“Bio-metals”: Uncovering ancient biological materials with nanoindentation size effects arising from manifold micromechanics
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The Nix-Gao nanoindentation size effect, where squared hardness scales with the inverse of the indentation depth, qualifies as a major hallmark of crystalline metals. The effect arises from the accumulation of dislocations, i.e., irregularities in the atomic lattice in the form of line defects, in the material located right below the indenter. We here show that the very same effect holds for a totally different type of material, namely the one making up the jaws of the bristle worm Perinereis cultrifera. After lysis and embedding into a two-component resin, both the resin surface and the jaw itself were abraded, providing a flat surface with a roughness of only 30 nanometers of the material found in the mid-plane of the jaw. Prior work suggests that this material is formed by an unusual class of proteins coordinated by transition metals and halogen ions. The flat surface was nanoindented down to six different indentation depths at eleven specific testing areas, covering the central and tip regions of the jaw. As a rule, 54 indentations were realized per indentation depth and area. Both in the tip and the central region, the hardness-to-depth relations clearly obey the Nix-Gao size effect law, which is classically linked to strain gradient plasticity. Interestingly, our experiments also reveal an elastic size effect. The latter is rationalized in the framework of “manifold micromechanics”: Concentrated microscopic volume forces of the Peach-Koehler type, situated at dislocation-type “folds” in the ion-coordinated structural protein matrix, induce non-negligible macroscopic strain gradients at the level of a representative volume element (RVE) of the matrix that we suggest referring to as “bio-metal”.
