Modeling intrinsic residual stresses built-up during growth of nanostructured multilayer NbN/CrN coatings

Composition modulated NbN/CrN nanostructured multilayer coatings were deposited onto martensitic stainless steel. The coatings were obtained by cathodic arc process, in an industrial-size physical vapor deposition (CAPVD) chamber. The coatings reached 15 μm thick, consisting of multilayers of different periodicities (4 nm ≤ Λ ≤ 20 nm), which were achieved by varying the rotating speed of the samples in a 2-fold rotation table. The obtained coatings were characterized by High Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Diffraction (SAD) in order to assess the crystal structure, epitaxy and degree of coherency of the NbN/CrN interface. The results showed that the coatings were formed by alternate individual layers of NbN and CrN separated by highly coherent interfaces, and revealed cross-contamination between the layers. Compositional variation measured across the multilayers, by Electron Energy Loss Spectroscopy (EELS) showed that the lower the coating periodicity (Ʌ), the greater is the cross-contamination in the NbN and CrN individual layers. A Finite Element Analysis (FEA) model was developed for assessing the intrinsic residual stresses due to differences of lattice parameters of the NbN and CrN structures, based on chemical composition gradients obtained from EELS measurements. The results show the build-up of stress gradients from the center of individual layers towards the interface, differently from other results published in literature. The smaller the periodicity the higher are the stress gradients developed near the NbN/CrN interfaces. The results are discussed based on the possible interaction of dislocations with stress fields developed near the NbN/CrN interfaces. The proposed model is consistent with former experimental results, which indicates that the smaller the periodicity, the greater is the coating hardness when periodicities range from 4 nm to 20 nm.