Electroless Ni–P/Ni–B duplex coatings: preparation and evaluation of microhardness, wear and corrosion resistance

The present work deals with the formation of Ni–P/Ni–B duplex coatings by electroless plating process and evaluation of their hardness, wear resistance and corrosion resistance. The Ni–P/Ni–B duplex coatings were prepared using dual baths (acidic hypophosphite- and alkaline borohydride-reduced electroless nickel baths) with both Ni–P and Ni–B as inner layers and with varying single layer thickness. Scanning electron microscopy (SEM) was used to assess the duplex interface. The microhardness, wear resistance and corrosion resistance of electroless nickel duplex coatings were compared with electroless Ni–P and Ni–B coatings of similar thickness. The study reveals that the Ni–P and Ni–B coatings are amorphous in their as-plated condition and upon heat-treatment at 450 °C for 1 h, both Ni–P and Ni–B coatings crystallize and produce nickel, nickel phosphide and nickel borides in the respective coatings. All the three phases are formed when Ni–P/Ni–B and Ni–B/Ni–P duplex coatings are heat-treated at 450 °C for 1 h. The duplex coatings are uniform and the compatibility between the layers is good. The microhardness, wear resistance and corrosion resistance of the duplex coating is higher than Ni–P and Ni–B coatings of similar thickness. Among the two types of duplex coatings studied, hardness and wear resistance is higher for coatings having Ni–B coating as the outer layer whereas better corrosion resistance is offered by coatings having Ni–P coating as the outer layer.