Electroless Ni–P Composite Coatings: The Effect of Heat Treatment on the Microhardness of Substrate and Coating

A possibility to increase materials performance for different applications is to protect them by coatings. Electroless nickel deposition represents an alternative method to obtain coatings on various substrates (metallic and nonmetallic). The process is based on a redox reaction in which the reducing agent is oxidized and Ni ions are reduced on the substrate surface. Once the first layer of nickel is deposited it is acting as a catalyst for the process. As a result, a linear relationship between coating thickness and time, usually occurs [1]. If the reducing agent is sodium hypophosphite, the obtained deposit will be a nickel–phosphorus alloy. The as-deposited Ni–P alloys were reported to have a non-equilibrium phase structure [2]. It is generally accepted that a microcrystalline, amorphous or a co-existence of these two phases can be obtained depending on phosphorus content [2–6]. A recent advance in electroless Ni–P deposition is the co-deposition of solid particles within coatings. These solid particles can be hard materials (SiC, B4C, Al2O3, diamond [7–10]) and dry lubricants (PTFE, MoS2 and graphite [11–13]). By this method, composite layers with very good characteristics for specific applications can be produced. Electroless Ni–P–SiC coatings are recognized for their hardness and wear resistance and can replace “hard chromium” in aerospace industry [1, 10, 14]. Also, the electroless Ni–P–PTFE films present excellent self-lubricating properties [12]. One of the more increasingly used substrate materials in electroless nickel deposition is aluminium and its alloys [15]. However, aluminium is a very reactive metal which easily form a hard to remove oxide film during rinsing or exposure to air. This oxide film represents a disadvantage for electroless nickel deposition where a metal-metal bond is required. To overcome this problem, a suitable pretreatment sequence including a deoxidizing step followed by a double immersion in a modified alloy zincate (MAZ) solution can be applied [15, 16]. To increase the hardness and the abrasion resistance of electroless Ni–P deposits, heat treatments are performed. Studies carried out in this direction [7] showed that a maximum hardness can be achieved after a heat treatment at 400°C for 1 h, when the hardness of the deposit increased from 500–600 up to 1000–1100 HV100. In this study, the electroless Ni–P and Ni–P–X (X 5 SiC, Al2O3 and B) coatings deposited on a heat treatable Al alloy (6063-T6) were investigated. Coating structure investigated by XRD and DSC, and Pergamon Scripta Materialia, Vol. 38, No. 9, pp. 1347–1353, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 1 .00 PII S1359-6462(98)00054-2