Deposition, structure, and hardness of polycrystalline transition-metal nitride superlattice films

Polycrystalline TiN/VN, NbN/VN, and TiN/NbN superlattices with periods Lamb da between 2 and 160 nm were deposited onto steel substrates using an oppos ed-cathode reactive magnetron sputtering system. The nitrogen partial press ure and the substrate bias values were optimized in order to obtain dense s toichiometric films, which yielded the highest Vickers hardnesses H-V. H-V for TiN/VN and TiN/NbN superlattices reached maximum values of approximate to 5000 kgf/mm(2) at Lambda approximate to 5-10 nm, compared with approxima te to 2000 kgf/mm2 for homogeneous TiN, NbN, and VN films. In contrast, H-V approximate to 2000 Kgf/mm(2) was obtained for VN/NbN superlattices indepe ndent of Lambda. Model calculations in which the hardness enhancement was p roportional to the difference in layer shear moduli gave good agreement wit h the data. The lack of hardness enhancement in VN/NbN indicates that any o ther hardening mechanisms, such as coherency strains and dislocation blocki ng by interfacial misfit dislocations, were not important.