Tension–compression asymmetry in mechanical properties of diamond nanopillars: molecular dynamics simulations

Introduction of twins into diamond can affect the hardness of diamond, but the underlying microcosmic mechanism remains unknown. Here we have compared the mechanical properties of diamond NPs with three different models, i.e. single crystal nanopillars (SC NPs), twin crystalline nanopillars (TC NPs), and five-fold twinned nanopillars (FT NPs), with diameters from 5 to 30 nm during both tension and compression by molecular dynamics simulations. Our study reveals that the mechanical properties of diamond NPs are closely related to the models of NPs, diameters, and loading modes. The stress–strain responses present significant asymmetry during tension and compression. And the yield strength and strain for FT NPs are always higher than those of TC and SC NPs due to the effect of five-fold twin boundary. The existence of plasticity in diamond NPs is confirmed by the abundance of dislocations after yield strain. The tension–compression asymmetry is also reflected by the differences in dislocation type, dislocation evolution processes, and the fracture shape of the NPs. Moreover, the typical characteristic during tension is that stacking faults are always found following the slip of dislocations, and during compression is that dislocation networks are observed for TC and FT NPs with diameters larger than 20 nm.