Abstract
We employ classical molecular dynamics to calculate elastic properties and to model the nucleation and propagation of deformation twins in calcite, both as a pure crystal and with magnesium and aspartate inclusions. The twinning is induced by applying uniaxial strain to the crystal and relaxing all stress components except the uniaxial component. A detailed analysis of the atomistic processes reveal that the twinning mechanism involves small displacements of the Ca ions and cooperative rotations of the CO3 ions. The volume of the twinned region expands under increased uniaxial strain via the propagation of steps along the twin boundaries. The energy cost of the twin boundaries is compensated by the reduced hydrostatic stress and strain energy. The presence of biogenic impurities is shown to decrease the strain required to induce twin formation in calcite and, thus, the yield stress. This increased propensity for twinning provides a possible explanation for the increased hardness and penetration resistance observed experimentally in biominerals.
Highlights
Biominerals produced by living organisms exhibit remarkable mechanical properties, rivalling those of engineered ceramics at high temperatures and pressures.[1,2,3] Understanding the mechanisms involved in the superior mechanical properties of biominerals is essential for the design of new materials that reproduce these properties, and has been the aim of several studies over the past few decades.[4,5,6,7] One of the most widely studied biominerals is calcite, the most stable polymorph of CaCO3
A detailed analysis of the atomistic processes reveal that the twinning mechanism involves small displacements of the Ca ions and cooperative rotations of the CO3 ions
In this study we examine twinning in calcite using molecular dynamics
Summary
Biominerals produced by living organisms exhibit remarkable mechanical properties, rivalling those of engineered ceramics at high temperatures and pressures.[1,2,3] Understanding the mechanisms involved in the superior mechanical properties of biominerals is essential for the design of new materials that reproduce these properties, and has been the aim of several studies over the past few decades.[4,5,6,7] One of the most widely studied biominerals is calcite, the most stable polymorph of CaCO3. Apart from being a rock-forming mineral abundant in the Earth’s crust, calcite is biogenic It often forms part of the skeleton or exoskeleton of invertebrates, where exceptional hardness and toughness are required for protection and support. These mechanical properties are strongly influenced by a range of additives, including inorganic ions and organic macromolecules.[8,9,10,11] These composite structures can effectively distribute stress and control crack propagation through the material, achieving the required toughness and penetration resistance.[12,13,14] One way that this is achieved is deformation twinning, as was recently shown using nanoindentation on a biomineralised calcite exoskeleton.[15]. We repeat the simulations in the presence of biogenic impurities known to affect mechanical properties (Mg ions and aspartate),[25,26,27] in order to investigate the superior mechanical properties of biogenic calcite
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