A high-strength and high-toughness nacreous structure in a deep-sea Nautilus shell: Critical role of platelet geometry and organic matrix

Abstract

To explore the differences in mechanical behavior of nacre between shells that live in different water depths, the microstructures, phase composition and related mechanical properties of nacre under indentation, three-point bending and shear tests in deep-sea Nautilus and freshwater Cristaria plicata shells were systematically investigated. It is found that the nacreous structure in Nautilus shell exhibits an outstanding combination of high strength and high toughness compared with that in C. plicata shell, attributing to its larger aspect ratio of platelet and interfacial shear resistance. Specifically, the interfacial resistance is mainly generated from the adhesion of organic matrix and friction caused by nano-asperities on platelet surfaces. According to the interfacial resistance model, the stiction force originated from organic matrix adhesion is sensitive to its content, and the friction force produced by nano-asperities presents a positive correlation with their distribution density and dimension. Hence, the higher content of organic matrix of nacre with denser and larger nano-asperities on platelet surfaces in Nautilus shell contributes to a higher interfacial resistance. Therefore, it is the coupled effects of platelet geometries (i.e. aspect ratio and nano-asperity) and organic matrix that result in the high-strength and high-toughness nacreous structure in Nautilus shell, which is thus more conductive to inhabit in the deep sea with extremely high pressure. The present research findings are expected to provide beneficial references for the design of strong and tough nacre-inspired materials with appropriate platelet geometry and content of soft phase.