Electroless Ni-B and Ni-P coatings with high-fretting resistance for electrical contact applications

Electroless nickel-phosphorus and nickel-boron plating have been tried on steel and brass substrates. By selected conditions of heat treatment in a high vacuum environment the plating can produce more than chromium equivalent hardness without the effluents of the hard chromium plating process. The resulting surfaces of Ni-P and Ni-B were examined and characterized under an optical and a scanning electron microscope. XRD analysis was also performed to investigate re-crystallization effects. The fabricated contact materials were also tested under corrosion conditions, and linear polarization measurements were performed. The electrical contact resistance of the deposits was also recorded during a 10,000 cycle test. The results show that after the heat treatment in a high vacuum environment the platings acquire a crystalline microstructure, with very good adhesion to the substrate materials (steel or brass). In addition, the microhardness of vacuum heat treated Ni-P coatings attained values of 1200 HV, while the nickel-boron coatings acquired an even higher microhardness, exceeding 1600 HV. The electrical performance of the fabricated layers showed that after thermal treatment the contact resistance remained almost unaffected. However, the Ni-B overlayers systematically exhibit lower contact resistance profiles. The above materials could be practically utilized whenever a good electrical performance of a hard engineering coating is required.