Impurities, in particularly oxygen, degrade the mechanical properties of superhard nc-TiN/a-Si3N4 nanocomposites. In the present paper we show that relatively small oxygen impurities also hinder the diffusion and already at a level of about >= 0.8 at.% apparently stabilize the Ti-Si-N solid solution to a high temperature of about 1000 degrees C, thus making it impossible to form stable and strong superhard nanocomposites. Therefore, a hardness enhancement to 30-35 GPa, which has been reported in many publications on a variety of Ti-Si-N and other transition metal nitrides with silicon, is likely to be due to a simple refinement of the crystallite size towards the so called "strongest size". Although the latter mechanism of strengthening is more universal than the design of superhard nanocomposites with strengthened interface, superhard nanocomposites can be prepared in this way only with a limited number of intrinsically very hard materials. (C) 2012 Published by Elsevier B.V.