In view of new environmental directives, hexavalent chromium baths can no longer be used to electroplate thick hard chromium deposits. To meet these industrial and environmental challenges, deposits are developed from trivalent chromium electrolytes. Cr(III) coatings are usually studied from the point of view of the use properties and hardness, but their intrinsic properties remain widely unknown. The novelty of this work consists in the mechanical characterisation of these coatings. Properties such as hardness, stiffness, yield strength, and toughness of trivalent chromium deposits are determined by combining instrumented hardness tests, in situ FEG–SEM observations, and finite element simulations. These are explained according to the microstructure of the deposits, which is determined by scanning electron microscopy and X-ray diffraction. Their composition was characterised by glow discharge spectrometry. The structure characterisation deposits showed a more severely fractured coating of trivalent chromium than in the case of hexavalent chromium. Non-post-treated trivalent chromium deposits have a higher hardness (13 ± 1.7 GPa) and yield strength (5 GPa) than hexavalent chromium deposits. However, their stiffness (191 ± 13 GPa) and toughness (1.37 ± 0.13 MPa√m) are lower. Its mechanical behaviour is elastofragile. These differences in mechanical properties can be explained by the amorphous structure of the deposits and their high carbon content.
Due to new environmental regulations, hexavalent chromium electrolytes can no longer be used for thick, hard chromium plating. In response to this industrial and environmental challenge, trivalent chromium electrolyte plating has been developed. In this paper, we propose a study of the adhesion of CrIII coatings based on the implementation of numerical models in comparison with an identified experimental scenario. The aim is to dissociate the influence of coating and substrate behaviours from the adhesion work by describing the intrinsic damage of the chromium layer and the coating–substrate interface. Two types of cracking were studied: transverse cracking and delamination. For the former, the crack density was higher for CrIII than for CrVI and increased with deformation and coating thickness. Microtensile tests with scanning electron microscopy (SEM) observations allowed us to highlight the cracking process in the coating (transverse cracking) and at the coating–substrate interface (delamination). The numerical simulation of the test allowed us to estimate a damage-initiation threshold normal stress of 1900 MPa, which occurred at an average applied strain of 2.5%. Delamination of the coating was complete at an average strain of 13.6% and an interfacial normal stress of 2600 MPa.
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