Enhanced mechanical hardness in epitaxial nonisostructural Mo/NbN and W/NbN superlattices

Epitaxial Mo/NbN and W/NbN superlattices with modulation wavelengths Λ ranging from 1.3 to 120 nm were grown on MgO (001) substrates by dc reactive magnetron sputtering in Ar/N₂ mixtures. The superlattices were shown to be epitaxial with nearly planar layers using high- and low-angle x-ray diffraction and transmission electron microscopy. Computer simulation fits of the x-ray data indicated that interface widths were ≤0.3 nm. The epitaxial relationship between the layers was (001)_metal∥(001)_NbN and [110]_metal∥[100]^NbN. The nanoindenter microhardness values from W/NbN and Mo/NbN superlattices with 50 vol % metal were nearly identical. The largest hardnesses were 30 GPa, observed at superlattice periods Λ = 2-3 nm, compared to rule-of-mixtures values of 10 GPa. The hardness decreased with increasing Λ above ≈ 3 nm, following the dependences H= 10.3+26.70Λ^-0.38 GPa for Mo/NbN and H=12.88 +22.1Λ^-0.3 GPa for W/NbN. Hardness versus metal volume fraction with Λ≈5 nm showed a flat-topped dependence. Brillouin scattering results for Mo/NbN superlattices showed a minor elastic anamoly at small A. The hardness results are compared with theories for strengthening of multilayers.