Formation and characterization of multilayers borohydride and hypophosphite reduced electroless nickel deposits

Abstract

Electroless nickel coatings are popular surface treatments for industrial application because they can provide either excellent corrosion protection (in the case of nickel-phosphorus − Ni-P − coatings) or excellent wear resistance and superficial hardness (in the case of nickel-boron − Ni-B − coatings). The properties provided by both types of coatings are thus complementary and recent work has focused on combining electroless Ni-P and Ni-B in the form of duplex coatings. This work goes further on this research by studying multilayer electroless Ni-P/Ni-B coatings. The Ni–P/Ni–B multilayer coatings were prepared using a commercial bath for the Ni-P coatings and a lab-developed alkaline bath based on sodium borohydride for the Ni-B deposits. Single layers of Ni-P and Ni-B as well as multilayers constituted of 10 layers, alternating between Ni-P and Ni-B electroless bath, were prepared. Scanning electron microscopy (SEM) observations of the cross-sections of the nickel duplex coatings reveal that coatings are uniform with proper compatibility between layers. When compared with the single layers of NiB and NiP the multilayer coatings have better wear and abrasion properties. Scratch test was used to qualify the adherence between the layers. Hardness and adhesion properties are closer to NiP.