Surface and Coatings Technology 124 (2000) 201–209 www.elsevier.nl/locate/surfcoat Autocatalytic electroless nickel-boron plating on light alloys F. Delaunois *, J.P. Petitjean, P. Lienard, M. Jacob-Duliere Faculte ´ Polytechnique de Mons, Service de Me ´tallurgie physique, Rue du Joncquois, 53 B-7000 Mons, Belgium Received 16 July 1999; accepted in revised form 16 October 1999 Abstract Light alloys are more and more widely used, notably in aeronautics and the car industry. To improve their superficial properties, they can be covered with a hard and wear resistant protective layer. In this article, we examine the possibility of depositing a thick autocatalytic electroless Ni–B plating on aluminium alloys. We propose an alkaline bath containing sodium borohydride as the reducing agent. The deposition rate is held constant by a regular replenishment of the Ni–B bath allowing a rate of about 25 mm/h. The Knoop microhardness of the Ni–B deposits after the bath reaches about 750 hk 100 . Appropriate heat treatment can increase the deposit microhardness up to more than 1000 hk 100 but must be applied with regard to the substrate properties. The study of these heat treatments will be published in a further paper. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Aluminium; Boron; Electroless Plating; Microhardness; Nickel 1. Introduction Since the discovery of autocatalytic electroless nickel plating by Brenner and Riddel in 1946, its use has continued to grow because of its useful combination of properties and characteristics. Indeed, electroless nickel plating offers unique deposit properties, including uni- formity whatever the substrate geometry. Other features are excellent corrosion, wear and abrasion resistances, good ductility, lubricity, solderability, excellent electrical properties and high hardness [1–4]. Electroless Ni–B plating allows extremely hard and uniform deposition of nickel boride, providing incredible Fig. 1. Equilibrium established at mixed potential. wear and abrasion resistance. Several substrates can be covered: carbon steels, stainless steels, iron, aluminium the other a reducing agent [2,3,5]: and aluminium alloys, glasses, plastics, etc. Men++Red 1 Me+Ox 1 . 2. General process and bath composition When the reducing agent is present in solution, ready to be oxidized, the process is an electroless reduction. It Electroless plating includes general processes which can lead to non-limited thickness of deposits when the produce deposits without the use of an electric current parameters are correctly maintained. The difficulty is (Fig. 1). Electrons are supplied by a chemical reaction preventing spontaneous metal deposition with solution in solution which involves an exchange between two decomposition ( loss of bath stability). oxido-reduction couples in which one is Me/ Men+ and In the case of catalytic deposition, the reduction of the metallic ions in solution is under control and the baths only deposit on metallic substrates. With the * Corresponding author. addition of complexing agents and stabilizers, the reduc- 0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0257-8972(99)00621-0