Artificial Nacre with High Toughness Amplification Factor: Residual Stress-Engineering Sparks Enhanced Extrinsic Toughening Mechanisms.

The high fracture toughness of mollusk nacre is predominantly attributed to the structure-associated extrinsic mechanisms such as platelet sliding and crack deflection. While the nacre-mimetic structures are widely adopted in artificial ceramics, the extrinsic mechanisms are often weakened by the relatively low tensile strength of the platelets with a large aspect ratio, which makes the fracture toughness of these materials much lower than expected. Here, it is demonstrated that the fracture toughness of artificial nacre materials with high inorganic contents can be improved by residual stress-induced platelet strengthening, which can catalyze more effective extrinsic toughening mechanisms that are specific to the nacre-mimetic structures. Thereby, while the absolute fracture toughness of the materials is not comparable with advanced ceramic-based composites, the toughness amplification factor of the material reaches 16.1 ± 1.1, outperforming the state-of-the-art biomimetic ceramics. The results reveal that, with the merit of nacre-mimetic structural designs, the overall fracture toughness of the artificial nacre can be improved by the platelet strengthening through extrinsic toughening mechanisms, although the intrinsic fracture toughness may decrease at platelet level due to the strengthening. It is anticipated that advanced structural ceramics with exceeding performance can be fabricated through these unconventional strategies.