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The recent development in the field of superhard materials with Vickers hardness of ⩾40 GPa is reviewed. Two basic approaches are outlined including the intrinsic superhard materials, such as diamond, cubic boron nitride, C3N4, carbonitrides, etc. and extrinsic, nanostructured materials for which superhardness is achieved by an appropriate design of their microstructure. The theoretically predicted high hardness of C3N4 has not been experimentally documented so far. Ceramics made of cubic boron nitride prepared at high pressure and temperature find many applications whereas thin films prepared by activated deposition from the gas phase are still in the stage of fundamental development. The greatest progress has been achieved in the field of nanostructured materials including superlattices and nanocomposites where superhardness of ⩾50 GPa was reported for several systems. More recently, nc-TiN/SiNx nanocomposites with hardness of 105 GPa were prepared, reaching the hardness of diamond. The principles of design for these materials are summarized and some unresolved questions outlined.

The recently reported strong effect of minor oxygen impurities on the degradation of mechanical properties of superhard nc-TiN / a-Si 3 N 4 can be, by analogy with similar effects known in metallic alloys, understood on the basis of a simple size effect of large O 2− impurity species incorporated into the Si 3 N 4 interface. The electronic effect of the O 2− sites within the Si 3 N 4 interface is also likely to play an important role in weakening the SiN and, possibly to a lesser extent, also TiN bonds. A simple model which assumes that the strength ͑and hardness͒ of these materials is approximately proportional to the reciprocal surface coverage of the TiN nanocrystals by oxygen impurities shows a surprisingly good agreement with the measured data.

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