Electrodeposition and properties of Ni-Co-W-(Mo-Cu) high/medium entropy alloy coatings deposited from an aqueous bath

Abstract

This study aimed to deposit multi-elemental coatings, including Ni-Co-Mo-W, Ni-Co-Cu-W and Ni-Co-Cu-Mo-W, with different configurational entropy from an aqueous bath by direct current electrodeposition. For this purpose, the effect of current density on the elemental composition was studied, and it was shown that the induced deposition of W was mainly controlled by Co, while the induced deposition of Mo was affected by both Ni and Co. In addition, Ni showed an activation deposition mechanism, while Mo suffered from concentration polarization for a prolonged electrodeposition process. All elements deposited homogeneously through the coatings' thickness except Cu showed some segregation. However, XRD spectra proved to have an amorphous structure for all coatings. Upon heat treatment, the crystallization of the coatings occurs, which results in an increase in hardness values from 5.6 to 11 GPa by increasing the heat treatment temperature up to 800 °C in the case of Ni-Co-Mo-W coating. The maximum hardness values were achieved at 500 °C in other coatings, and then a slight decrease was observed. Polarization results of the as-deposited coatings showed the potential for passivation of these coatings. However, due to the defects in the coatings, they were not completely protective towards the substrate. The results suggest that the presence of Cu had the main impact on the coating's properties since it decreased Co deposition, the hardness and the thermal stability of the coatings.