Influences of Al particles and current density on structural, mechanical and anti-corrosion properties of electrodeposited Ni–Co/Al composite coatings

Abstract

Ni–Co/Al composite coatings have been synthesized via electrochemical co-deposition routes. The effects of Al particle concentration upon the co-electrodeposition and performance of Ni–Co/Al composite coatings have been evaluated. The prepared coatings present compact and flat surfaces, displaying nodular-like morphology that consists of grain clusters. The grain sizes of the coatings were refined from 30 nm to 25 nm when Al particles were added but enlarged from 27 nm to 38 nm with the increase of current density from 2 to 8 A dm−2. The mean value of Ra of the surface prepared at 2 A dm−2 and 6 A dm−2 was 191 nm and 302 nm, respectively, indicating large current density promoted the rougher surface. The Ni–Co/Al obtained at 2 A dm−2 and 20 g L−1 Al particles contains about 13.77 wt% Al. Microhardness was improved as Al particles increased in the Ni–Co matrix, but coefficients of friction (COF) decreased in the presence of Al particles. As Al particle concentration increased, the corrosion resistance first enhanced and then declined when Al surpassed 40 g L−1. Due to the competition of reduction deposition between Ni and Co ions in electrolytes, it was found that Co content increased and Ni content decreased for Ni–Co/Al deposits. The Ni–Co/Al composite coating electrodeposited at 2–4 A dm−2 with 20–40 g L−1 Al particles exhibited the best mechanical and anti-corrosion performance.