Mechanical behavior of ultrastructural biocomposites

Abstract

For numerous centuries nature has successfully developed biocomposite materials with detailed multiscale architectures to provide a material stiffness, strength and toughness. One such example is nacre, which is found in the shells of many mollusks, and consists of an inorganic phase of aragonite tablets 5-8jim in planar dimension and 0.5-1gm in thickness direction and an organic phase of biomacromolecules. High resolution microscopy imaging was employed to investigate the microscale features of seashell nacre to reveal the nucleation points within tablets, the sector boundaries and an overlap between tablets of neighboring layers of [approx.] 20 %. Aragonite, the mineral constituting the inorganic phase of nacre, is a calcium carbonate mineral that is ubiquitous in many natural systems, including both living organisms and geological structures. Resistance to yield is an important factor in the ability of aragonite to provide both strength and toughness to numerous biological materials. Conversely, plastic deformation of aragonite is a governing factor in the formation and flow of large scale geological structures. The technique of nanoindentation combined with in-situ tapping mode atomic force microscopy imaging was used to show the anisotropic nanoscale plastic behavior of single crystal aragonite for indentations into three mutually orthogonal planes.

(cont.) Force vs. indentation depth curves for nanoindentation coaxial to the orthorhombic crystal c-axis exhibited distinct load plateaus, ranging between 275-375gN for the Berkovich indenter and 400-500 [mu]N for the cono-spherical indenter, indicative of dislocation nucleation events. Atomic force microscopy imaging of residual impressions made by a cono-spherical indenter showed four pileup lobes; residual impressions made by the Berkovich indenter showed protruding slip bands in pileups occurring adjacent to only one or two of the Berkovich indenter planes. Anisotropic elastic simulations were used to capture the low load response of single crystal aragonite, with the elastic simulations for the (001) plane matching the experimental data up until the onset of plasticity. Numerical simulations based on a crystal plasticity model were used to interrogate and identify the kinematic mechanisms of plastic slip leading to the experimentally observed plastic anisotropy. In particular, in addition to the previously reported slip systems of the {100}<001> family, the family of {110}<001> slip systems is found to play a key role in the plastic response of aragonite.