Mohamed HAMIDOUCHE, Nafissa KHENNAFI-BENGHALEM

hamidouchemohamed@yahoo.fr

Abstract

In order to improve the performances of nickel coatings, different approaches have been applied such as alloying with other elements, crystallites refinement using additives, and deposition of composite coatings by dispersing reinforcing particles inside the Ni matrix. The alloying of Ni with cobalt was one of the most commonly techniques used for improve the properties of nanocrystalline nickel coatings. Ni-Co alloys have been investigated as important engineering materials for several decades due to their unique properties, such as high resilience, good corrosion resistance, thermal conductivity, electro-catalytic activity and their magnetic properties.

In this work, we will try to give more value to these coatings, and this by proof of their good tribological properties. For this, a comparison was made between the microstructural and morphological properties of high cobalt content nanocrystalline Ni-Co alloy coating (87% Co) with a nanocrystalline nickel coating produced electrochemically. Thus, a correlation between these properties and the mechanical and tribological properties of these deposits was made. The observation of the coatings made by scanning electron microscopy (SEM) shows that nickel coatings have a smooth morphology whereas Ni-Co alloy deposits have a lens shaped morphology with a considerable increase in the grains size of Ni-Co alloy deposits compared to Ni coatings. The results of XRD show that nickel coatings having fcc phase structure while Ni-Co coatings have hcp phase structure. The study of coatings microhardness indicates that the latter follows Hall Petche effect where nickel deposits which have the smallest grain size show a higher micro-hardness in comparison with Ni-Co coatings. Pin on disk tribometric analysis under unlubricated conditions illustrates a great improvement of the wear resistance by the addition of high cobalt content on nickel coatings, where the friction coefficient and the wear rates are significantly reduced in Ni-Co coatings compared with nickel coatings.

Keywords: Nickel coatings, Ni-Co coatings, Microstructure, Morphology, Micro-hardness, Tribological properties.