Calcium carbonate with nanogranular microstructure yields enhanced toughness

The presence of nanogranular microstructures is a widely reported feature of biominerals that form by classical and non-classical mineralization pathways. Inspired by nature, we have synthesized amorphous calcium carbonate nanoparticles with nanogranular microstructures, whose grain size is tuned by varying the polymer concentration. The response to indentation of single calcium carbonate nanoparticles proceeds via an intermittent stick-slip that reflects the characteristics of the nanogranular microstructure. A two-fold mechanism is thus proposed to enhance the toughness of the nanoparticles, namely nanogranular rearrangement and intergranular bridging by an organic phase and/or hydration. This work not only provides a synthesis route to design biologically inspired mineral nanoparticles with nanogranular structure, but also helps in understanding toughening mechanisms of biominerals arising from their nanoscale heterogeneity.