Mechanical Behaviour of Hard Chromium Deposited from a Trivalent Chromium Bath

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 root 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.

Automated summary

Due to new environmental directives, hexavalent chromium baths are no longer allowed for electroplating thick hard chromium deposits. This study focuses on the mechanical characterization of trivalent chromium deposits and investigates properties such as hardness, stiffness, yield strength, and toughness. The microstructure of the deposits, determined by scanning electron microscopy and X-ray diffraction, and the composition characterized by glow discharge spectrometry, are considered in explaining the differences in mechanical properties between trivalent chromium and hexavalent chromium deposits.