Annealing of Electrodeposited Nickel Coatings on Aluminum Alloy: Microstructural Properties and Wear Resistance of Nickel-Rich Aluminide Layers

Nickel coatings are electrodeposited onto Al 0.95 Mg 0.05 substrates and heat treated to improve the mechanical properties of aluminum alloys. Al 0.95 Mg 0.05 samples were electroplated for varying durations in a stirred nickel-plating Watts bath. The coating adhesion and surface mechanical properties were then improved by heat treatment at 450, 500, and 550 °C with annealing times ranging from 1 to 40 h depending on the diffusion temperature. Microstructural studies using optical microscopy, scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy, and x-ray diffraction reveal that heat treatment results in varying amounts of Al 3 Ni 2 and Al 3 Ni intermetallic phases. Berkovich microindentation was used to determine sample hardness, and wear tests were performed with a ball-plane tribometer with alternating translation motion. The results show that as-treated substrates have a higher hardness than the original Al 0.95 Mg 0.05 substrates due to the various (Al, Ni) intermetallic growths. The time of Ni deposition in the bath and the temperature/time of annealing influence the intermetallic type, thickness, and crystallinity. In addition, wear behavior demonstrates that unreacted Ni on the top surface is harmful and causes the running-in stage and that after several runs, the coefficient of friction of the as-prepared samples stabilizes between 0.3 and 0.36.