Mechanical properties of epitaxial TiN/(V_0.6Nb_0.4)N superlattices measured by nanoindentation

We have used nanoindentation to measure the mechanical properties of epitaxial TiN/(V_0.6Nb_0.4)N superlattices, grown on MgO(100), as a function of the wavelength λ. The V/Nb ratio within the VNbN layers was chosen to provide a lattice match with TiN, minimizing effects resulting from coherency strains. For λ≥4 nm, the hardness was found to be significantly enhanced relative to a homogeneous reference film of the same average composition. For λ<4 nm, the hardness decreased to a value close to that of the reference film. The elastic modulus was found to be constant for λ≥4nm, at a value close to that predicted by the law of mixtures. For samples with λ=2.3 and 2.8 nm, there was a 15% decrease in modulus. The observed variations appear not to be an effect of interfacial strain. Possible mechanisms are discussed.