Nanoscale toughening mechanism of nacre tablet.
Nacre has attracted widespread interest because its unique hierarchical structure, which is assembled by 95 wt% brittle aragonite and 5 wt% soft organic materials, leads to several orders of improvement in fracture toughness. Apart from the well proposed toughening mechanisms such as mineral bridges and tablets interlocks, the organic materials including biopolymers between tablets and proteins exist within a tablet can also potentially improve the toughness. In this work, we employ a novel approach combining steered molecular dynamics (SMD) and classical molecular dynamics (MD) to build a model of mineral-protein composite to mimic nacre tablet. The critical role of protein in improving the fracture toughness of nacre is investigated for the first time. MD simulations of single crystalline aragonite, polycrystalline aragonite and mineral-protein composite under uniaxial tensile loading are performed, and the obtained constitutive responses are compared with experimental measurements of nacre under tension. It is shown that the fracture toughness of mineral-protein composite is significantly larger than that of single crystalline or polycrystalline aragonite. Detailed atomic configuration analyses reveal that the fracture of individual computer model is governed by its unique failure mechanisms. Dislocation motion and phase transformation are observed during the failure of single crystalline aragonite. Polycrystalline aragonite fails by the inter-granular cleavage, as well as phase transformation within grain. It is surprisingly noted that other than the stretching of protein chains on grain boundaries, intra-granular fracture is triggered in mineral-protein composites. Proteins serve as strong glue between the inorganic nanograins. It is believed that the strong electrostatic interaction between protein and aragonite nanograins, combined with the remarkable plastic ductility of protein lead to the intra-granular failure, which consequently enhance the fracture toughness of the whole specimen.
Department of Civil and Environmental Engineering, Rice University, Houston, TX 77005, USA. Electronic address: ningzhang@ufl.edu.
Ning
N
Zhang
Department of Material Science, University of California, San Diego, CA 92093, USA.
Shengfeng
S
Yang
Department of Aerospace Engineering, Iowa State University, Ames, IA 50011, USA.
Liming
L
Xiong
Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA.
Yu
Y
Hong
Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA.
Youping
Y
Chen
D032701:Biomimetics / Q000379:methods*; D002119:Calcium Carbonate / Q000737:chemistry; D006244:Hardness; D056004:Molecular Dynamics Simulation; D060734:Nacre; D036103:Nanotechnology / Q000379:methods*; D011487:Protein Conformation; D011506:Proteins / Q000737:chemistry; D013314:Stress, Mechanical
D060734:Nacre; D011506:Proteins; D002119:Calcium Carbonate
Aragonite; Electrostatic interaction; Molecular dynamics; Nacre tablet; Protein–mineral interaction; Toughening mechanism