Grain size dependence of tensile properties in nanocrystalline diamond

Nanocrystalline diamond (NCD) is a promising material due to its extraordinary mechanical properties, however, the research on the dependence of mechanical properties on the grain size (d) of NCD is still limited. In this paper, the mechanical behavior of 3D NCD with various d is investigated using molecular dynamics (MD) simulations. It was found that the mechanical properties of NCD are sensitive to d. The Young's modulus (E) increases with the increase of d due to the increasing fraction of grain interiors (GIs), while the failure strain (ε_f) decreases with the increase of d due to the decreasing fraction of grain boundaries (GBs). It was also found that the failure strength (σ_f) decreases with the increase of d, which could be attributed to that for larger d the stress concentration in the GBs is severer, which may make cracks initiate more easily. Similar to ε_f and σ_f, the deformation work density was found to increase with the decrease of d, indicating the enhancement of toughness of NCD when d is small. For the samples of different d, the nucleation and propagation of both transgranular and intergranular cracks are the main failure mechanisms, which is consistent with experimental observations.