Growth, structure, and microhardness of epitaxial TiN/NbN superlattices

Epitaxial TiN/NbN superlattices with wavelengths/ranging from 1.6 to 450 nm have been grown by reactive magnetron sputtering on MgO(100). Cross-sectional transmission electron microscopy (XTEM) studies showed well-defined superlattice layers. Voided low-angle boundaries, aligned perpendicular to the film plane, were also present. High-resolution images showed misfit dislocations for Λ = 9.4 nm, but not Λ = 4.6 nm. Up to ninth-order superlattice reflections were observed in diffraction, indicating that the interfaces were relatively sharp. Analysis of the first-order x-ray superlattice reflection intensities indicated that the composition modulation amplitude increased and the coherency strains decreased for Λ increased from 2 to 10 nm. Vickers microhardness H was found to increase rapidly with increasing Λ, from 1700 kg/mm² for a TiN–NbN alloy (i.e., Λ = 0) to a maximum of 4900 kg/mm² at Λ = 4.6 nm. H decreased gradually for further increases in Λ above 4.6 nm, to H = 2500 kg/mm² at Λ = 450 nm. The hardness results are compared with theories for strengthening of multilayers.