Microstructure and hardening mechanisms in a-Si3N4/nc-TiN nanostructured multilayers

Amorphous/nanocrystalline Si3N4/TiN nanostructured multilayer films were fabricated by radio-frequency reactive magnetron sputtering. The microstructure and properties of these films were measured using an X-ray diffractometer, X-ray photoelectron spectroscope, high-resolution transmission electron microscopy and nanoindenter. The superhardness effect was found in Si3N4/TiN multilayers. The hardness of Si3N4/TiN multilayers is affected not only by modulation periods, but also by layer thickness ratio and deposition temperature. The hardness value is about 40% higher than the value calculated from the rule of mixtures at a deposition temperature of 500 °C and a layer thickness ratio (lSi3N4/lTiN) of 3/1. The hardening mechanisms in this system are discussed in the light of our experimental results. Results of calculation of the theoretical stress distribution in the multilayers suggests that alternating stress fields caused by thermal mismatching between Si3N4 and TiN is one of the main reasons for the superhardness effect observed in Si3N4/TiN multilayers.