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

Epitaxial Mo/NbN and W/NbN superlattices with modulation wavelengths Lambda ranging from 1.3 to 120 nm were grown on MgO (001) substrates by de reactive magnetron sputtering in Ar/N-2 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 less than or equal to 0.3 nm. The epitaxial relationship between the layers was (001)(metal)parallel to(001)(NbN) and [110](metal)parallel to[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 Lambda = 2-3 nm, compared to rule-of-mixtures values of 10 GPa. The hardness decreased with increasing Lambda above approximate to 3 nm, following the dependences H = 10.3 + 26.70 Lambda(-0.38) GPa for Mo/NbN and H = 12.88 +22.1 Lambda(-0.3) GPa for W/NbN. Hardness versus metal volume fraction with Lambda approximate to 5 nm showed a flat-topped dependence. Brillouin scattering results for Mo/NbN superlattices showed a minor elastic anamoly at small Lambda. The hardness results are compared with theories for strengthening of multilayers. (C) 1998 American Institute of Physics. [S0021-8979(98)01914-8]