Influence of heat treatment on microstructure and adhesion of Al2O3 fiber-reinforced electroless Ni–P coating on Al–Si casting alloy

Influence of heat treatment regime on microstructure, phase composition and adhesion of Al2O3 fiber-reinforced Ni–P electroless coating on an Al–10Si–0.3 Mg casting alloy is investigated in this work. The pre-treated substrate was plated using a bath containing nickel hypophosphite, nickel lactate and lactic acid. Al2O3 fibers pretreated with demineralised water were placed into the plating bath. Resulting Ni–P–Al2O3 coating thickness was about 12 μm. The coated samples were heat treated at 400–550 °C/1–8 h. LM, SEM, EDS and XRD were used to investigate phase transformations. Adhesion of coating was estimated using scratch test with an initial load of 8.80 N. It is found that annealing at high temperatures (450 °C and above) leads to the formation of hard intermetallic products (namely Al3Ni and Al3Ni2 phases) at the substrate–coating interface. However, as determined by the light microscopy and by the scratch test, these phases reduce the coating adhesion (compared to coatings treated by the optimal annealing regime 400 °C/1 h). The analysis of scratch tracks proves that fiber reinforcement significantly reduces the coating scaling. However, due to the formed intermetallic sub-layers, partial coating delamination may occur on the samples annealed at 450 °C and above.