Growth, microstructure and mechanical properties of (Ti, Al)N/VN nanomultilayers

(Ti, Al)N/VN nanomultilayer films with modulation period (Λ) ranging from 1.6 nm to 11.9 nm have been grown by reactive magnetron sputtering on high-speed steel substrates. The growth, microstructures and mechanical properties of these films were characterized with energy dispersive spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy and nanoindentation. The results showed that when Λ was less than 8.8 nm, multilayers grew in a polycrystalline epitaxial growth mode with a (111) preferred orientation. Tensile and compressive alternating strain fields were formed due to the lattice mismatch between (Ti, Al)N and VN layers. Correspondingly, multilayers showed hardness enhancement with maximum hardness values of 38.4 GPa. The inhibition of dislocation motion by alternating strain fields is probably responsible for hardness enhancements in the multilayers. Pronounced composition intermixing occurred in the multilayers when Λ < 3.4 nm; and incoherent interfaces emerged after Λ increased to 8.8 nm. Both of the two cases would gradually decrease the hardness of the multilayers.