Effect of micro-droplets on the local residual stress field in CAE-PVD thin coatings

The effect of surface defects on the residual stress local distribution in thin coatings has been investigated by an innovative high resolution methodology, which mainly consists of incremental focused ion beam (FIB) micron-scale ring-core milling (IμRCM), combined with high-resolution in situ SEM-FEG imaging of the relaxing surface and in-plane strain analysis by digital image correlation (DIC). The depth profile of the residual stress can be obtained from the experimentally measured surface strain with sub-micron spatial resolution (both lateral and in-depth) by using a finite element modeling (FEM) reverse calculation procedure. A chromium nitride (CrN) coatings was produced by Cathodic Arc Evaporation Physical Vapor Deposition (CAE-PVD) on AISI 316L steel substrate. Preliminary characterization of coatings consisted of nanoindentation testing, FIB cross section microstructural analysis and XRD (sin2ψ) average residual stress measurement. Local residual stress was measured nearby to surface micro-droplets, with the main aim of investigating the effect of such defects on the residual stress in-depth distribution. A series of stress measurements were also performed on the homogeneous defect-free coating. Significant differences were found in terms of residual stress and stress gradient in proximity of surface defects, in comparison with the residual stress measured on the defect-free areas. In case of micro-droplets, such differences were attributed to significant modification of growth mechanisms and microstructure of the coating, that lead to a different competitive mechanism of residual stress build-up. This assumption was supported by the microstructural FIB-SEM observation, which showed anomalous grain growth and changes in crystal orientation in correspondence of a droplet.