Effect of nanometer-sized grains on the superhardness of c -BC5: A first-principles study

Using ab initio density-functional theory to calculate the anisotropic ideal strengths and electronic structure of c -BC5 with different distribution of boron in the diamond lattice we show that the recently reported load-invariant hardness of 71 GPa cannot be explained by the relatively low intrinsic shear strength of this material, but it is, in agreement with many reports on nanosize effect, extrinsically enhanced by the small crystallites size of 10-15 nm of the samples. It is further shown that random distribution of boron atoms within the diamond lattice results in higher strength than in heterostructures consisting of repetitive stacking of one B and five C layers along the <111> direction, which were used by other researchers in their modeling of c -BC5 solid solution