Search for superhard materials: Nanocrystalline composites with hardness exceeding 50 GPa

The search for superhard materials with Vickers hardness H ≥ 40 GPa (about 4000 kg/mm²) concentrates mainly on polycrystalline diamond, cubic c-BN and C₃N₄ or the substoichiometric CN_x. This approach is based on the theoretical strength which is proportional to the bulk modulus. However, the practically achievable strength (and hardness) of engineering materials is two to four orders of magnitude smaller because their failure occurs due to flaws, and it is determined by their microstructure. Therefore, an alternative approach deals with the design of materials with an appropriate microstructure, such as heterostructures.Recently, we have developed new superhard nanocrystalline composites $\text{nc-Me}{}_{\mathrm{n}}\text{N}\text{a-Si}{}_3\text{N}{}_4$ (Me = Ti, W; V,…). These materials consist of ≤ 4 nm small nanocrystals of a hard transition metal nitride embedded into < 1 nm thin matrix of amorphous silicon nitride. Unlike pure nanocrystalline metals and the heterostructures which show softening when the crystallite size or lattice period decreases below 5–6 nm, the hardness of our composites strongly increases with decreasing crystallite size in that range and approaches the hardness of diamond. In this paper we shall briefly summarize the concept for the design of these materials and experimental results achieved so far. New results to be reported concern the surprising structural stability of these composites and a discussion of the possible origin of the superhardness.