In Nature, the combination of brittle minerals and organic molecules in highly sophisticated structures and designs results in materials (such as bone, wood or nacre) whose mechanicals properties far exceed those that can be achieved by a simple mixture of their components.
Producing new high performance materials with enhanced strength and toughness by copying and mimicking nature's hidden marvels is the essence of biomimicry. Biomimicry is a very appealing idea that has yielded only a few practical advances, until a team of researchers from the Lawrence Berkeley National Laboratory and the University of California recently managed to successfully mimic the structure of nacre to create one of the toughest ever produced ceramic [Munch et al., Science (2008) 322, 1516].
Through directional freeze casting of ceramic-based suspensions in water (Al2O3) and subsequent infiltration with a polymer (polymethylmethacrylate, PMMA), Munch et al. produced lamellar ceramic scaffolds. Pressing the lamellar structure perpendicular to its direction of extension and further sintering, result in a collapse of the entire edifice leading to the formation of a ‘brick-and-mortar’ structure in which the bricks are held together by ceramic bridges. While trying to replicate the microstructural design of nacre to perfection, Munch and co-workers have worked towards reducing the lamellae thickness and used some additive to the ceramic slurries in order to modify the viscosity and phase diagram of the solvent, thus leading to ice crystals with a microscopic roughness and bridge density similar to that existing in nacre.
They produced a series of Al2O3/PMMA hybrid composites with hierarchical structures spanning multiple length scales. By combining those two relatively ordinary phases (Al2O3 and PMMA) the authors have synthesized a bio-inspired ceramic-based material that exhibits a toughness that can be up to 300 times (in energy terms) that of its initial constituents and whose properties match those of engineering aluminium alloys.
While ‘these results highlight the tremendous potential of the biomimetic approach’ as stated by Munch et al., the next step of this research is to boost the ceramic content of the hybrid material up to 95 vol. % ceramic and decrease the lamellae thickness down to 2-3 nm, to achieve the unprecedented characteristics of nacre and provide additional nanoscale thoughening mechanisms similar to those acting in natural materials.