Exploring the link between microstructure, order, and toughness in bioinspired composites

Composites with intricate microstructures are ubiquitous in the natural world where they fulfil the specific functional demands imposed by the environment. For instance, nacre presents a fracture toughness 40 times higher than its main constituent, a crystalline form of calcium carbonate. This relative increase in toughness value is obtained as a crack propagating within this natural brick-and-mortar structure must interact with multiple reinforcing mechanisms, leading to a millimetre-sized process zone. The boost in performance obtained has pushed scientists for a few decades to use nacre as a blueprint to increase the toughness of synthetic ceramics and composites. Our ability to reproduce accurately the structure of nacre from the nanometre to the millimetre scale has improved with the introduction of Magnetically-Assisted Slip Casting (M.A.S.C.), a technique that combines an aqueous-based slip casting process with magnetically-directed anisotropic particle assembly. Using this technique, we can now fine tune the structural properties of nacre-inspired alumina-based composites to reach strengths up to 670 MPa, KIC up to 7 MPa.m1/2 with subsequent stable crack propagation and this even at temperature up to 1200°C. While these materials already present interesting properties for engineering applications, we fail to see the large process zone that are acting in natural nacre. This led us to work on a new composite system, using this time monodisperse silica rods that can self-assemble into bulk colloidal crystals to finally test the effect of order in the microstructure on the toughness. The presence of this regularity in the microstructure proves crucial in enabling a large process zone. We obtained a 40-fold increase in toughness compared with the polymer use as a matrix in a composite made of 80% in volume of ceramic, all of which is processed at room temperature. From these two studies, we can extract the role of the interface and grain morphology in tough bioinspired composites and what will be the next steps for these materials.