Effect of Cobalt Content on the Properties of Nickel-Cobalt Alloy Coatings

Abstract

A batch of nickel-cobalt alloy coatings was pulse-electroplated on gun steel (PCrNi3MoVA) surface, in which weight fractions of Co element are 39.9 %, 55.5 %, 67.5 %, and 79.8 %, respectively. The formations of phase and crystal structure in nickel-cobalt coatings were investigated by X-ray diffraction analysis. The layers' surface morphology and chemical composition were analyzed by scanning electron microscopy coupled with energy dispersive spectrometry. The tribological and nano-mechanical performances of the coatings were used to evaluate by high-speed reciprocating friction and wear tester and nanoindentation tester, respectively. The results showed that the preferred orientation of the coatings gradually changed from (111) to (220) with the rising cobalt content, resulting in changes in the surface morphology of the coatings from pyramidal grains to spherical particles and then to a triangular pyramid structure. As the Co content was 55.5 wt.% in the nickel-cobalt coating, its crystalline grain size reached the smallest value of 11.53 nm, enhancing the hardness and wear resistance.