Abstract：Although composed of relatively weak constituent materials (stiff but brittle minerals and tough but soft biopolymers), nacre can achieve excellent mechanical properties through fabulous designs of multiple structural hierarchies. At microscale, nacre exhibits the well-known “brick-and-mortar” structure, which has attracted considerable research attention. At nanoscale, the mineral platelet (i.e., “brick”) surfaces are featured with distributed nanoscale asperities, which is believed to play an important role in the interface strengthening and toughening. Based on the kinetic and contact analysis, a theoretical model was established herein to characterize the friction and energy dissipation behaviors between the neighboring mineral platelet surfaces in nacre. In the model, the equivalent interface friction coefficient, shear strength, and energy dissipation can be expressed as functions of two dimensionless geometrical parameters: the asperity aspect ratio α=A/l, and the ratio of asperity height to platelet thickness β=A/D. The theoretical predictions were compared with the finite element simulation results, and the good agreement validated the theoretical model. Further studies via the theoretical model led to the following three major findings: (1) the presence of asperities on the mineral platelet surfaces significantly enhances the interface friction, strength and energy dissipation during their relative sliding; (2) the equivalent interface friction coefficient increases with α, but is independent of β; (3) the increases of α and β can both improve the equivalent shear strength and frictional energy dissipation. These results and conclusions are of great significance not only for understanding the interface strengthening and toughening mechanism in nacre but also for guiding the design of related biomimetic composite materials. It is worth noting that the current work mainly focused on the role of mineral asperity, and did not take into consideration the influence of mineral platelet deformation along the interface, biopolymer matrix, mineral bridge, etc. To better understand the interface strengthening and toughening mechanism of nacre, the individual and synergistic roles of these possible factors should be further clarified, which correspondingly asks for more complicated theoretical and numerical models.
2021, Vol. 42 
