Materials Chemistry and Physics 82 (2003) 771–779 Electroless Ni–P/Ni–B duplex coatings: preparation and evaluation of microhardness, wear and corrosion resistance T.S.N. Sankara Narayanan a,∗ , K. Krishnaveni a , S.K. Seshadri b a National Metallurgical Laboratory, Madras Centre CSIR Complex, Taramani, Chennai 600113, India b Department of Metallurgical Engineering, Indian Institute of Technology, Chennai 600036, India Received 24 October 2002; received in revised form 20 June 2003; accepted 11 July 2003 Abstract 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. © 2003 Elsevier B.V. All rights reserved. Keywords: Amorphous materials; Electroless plating; Duplex coating; Microhardness; Wear resistance; Corrosion resistance 1. Introduction Electroless deposition process experienced numerous modifications to meet the challenging needs of a variety of industrial applications since Brenner and Riddell in- vented the process in 1946 [1]. Among the various types of electroless plating, electroless nickel has gained immense popularity due to their ability to provide a hard, wear and corrosion resistant surface [2–4]. Hypophosphite-reduced electroless nickel plating has proved its supremacy in providing improved hardness, corrosion and wear resis- tance. Recently, much attention is being paid towards borohydride-reduced electroless nickel plating. The prop- erties of sodium borohydride-reduced electroless nickel coatings are often superior to those of deposits reduced with other boron compounds or with sodium hypophos- phite. The principal advantages of borohydride-reduced electroless nickel deposits are its hardness and superior wear resistance in the as-deposited condition [5]. Elec- ∗ Corresponding author. Tel.: +91-44-2254-2077; fax: +91-44-2254-1027. E-mail address: tsnsn@rediffmail.com (T.S.N.S. Narayanan). troless Ni–B coatings are more wear resistant than tool steel and hard chromium coatings and it can replace gold in electronic industries. The columnar structure of Ni–B coatings is useful in retaining lubricants under conditions of adhesive wear. The rapid changing needs of engineering industries warrant the development of coatings that possess high hardness and good wear and abrasion resistance and in this respect one such coating which seems promising is the borohydride-reduced electroless nickel deposit. The major limitation of Ni–B coating is its relatively poor corrosion resistance compared to electroless Ni–P deposits. The cor- rosion resistance of electroless Ni–P and Ni–B deposits is found to increase with the incorporation of an additional alloying element such as Cu, Zn, W, Mo, etc. or with the incorporation of second phase particles, such as silicon ni- tride, ceria and titania in the metal matrix [6–9]. Ni–P–B alloy deposits are prepared using both the reducing agents in the same plating bath [10]. As borohydride is stable only under alkaline condition, Ni–P–B alloy deposits are generally prepared by the addition of hypophosphite in the borohydride-reduced electroless nickel bath. Since borohy- dride is a powerful reducing agent than hypophosphite and the plating bath is alkaline, the amount of phosphorous in 0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0254-0584(03)00390-0