The recently reported strong effect of minor oxygen impurities on the degradation of mechanical properties of superhard nc-TiN/a-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. (c) 2005 American Vacuum Society.