Some Knowledge of Composite Electroless Nickel Plating

Electroless nickel plating (EN) is an auto-catalytic chemical technique used to deposit a layer of nickel-phosphorus or nickel-boron alloy on a solid workpiece, such as metal or plastic. The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPO2H2·H2O) which reacts with the metal ions to deposit metal. The alloys with different percentage of phosphorus, ranging from 2-5 (low phosphorus) to up to 11-14 (high phosphorus) are possible. The metallurgical properties of alloys depend on the percentage of phosphorus.

Electroless nickel is an alloy of nickel and phosphorus. It is an autocatalytic coating, which simply means it will deposit from solution on certain substrates without any external source of electricity. Electroless nickel coatings are produced by the controlled chemical reduction of nickel ions onto a catalytic surface. The reaction continues as long as the surface remains in contact with the electroless nickel solution. Because the deposit is applied without an electric current, its thickness is uniform on all areas in contact with fresh solution. All electroless nickel coatings have the distinct advantage of being able to evenly coat the substrate, both inside and out, as long as the solution flows uniformly. Electrolytic coatings, vapor coatings, and thermal-spray coatings typically cannot achieve uniform coating thicknesses across a broad range of part geometry. With some of these methods, a final 0.0005 inch thickness on a part interior may require depositing 0.001 inch or more on the exterior. Others cannot deposit on the interior of parts at
all. This can be a major cost advantage for electroless nickel coatings, and also makes it the only real choice in certain applications.  Electroless nickel plating can be divided intothree main types: low phosphorus (1 to 4 wt.% P),*Member of ASM International mid phosphorus (4 to 10 wt% P), and high phosphorus (>10.5 wt% P). Without the composite element, each subset has distinct uses and properties. Some properties of non-composite electroless nickels are presented in Table 1.This article will examine the performance and cost advantages possible with electroless nickel composite coatings. It will focus on four specific types of composite electroless nickels: diamond, silicon carbide, boron nitride, and polytetrafluoroethylene(PTFE).

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