Is it valid to apply this rule to covalent materials? To answer these questions, a detailed study on interactions between dislocations and twin planes in nt-diamond is necessary. The Hall-Petch effect is an experimental rule deduced from metals ( 18). As carriers of plastic deformation ( 17), dislocations in diamond must also be different from those in metals. Diamond is a covalent material, and its elastic and plastic properties are largely different from those of metals. Can the hardness be further increased? Understanding the origin of hardness in nt-diamond is an important scientific problem, as it may provide a new strategy for designing new materials with even higher hardness.Īlthough it has been speculated that the unprecedented hardness in nt-diamond may be due to combined Hall-Petch effect ( 10, 11) and quantum confinement effect ( 12– 14), alternative mechanisms have also been proposed ( 15, 16). Recently, nanotwinned diamond (nt-diamond) synthesized by compressing onion carbon has achieved hardness values more than twice that of natural diamond (180 to 200 GPa) ( 8, 9), setting a new world record. The hardness of synthetic nanograined diamond (110 to 140 GPa) is higher than the average hardness of single crystal diamond (96 GPa) ( 7). Similar to metals and alloys, diamond can be hardened by grain refinement. Therefore, numerous experimental and theoretical studies have been performed to understand its origin of superhardness ( 4, 5) and improve its hardness further ( 6– 9). This work provides new insights into hardening mechanisms in nt-diamond and will be helpful for developing new superhard materials in the future.ĭiamond, the hardest material available ( 1), is of great interest both scientifically, for understanding the origin of its outstanding mechanical properties ( 2), and technologically, for its direct industrial and research applications as cutting and polishing tools, diamond knives, diamond anvil cells, etc. Both factors stem from the low activation volumes and high activation energy for dislocation nucleation and propagation in diamond twin planes. We show that two factors contribute to the unusually high hardness of nt-diamond: high lattice frictional stress by the nature of carbon bonding in diamond and high athermal stress due to the Hall-Petch effect. On the basis of the Sachs model, twin thickness dependence of nt-diamond hardness is evaluated, which is in good agreement with the experimental data. In this report, we categorize interaction modes between dislocations and twin planes in nt-diamond and calculate the associated reaction heat, activation energies, and barrier strength using molecular dynamics. Experimental results (Huang et al.) indicated that nanotwinned diamond (nt-diamond) has unprecedented hardness, whose physical mechanism has remained elusive.

Diffrent typ o dimonad structers