Mechanical properties of compositionally modulated epitaxial VN(001)/VC(001) films

Epitaxial 1.5-μm-thick VN(001) and VC(001) films are deposited on MgO(001) by reactive magnetron sputtering in Ar/N₂ and Ar/CH₄ gas mixtures at T_s = 1000 °C, respectively. Cyclic switching between the Ar/N₂ and Ar/CH₄ gas yields compositionally modulated vanadium nitride/carbide films which exhibit a single-crystal epitaxial rock-salt structure matrix but also contain misoriented grains resulting in strongly facetted triangular 375–500 nm wide surface protrusions. The gas switching is adjusted to obtain films containing nominally identical amounts of VN and VC and various modulation periods Λ = 1.9 - 30 nm. They exhibit a stoichiometric anion-to-cation ratio but contain more carbon than nitrogen, suggesting that surface carbon reduces the N₂ sticking coefficient and the nitrogen incorporation rate. X-ray diffraction θ-2θ scans and reciprocal space maps indicate an in-plane compressive strain that decreases from -0.4 % to -0.1 % with increasing Λ, and a slightly decreasing relaxed lattice constant a_o = 4.160 - 4.155 Å. Hardness and elastic moduli measured by nanoindentation are H_VN = 5.1 GPa and E_VN = 226 GPa for VN(001), and H_VC = 19.4 GPa and E_VC = 323 GPa for VC(001). The hardness decreases from H = 16.4 – 11.1 GPa with increasing Λ = 1.9 - 30 nm, while the measured E = 260 ± 20 GPa is independent of Λ. Both H and E values are close to the theoretical isostress rule of mixtures (ROM) prediction, revealing that the modulated VN/VC films exhibit no hardness enhancement. This suggests that neither nitrogen-carbon intermixing nor composition modulation results in dislocation pinning during mechanical deformation. This is attributed to the relatively small 0.8 % VN/VC lattice mismatch which results in negligible local strain variations or coherency strains.