A critical comparison between XRD and FIB residual stress measurement techniques in thin films

Abstract

Residual stress has a significant effect on the performance of thin films, in terms of adhesion, hardness, wear and fatigue resistance. Thus, when assessing innovative coatings or new deposition technologies, it is important to perform residual stress evaluation by means of a suitable and reliable technique.X-ray diffraction (XRD) is one of the commonly used techniques, because it is non-destructive, surface sensitive and phase selective. However, it is subject to certain limitations: X-ray diffraction allows stress evaluation (i.e. its indirect deduction from the measured diffraction profile) only in case of crystalline materials, and the results may be subject to aberrations in the presence of texture or stress gradients often occurring in thin films.Recently, a new class of methods for residual stress evaluation has been proposed, based on incremental focused ion beam (fib) milling, combined with high-resolution in situ scanning electron microscopy (SEM) imaging and full field strain analysis by digital image correlation (DIC).The aim of the present paper is to explore in some detail the significance of the stress values obtained for the same coating by X-ray diffraction and focused ion beam milling, and to demonstrate that the analysis of residual stress depth gradients is possible by using FIB-DIC techniques. Finally, a comparative assessment of the weaknesses and strengths of the two techniques will be carried out. For this purpose, a chromium nitride highly textured thin film sample was chosen. The residual stress state evaluation by the two methods was carried out for the coating deposited by the cathodic arc evaporation (CAE-PVD) technique.Although many significant differences were identified between the X-ray diffraction and focused ion beam milling methods, careful consideration of the gauge volumes and weighting demonstrated that satisfactory agreement was obtained. The analysis highlighted the importance of the issues related to (a) probe-to-sample interaction volume (gauge volume), (b) film texture, and (c) the elastic anisotropy. All these factors must be taken into account in order to enable valid comparisons to be drawn.