Hardness of nanocrystalline diamonds

The predicted static and dynamical properties of a recently reported 6H polymorph of diamond based on first-principles electronic structure calculations are reported. Although 6H diamond is less stable than the cubic 3C phase, it is found to exhibit similar mechanical properties. The hardness predicted using a semiempirical theory developed recently is found to be comparable to cubic diamond. This theory, extended to nanocrystals by including quantum confinement effects, elucidates the superhardness recently reported in nano 3C and 6H diamond crystallites that were shown to exceed that of the bulk diamond. The method provides a theoretical tool for the characterization and design of other nanohardened materials.