Hardness of nickel electrodeposition on aluminium alloys : influence of temperature and duration of heat treatment (original) (raw)

Aluminium and its alloys have a great potential for application in aerospace and automotive industries because of low cost, lightweight and good strength. Depositing hard coatings on Al alloys substrate is often used to improve the poor Al wear resistance. In this study, the way to improve the mechanical behaviour of Al alloys is to deposit a nickel layer followed by a heat treatment. Al-5%Mg alloy samples were electroplated using pure nickel. The deposition was carried out in a stirred standard nickel-plating Watts bath, with a current density of 2 A.dm-2 , a pH of 4.2 and a bath temperature of 45°C with different durations: 10, 15 and 20 min. A heat treatment was then performed to improve coating adhesion and surface mechanical properties at different temperatures and durations (450°C / 24h, 450°C / 40h, 500°C / 5h, 500°C / 10h and 550°C / 1h). To determine the hardness of the samples, Berkovich microindentation tests were performed and the Loubet's method was used to take into account the sink-in behaviour. The data processing allowed to find values of elastic modulus EIT = 69.6 ± 6.1 GPa and hardness HIT = 0.76 ± 0.03 GPa for the aluminium substrate closed to the theoretical values. Microhardness results of the non-treated Ni coatings deposited on Al alloy substrate confirm that their hardness is greater than that of substrate alone. For samples with Ni deposition time of 10 minutes, the highest hardness is obtained after annealing at 450°C during 40h and 500°C during 5h because of the formation of intermetallic phases between substrate and coating determined by X-ray diffraction (XRD). For annealing at 450°C during 24h, Energy Dispersive X-ray emission spectroscopy and XRD show that the time of the heat treatment is not sufficient to diffuse the nickel into the aluminium substrate. The hardness is related to the various phase of (Al,Ni) formed during annealing which depend on one hand of the Ni deposition time in the Watts bath and on the other hand of the pair temperature / time of heat treatment. Finally, a discussion is conducted on the intermetallic phases Al3Ni2 and Al3Ni obtained during heat treatments.