Enhanced hardness in lattice-matched single-crystal TiN/V _0.6Nb_0.4N superlattices

Single-crystal TiN/V_0.6Nb_0.4N superlattice films were deposited on MgO(100) substrates using ultrahigh vacuum reactive sputtering from two magnetron sources with a computer-controlled shutter. X-ray diffractometer measurements showed first- and second-order superlattice peaks while electron diffraction showed up to seventh-order superlattice reflections. Direct observations using cross-sectional transmission electron microscopy showed that the superlattices had flat, abrupt interfaces, but contained threading dislocations. Microhardness values for the superlattices ranged from to 3100 to 4100 kg/mm², depending on the superlattice period (λ), compared with 1500 for an alloy with equal average composition. The hardness values for these lattice-matched superlattices agreed with those previously reported for TiN/VN superlattices with a lattice mismatch of 2.4%, although the hardness peak was less pronounced. The present results indicate that coherency strains do not play the major role in enhancing the hardness of these nitride superlattices.