Crystal structure and mechanical properties of the nacre and prismatic layers in abalone shells

Advancements in the research of natural biocomposites are enhancing the design of bioinspired materials. However, to determine safe operational limits for high-temperature engineering applications, it is essential to comprehensively understand their mechanical performance under thermal loads. This study examined the lattice expansion behavior and mechanical properties of the nacre and prismatic layers of heat-treated abalone shells. Anisotropic expansion along the three crystallographic axes was observed in aragonitic nacre at room temperature, which was associated with deformation of the V[CaO₉] polyhedron. Several Ca-O bonds contributed to the deformation of this polyhedron. The prismatic layer also exhibited anisotropic expansion at temperatures up to 623 K. The V[CaO₆] polyhedron underwent deformation, and the Ca-O bond within the polyhedron was associated with anisotropic expansion along the c-axis. The anisotropic expansion of the nacre relaxed at 473 K, whereas that of the prismatic layer persisted up to 623 K. Across all heat-treatment temperatures, nanoindentation measurements showed that the nanohardness of the prismatic layer exceeded that of the nacre. A similar trend was observed for the elastic modulus. A comparative analysis of the mechanical properties of the nacre and prismatic layers revealed that nacre has superior toughness, whereas the prismatic layer exhibits greater hardness. Both properties substantially deteriorated after heat treatment at 473 K.