Understanding the mechanical characteristics of nanotwinned diamond by atomistic simulations

As is known to all, diamond is the hardest natural substance. The synthesis of harder materials than the natural diamond is very favorable but challenging. It has been found that the introduction of nanotwinned (NT) structure into diamond could be an effective approach to improve the hardness of diamond. Nevertheless, the fundamental understanding of the strengthening characteristics by NT structure is still ambiguous. In this work, we unveil the effect of NT structure on the deformation mechanism and mechanical properties of diamond at the atomic scale by virtue of atomistic simulations. Interestingly, the hardness measured by nanoindentation shows a positive correlation to the twin spacing instead of the inverse Hall-Petch effect extensively discovered in many traditional metal materials. However, it is not the smallest twin spacing achieving the highest tensile strength, indicating the existence of twin-spacing-induced inverse Hall-Petch effect in NT diamond during tensile deformation. The specific deformation mechanisms are investigated which are capable of providing an explicit explanation for the observed mechanical characteristics.