To study the atomic structure of Ti-Si-N superhard nano-composite coatings, total energy calculations for some different configurations of TiN with Si addition have been conducted with the ab initio method. The investigation of silicon additions into the titanium nitride crystallite shows that (a) there is no silicon interstitial solid solution in the titanium nitride crystallite, and (b) if some titanium atoms or nitrogen atoms are missing in the titanium nitride crystallite, it is possible for silicon atoms to occupy the vacant sites and to form the substitution solid solution. The studies of the silicon addition as an interface sandwiched between the B1-TiN (001) oriented crystallites indicates that compared with the substitution interface structure, the structure with the interstitial interface is a low energy structure. (C) 2011 Elsevier B.V. All rights reserved.