in the plating bath, so that the growing metal layer will surround and cover them. This procedure was initially developed by Odekerken in 1966 for electrodeposited nickel-chromium coatings. In that study, in an intermediate layer, finely powdered particles, like aluminum oxide and polyvinyl chloride (PVC) resin, were distributed within a metallic matrix. By changing the baths, the procedure can create coatings with multiple layers of different composition. The first commercial application of their work was electroless nickel-silicon carbide coatings on the Wankel internal combustion engine. Another commercial composite in 1981 incorporated polytetrafluoroethylene (nickel-phosphorus PTFE). However, the co-deposition of diamond and PTFE particles was more difficult than that of aluminum oxide or silicon carbide. The feasibility to incorporate the second phase of fine particles, the size of a nanometer to micrometer, within a metal-alloy matrix has initiated a new generation of composite coatings. Characteristics Advantages and disadvantages Compared to the electrolytic process, a major advantage of electroless nickel plating is that it creates an even coating of a desired thickness and volume, even in parts with complex shape, recesses, and blind holes. Because of this property, it may often be the only option. Another major advantage of EN plating is that it does not require electrical power, electrical apparatuses, or sophisticated jigs and racks. If properly formulated, EN plating may also provide a less porous coating, harder and more resistant to corrosion and hydrogen absorption. Electroless nickel plating also can produce coatings that are free of built-in mechanical stress, or even have compressive stress. A disadvantage is the higher cost of the chemicals, which are consumed in proportion to the mass of nickel deposited; whereas in electroplating the nickel ions are replenished by the metallic nickel anode. Automatic mechanisms may be needed to replenish those reagents during plating. The specific characteristics vary depending on the type of EN plating and nickel alloy used, which are chosen to suit the application. Types The metallurgical properties of the alloy depend on the percentage of phosphorus. Low-phosphorus coatings have up to 4% P contents. Their hardness reaches up to 60 on the Rockwell C scale. Medium-phosphorus coatings, the most common type, are defined as those with 4 to 10% P, although the range depends on the application: up to 4–7% for decorative applications, 6–9% for industrial applications, and 4–10% for electronics. High-phosphorus coatings have 10–14% P. They are preferred for parts that will be