Electroless nickel-boron plating
Electroless nickel-boron coating (often called NiB 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 nickel salt 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 substrate shape on the electric field at its surface. Moreover, electroless plating can be applied to non-conductive surfaces. The plating bath usually also contains buffers, complexants, and other control chemicals. History. Electroless nickel-boron plating developed as a variant of  the similar nickel-phosphorus process, discovered accidentally by Charles Adolphe Wurtz in 1844. In 1969, Harold Edward Bellis from DuPont filed a patent for a general class of electroless plating processes using sodium borohydride, dimethylamine borane, or sodium hypophosphite, in the presence of thallium salts, thus producing a metal-thallium-boron or metal-thallium-phosphorus; where the metal could be either nickel or cobalt. The boron or phosphorus contents was claimed to be variable from 0.1 to 12%, and that of thallium from 0.5 to 6%. The coatings were claimed to be "an intimate dispersion of hard trinickel boride () or  nickel phosphide () in a soft matrix of nickel and thallium". Several versions of the process were patented by Charles Edward McComas in the following years: Types. The earliest variants of electroless nickel-boron plating included thallium salts in the plating bath, and actually created nickel-thallium-boron coatings ("Type 1" in the AMS classification). Eventually formulations were devised that were free from the toxic thallium ingredients, resulting in true nickel-boron ("Type 2") coatings. 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 electroless nickel-boron coating include saw blades, ship propellers, down-hole crude oil pumping equipment, bushings, thrust washers, paper guide plates, and greaseless guns.