Monolithic TaN，NbN, TiN films and TaN/TiN, NbN/TiN multilayers with different modulation periods were prepared by reactive magnetic sputtering. The films were characterized by X-ray diffraction, high-resolution transmission electron microscopy and nanoindentation. Results showed that there are coherent interfaces between the layers of TaN and TiN in TaN/TiN multilayers and the layers of NbN and TiN in NbN/TiN multilayers within a modulation period. The superhardness effect happened and the maximum hardness values of the two multilayers are nearly equal. The superhardness mechanism was discussed. The lattices mismatch between NbN/TiN and TaN/TiN multilayers is similar, but the difference in modulus of NbN/TiN multilayers is larger than that of TaN/TiN multilayers. It shows that stress field induced by coherent epitaxial growth is the main reason of superhardness effect.
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