Construction of nacre-mimetic composites with a “brick-and-mortar” architecture based on structural defects in ice-templating

The construction of a hierarchically ordered "brick-and-mortar" structure with a high content of "brick" phase is a key challenge in the preparation of nacre-mimetic composites. Herein, inspired by the transverse cracks in the ceramic lamellae after freeze-drying caused by the secondary nucleation of ice crystals during directional freezing of water-based ceramic slurry, we propose a new idea to use these structural defects and an in-situ reaction to construct composites with a nacre-like "brick-and-mortar" structure and high hard-phase content. This uses ice-templating to first prepare lamellar TiO2 scaffolds with transverse cracks that separated the ceramic lamellae into "brick" whose size was controlled by adjusting the initial ceramic particle size (50, 300, and 700 nm). After the reactive infiltration, these cracks were filled with molten Al, leading to the formation of a "brick-and-mortar" structure with a high hard-phase content (86 vol%). The increasing ceramic particle size reduced the hard-phase content but significantly increased the aspect ratio (height/width) of the ceramic "brick" and the fracture toughness. The excellent damage tolerance was attributed to multiple toughening mechanisms, i.e., multiple cracking, crack deflection, and uncracked-ligament bridging of the metal layers. The existence of "mortar" increased the probability of the formation of multiple cracking.