The superhardness effect of Ti-Si-N nanocomposite films is closely related to the thickness of the interfacial phase, Si3N4. The influence of the thickness of Si3N4 on the growth structure and mechanical properties of Ti-Si-N nanocomposite films was determined by studying TiN/Si3N4 multilayers with different thicknesses of Si3N4 layers to reveal the strengthening mechanism of the nanocomposite films. The results show that Si3N4, which is amorphous in single film, can exist as a crystalline structure in TiN/Si3N4 multilayers when the thickness of,the Si3N4 layers is less than 0.7 nm because of the "template effect" of TiN. As a result, TiN and Si3N4 layers form a coherent epitaxial growth with a preferred orientation. As a result, the multilayers exhibit a superhardness effect. When the thickness is greater than 0.7 nm, Si3N4, exists as an amorphous structure and prevents TiN from coherent epitaxial growth with it, resulting in the decrease of the hardness and elastic moduli of the multilayers. The experiments reveal that when the thickness is less than 0.7 nm, the Si3N4 interfacial phase in Ti-Si-N nanocomposite films may have the same structure as that in TiN/Si3N4 multilayers. The strengthening mechanism of Ti-Si-N nanocomposite films may be the same as that of TiN/Si3N4 nanomultilayers formed by the coherent epitaxial growth of two layers with different moduli. (C) 2005 American Vacuum Society.