HRTEM image and electron diffraction pattern of the impact compressed nacre. (A) Partial dislocations as indicated by the white box. The insert is the diffraction pattern from the HRTEM image. (B) Partial dislocations with stacking faults. (C) Schematic analysis.
Image Credit:ARO-in-Review - 2009
Nacre pronounced “neɪkər” is known as mother of pearl. It is an organic-inorganic composite material produced by some mollusks as an inner shell layer and also is what makes up pearls. It is very strong, resilient, and iridescent.
The fascinating mechanical properties associated with nacre have conventionally been attributed to crack deflection and biopolymer bonding at the interfaces between aragonite platelets. Recent atomic force microscopy (AFM) observations conducted by researchers, led by Professor Xiaodong Li at the University of South Carolina, reveal that an aragonite platelet is, in fact, composed of a large number of nanoparticles with an average size of 32-44 nanometers, and that these nanoparticles rotate in the deformation of nacre under high strain rate (~103/s) compression. The research is partially sponsored by the U.S. Army Research Office and is presented in ARO 2009 in Review which notes a number of U.S. Army nanomaterials and nanotechnology research projects.
Careful analysis has led to the discovery of new deformation mechanisms in nacre under impact loading. Twins and partial dislocations were found in the plastically deformed aragonite nanoparticles. Twining and partial dislocation formation makes the plastic deformation possible when rotation of the aragonite nanoparticles encounters resistance.
Furthermore, the results demonstrate that deformation in nacre is non-uniform at both the platelet and nanoparticle level; in some aragonite platelets, a negative Poisson’s ratio was even found. These new findings provide tremendous new insights into nacre’s strengthening and toughening mechanisms, and offer new guidelines for developing unprecedented biomimetic materials.
The research in the nanomaterials subfield addresses Army needs for: lightweight alloys and composites for vehicle structures, lightweight armaments, airframes, and bridging; advanced ceramics for improved armor; improved materials and processes for joining of components; high-density metals for kinetic energy penetrators; fabrics and polymeric body armor; thermal and acoustical insulating foams; materials for gun tubes, and directed energy weapons.