Visualizing the toughening origins of gel-grown calcite single-crystal composites

Biogenic single crystals have been widely demonstrated to incorporate macromolecules to achieve extra damage tolerance, spurring investigations on their synthetic analogs with enhanced mechanical properties as well as the enhancement mechanism(s) behind. And the investigations rely on both rational design of the single-crystal composites and, equally importantly, nanoscale and in-situ characterization strategy. Here, composite structures are constructed inside the calcite single-crystal host by incorporating guest materials of agarose fibers, multi-walled carbon nanotubes (MWCNTs), and graphene oxide (GO), through crystallization in agarose gel media. Further, transmission electron microscopy-scanning probe microscopy (TEM-SPM) method, coupling compression measurements with nanoscale imaging, shows that the obtained single-crystal composites exhibit improved toughness, compared to the solution-grown pure single crystals. Particularly, the rupture time increases by 1.25 times after the gel-networks and MWCNTs are incorporated. More importantly, the in-situ observation of the crystal deformation suggests that the guest incorporation toughens the single-crystal host by the shielding effect of nanofiber on crack-bridging at nanoscale. As such, this work may have implications for understanding the damage tolerance of biominerals as well as towards the development of new mechanically reinforced single-crystal composite materials.