An Estimation of Local Residual Stresses in Amorphous and Crystallized Trivalent Chromium Coatings

Abstract

Hard chromium coatings have demonstrated their performance for several decades, particularly for their wear and corrosion resistance properties. However, the traditional process using chromium trioxide (CrO3) is very toxic and is intended to disappear in the European Union by 2024, thanks to the REACH regulation. This study aimed to determine the residual stress of amorphous chromium deposits elaborated from baths containing trivalent chromium ions. However, the amorphous structure of these deposits does not allow the determination of residual stresses by conventional means. In this study, we propose adapting a recent method called “FIB-DIC”, which is not limited by the material’s crystalline structure compared to the classical X-ray diffraction (XRD) analysis. The method is based on the measurement of relaxation-induced displacement fields following the ablation of material on a very local scale. The results obtained by the classical (XRD) and the FIB-DIC method on crystallized heat-treated Cr(III) deposits are 296 ± 45 and 377 ± 275 MPa. Then, the FIB-DIC technique allowed us to evaluate a residual stress level of 479 ± 359 MPa for amorphous trivalent chromium coating, which has never been performed before.

Damage Mechanism of Trivalent Chromium Coatings under Tensile Stress

“…On the other hand, in the early stages of deformation, the specimens subjected to post-heat treatment at 300 and 700 °C showed a stable crack density (up to 0.7% substrate strain). The authors' previous work shows that the application of heat treatment reduces the level of residual stresses in these coatings, where they can vary from 479 MPa for untreated amorphous samples to low compressive stresses after 1 h at 700 • C [48]. Therefore, the stresses in the layer are lower than for the degassed specimens.…”