Orientation Dependent Dislocation Generation in Shocked Diamond

Diamond is, by virtue of its high bonding forces and Peierls-Nabarro barrier stresses, among the hardest materials on earth. This extreme hardness is a direct consequence of the difficulty to generate and move dislocations. Thus, dislocations are rarely formed under conventional deformation and extreme regimes of loading are required. Here we demonstrate, using molecular dynamics calculations, that the generation of defects is highly dependent on the loading orientation. Shock loading single-crystal diamond along [001] to a shock stress of 137 GPa did not reveal dislocations. On the other hand, loading along [011] and [111] generated profuse dislocations at this shock stress, and at even lower stresses down to 72.6 GPa when a nanoscale void was introduced. Two slip systems were identified: <011>{100} and <112>{111}. These results demonstrate that the threshold for plastic deformation in diamond is orientation dependent and significantly lower than thought as a fraction of the shear modulus: 0.17 G. While no dislocations were formed in [001] loading, in the case with a void its collapse generated localized amorphization, demonstrating that the full elastic-plastic transition requires not only shock stress but superposed shear.