Impurities such as oxygen and chlorine can strongly decrease the hardness of superhard nc-TiN/a-Si3N4 and similar nanocomposites when incorporated into the coatings during their deposition. It is shown that 1-1.5 at. % of oxygen causes a hardness decrease to about 30 GPa, as compared to 45-55 GPa for the pure material. This may explain some of the contradictory results found by other authors, particularly for coatings deposited by physical vapor deposition at relatively low nitrogen pressure, deposition temperature, and deposition rates. (C) 2004 American Vacuum Society.