Tribological properties of heat-treated electroless Ni–P coatings on AZ91 alloy

Influence of heat treatment regime on adhesion and wear resistance of Ni–P electroless coating on AZ91 magnesium alloy is investigated in this work. The pretreated substrate was plated using a bath containing nickel sulphate, sodium hypophosphite and sodium acetate as main constituents. The coated samples were heat treated at 400–450 °C for 1–8 h. Adhesion of coating was estimated from the scratch test with an initial load of 8.80 N. Wear resistance was studied using the pin-on-disc method. It was found that there is no significant dependence of the coating wear resistance on heat treatment regime, as the formation of Al–Ni intermetallic sub-layers that reduce coating adhesion is limited to regions where Al17Mg12 phase is present in the substrate. Moreover, the coating shows good sliding properties due to the formation of oxide glazes in the wear track.