Electroless nickel-boron coating is a metal plating process that can create a layer of a nickel-boron alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing nickelsalt and a boron-containing reducing agent, such as an alkylamineborane or sodium borohydride. It is a type of electroless nickel plating. A similar process, that uses a hypophosphite as a reducing agent, yields a nickel-phosphorus coating instead. Unlike electroplating, electroless plating processes in general not require passing an electric current through the bath and the substrate; the reduction of the metal cations in solution to metallic is achieved by purely chemical means, through an autocatalytic reaction. Thus electroless plating creates an even layer of metal regardless of the geometry of the surface – in contrast to electroplating which suffers from uneven current density due to the effect of subtrate shape on the electric field at its surface. Moreover, electroless plating can be applied to non-conductive surfaces. The plating bath usually contains also buffers, complexants, and other control chemicals.
Bellis's formulation was modified by adding stronger chelating agents. A method for applying nickel-thallium-boron to titanium without fatiguing the titanium substrate was also developed and commercialized by Purecoat International under the brand "NiBRON".
In 1986, McComas patented an improved formulation for nickel-cobalt-thallium-boron coating, that further increased stability and repeatability of the process. The patent claims that the coating consisted of "hard, amorphous alloy nodules of high nickel content dispersed or rooted in a softer alloy of high cobalt content".
In 1994 McComas developed another formulation, patented in 1998, that allowed consumed chemicals to be replenished during the plating, making it into a continuous process rather than a batch one.
The earliest variants of electroless nickel-boron plating included thallium salts in the plating bath, and actually created nickel-thallium-boron coatings. Eventually formulations were devised that were free from the toxic thallium ingredients, resulting in true nickel-boron coatngs.
Characteristics
As-plated grains of amorphous nickel boron deposit in a columnar structure with the columns being perpendicular to the substrate surface and forming a nodular topography on the surface. Coating will contain 2.5–8% boron by weight. The nodular structure reduces surface-to-surface contact of two mating/sliding surfaces, thus reducing friction and improving heat dissipation. It is also claimed to reduces drag in both gas and liquid flows.
Applications
Actual and potential applications of eectroless nickel-boron coating include saw blades blades, ship propellers, down-hole crude oil pumping equipment, bushings, thrust washers, paper guide plates, and greaseless guns.
