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

Polycrystalline TiN/VN, NbN/VN, and TiN/NbN superlattices with periods Λ between 2 and 160 nm were deposited onto steel substrates using an opposed-cathode reactive magnetron sputtering system. The nitrogen partial pressure and the substrate bias values were optimized in order to obtain dense stoichiometric films, which yielded the highest Vickers hardnesses H_V. H_V for TiN/VN and TiN/NbN superlattices reached maximum values of ≈5000 kgf/mm² at Λ ≈ 5–10 nm, compared with ≈2000 kgf/mm² for homogeneous TiN, NbN, and VN films. In contrast, H_V ≈ 2000 kgf/mm² was obtained for VN/NbN superlattices independent of Λ. Model calculations in which the hardness enhancement was proportional to the difference in layer shear moduli gave good agreement with the data. The lack of hardness enhancement in VN/NbN indicates that any other hardening mechanisms, such as coherency strains and dislocation blocking by interfacial misfit dislocations, were not important.