The recently reported strong effect of minor oxygen impurities on the degradation of mechanical properties of superhard nc-TiNa-Si3N4 can be, by analogy with similar effects known in metallic alloys, understood on the basis of a simple size effect of large O2 impurity species incorporated into the Si3N4 interface. The electronic effect of the O2 sites within the Si3N4 interface is also likely to play an important role in weakening the Si–N and, possibly to a lesser extent, also Ti–N bonds. A simple model which assumes that the strength (and hardness) 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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