Possible role of oxygen impurities in degradation of $nc-TiN∕a-Si3N4$ nanocomposites

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.