Twin-size effects on the hardness and plastic deformation mechanisms of nanotwinned diamond

Although the recently synthesized nanotwinned diamond was found with Vickers hardness exceeding 200 GPa, almost twice the commonly quoted hardness of single crystal diamond, there are still some controversies about the twin-size effects on its hardness and the related plastic deformation mechanisms under Vickers indentations. Through molecular dynamics simulations, we have investigated the behaviors of nanotwinned diamond models under Vickers indentations. Our results indicate that the hardness of nanotwinned diamond would increase with the decreasing of twin boundary separations, but the increasing rate would be reduced due to the concentrated twin boundaries. A formula was given to depict the relationship between hardness and twin thickness, and was consistence with the simulation results. We found that the hardness of nanotwinned diamond is mainly resulted from the competition between strengthening and softening effects of twin boundaries, and the related mechanisms are qualitatively discussed. This work not only provide an answer for the questions about the hardness of nanotwinned diamond versus twin boundary separations, but also offer insights into the plasticity of nanotwinned diamond, which could be beneficial to the design of novel nano structured ultrahard materials and evaluate their origins of hardness and strength.