Mechanical properties of VC/ZrC and VC/HfC superlattices

Face-centered cubic transition metal carbides exhibit high melting points and hardness, making them prominent candidates for protective coating applications. Vanadium carbide (VC) has typical characteristics of transition metal carbides. It serves as model material in this study, in which we showcase the effect of superlattice architecture on mechanical properties and fracture toughness. While beneficial effects of superlattice arrangement have been demonstrated for a series of nitride-based coatings, transition metal carbides remain largely unexplored territory. Following density functional theory based ab initio predictions on lattice and shear modulus mismatch, we develop VC/ZrC and VC/HfC superlattice coatings synthesized via pulsed DC sputter deposition. The bilayer periods (Λ) of our fully fcc-structured polycrystalline coatings range between 2–50 nm (indicated by X-ray diffraction, transmission and scanning electron microscopy). The chemical composition is close to 1:1 stoichiometry (from X-ray fluorescence, elastic back-scattering spectrometry and elastic recoil detection analysis). Both superlattice series exhibit a strong dependence of hardness, elastic modulus, and fracture toughness on their bilayer periods, which can only be correlated with the in-plane stress variations for VC/ZrC. The VC/HfC superlattices provide their peak-hardness of 36.0 ± 1.6 GPa for Λ = 6 nm and their peak in fracture toughness of 3.5 ± 0.5 MPa√m for Λ = 10 nm.