Electroless coatings, which utilize chemical catalysis and reduction reactions to solve the problems of surface treatment, are widely used in the aerospace, semiconductor, machinery, petroleum, electronic and chemical industries . Electroless nickel coating, which does not require external current, is one kind of electroless coating process. The principle is to use the charge released by the reducing agent in the plating solution to supply the surrounding nickel ions. The reaction surface must have catalytic activity to reduce the nickel metal ions on the surface of the coated parts. The application and research of electroless coating are promoted with plating bath regeneration technology, adding additives and stabilizers to extend the working life of the plating solution and improve the quality of the coating products. Due to the development of high-tech and defense industries, the material properties and hardness requirements have been continuously improved. Thin film coatings meeting the properties required by high-tech and defense industries have been prepared to improve the wear resistance and corrosion resistance of materials , and new plating solution compositions, the performance and processing conditions, reduction of preparation costs and improvement of electroless coating technology are electroless coating technology research topics for the future.
Non-heat-treated electroless nickel coating film is metastable, and the structure of the metastable film is related to the phosphorus content. When the phosphorus content is high, the coating is amorphous, and the low phosphorus content can form microcrystalline. Gutzeit  reports the phosphorus content is less than 7 wt.%, and nickel-phosphorus alloy is a supersaturated solution of phosphorus in nickel crystals. Park and Lee  mentioned phosphorus content above 10 wt.% is completely amorphous. When Jones et al.  studied amorphous nickel-phosphorus alloys, they found the main effect of phosphorus in nickel-phosphorus alloys is the overpassive behavior shifts to low potential. This phenomenon also increases the current density in the passive and overpassive regions. Bai et al.  showed the adhesion strength between DLC films and nitrile-butadiene rubber first increases and then decreases along with the change in Ar plasma pretreatment time. Hsu et al.  found Ni-P films have greater hardness and fatigue life as the concentration of phosphorus gradually decreases from 17.07 to 16.83 at.%. Czagány et al.  found the microhardness increases with increased P content in Ni-P coating after annealing treatment. Many studies mention electroless nickel coating has better corrosion resistance because electroless nickel coating is an amorphous coating and has better corrosion resistance than the crystalline phase [9–11]. Following proper heat treatment of the phosphorus content of the coating film below 15 wt.%, the coating structure changes from amorphous to crystalline structures of Ni and Ni3P. In Allen and Vander Sande research , the amorphous nickel phosphorus matrix was replaced by nickel FCC crystals at 247oC, and Ni3P was precipitated and grown above 367oC.
Ever increasing demands of structures and components with regard to aerospace, military and transport industries require the development of advanced transmission mechanisms. The bearing, which controls the high accuracy and stability of machinery, is an indispensable key supporting component in the rotating or linear moving mechanism of machinery. In general, bearing operating modes can be divided into gas, magnetic levitation, hydrostatic pressure, hydrodynamic pressure, and rolling types. However, the efficiency requirements of industrial equipment are getting higher and higher, and the speed of the rotating mechanism is continuously increasing. When the speed increases, the micro-bearing operating inside the rotating mechanism will reduce the working life and increase the amount of vibration. The low working life of the bearing will lead to increased manufacturing costs and vibration in power transmission. Therefore, a metal nickel layer is prepared using the electroless coating method to achieve the effects of high hardness, corrosion resistance and wear resistance on the smaller copper bearing. However, the amount of phosphorus in the electroless nickel coating layer depends on the pH value of the plating solution, the deposition time, the temperature of the plating solution, the content of the nickel salt and the reducing agent and other supplemental liquid content factors. In this study, the electroless coating parameters (pH value, deposition time and bath temperature) were changed to find the optimal parameters with the smallest micro journal bearing wear. Nickel-coated micro-bearing are annealed under various temperatures (100, 200, 300, 400, 500, 600 and 700oC) at the same time to analyze the effects of the film surface microstructure, crystal properties, microhardness, roughness and abrasion tests.