different length scales and reveal the structure-function correlation.