Multiscale mechanical studies of nacre from gastropod mollusk Trochus Nilocitus

Abstract

(cont.) revealed jagged and branched crack fronts at plate interface, tortuous crack paths, non-uniform angles of polygons (suggesting possible intrinsic deformability and displacement/sliding). The technique of nanoindentation was carried out on individual aragonite tablets using a diamond-coated Berkovich probe tip (end-radius of 70 nm, tip angle of 142.3 degrees), at a rate of indentation of 10 [micro]N/s (load controlled), forces from 10 to 1000 [micro]N and indentation depths from 10 to 97 nm. AFM inspection of the indented region showed the existence of extensive plastic deformation within the tablet and suggested that occluded biomacromolecules may play a significant role in the deformation at loads below 100 [micro]N. Using the contact elastic theory, a Young modulus of 112.3 GPa and a hardness of 10.5 GPa were found for an individual platelet. This study shows that a biocomposite principally composed of a poor ceramic (aragonite) can achieve surprisingly good macroscopic mechanical properties thanks to a complex hierarchical structure allowing an extraordinary variety of energy-dissipating mechanisms. Our aim is to continue to formulate multiscale structure-property relationships to eventually aid in the design and advancement of new synthetic biologically inspired lightweight, hard body armor technologies.

The inner columnar nacreous layer of the gastropod mollusk Trochus Niloticus is a nanostructured biocomposite with outstanding and unique mechanical properties. It is composed of [approximately]95% wt of hexagonal aragonite plates (width=5.8±0.4 [micro]m, thickness=0.87±0.07 [micro]m), stacked [approximately]40 nm apart, and [approximately]5% wt of a biomacromolecular "glue" which exists between and within the individual plates. Atomic force microscopy (AFM) revealed a dense array of nanoasperities on the top and sides of the aragonite plates ([approximately]120 nm wide). A multiscale theoretical and experimental approach was taken to identify, understand, and predict the complex deformation mechanisms and mechanical behavior of this fascinating material. Macroscopic 3-point bend tests yielded an in-plane Young modulus of 68.0 ± 11.4 GPa and 65.4 ± 9.6 GPa for freshly cut samples and samples soaked for 10 weeks respectively. A fracture strength of 231 ± 34 MPa and 213 ± 42 MPa respectively were measured. Samples soaked for 10 weeks, even if slightly less strong, exhibited major non-linearities in the stress-strain curves, emphasizing the greater toughness of hydrated nacre. Uniaxial compression yielded Young moduli of 63.8 ± 14.7 GPa for samples with the brick layers oriented parallel to the load, 19.1 ± 3.4 GPa when oriented perpendicular to it and fracture strengths of respectively 225 ± 44 and 663 ± 71 MPa. The discrepancy between the compression moduli emphasizes that very distinct deformation mechanisms prevail during these tests, which is confirmed by the fact that fracture occurs also in three different ways (respectively through thickness, interlaminar and shatter). Scanning electron microscopy (SEM) and AFM of fractured samples