Nickel–tungsten alloy brush plating for engineering applications

Abstract Hard coating can improve the surface properties of a material beyond the capability of the substrate. For a long time, coatings such as hard nickel, hard chromium, hard alloys and hard composites have been developed for engineering applications due to the improved wear resistance. A newly-developed nickel–tungsten brush plating process has the potential as an alternative of electroplating of hard chromium for engineering application. The nickel–tungsten solution is based on the ammoniacal citrate bath, which can be conveniently brush plated just as brush plating of other metals or alloys. On the other side, hard chromium cannot be brush plated due to the exposure of hazardous hexavalent chromium exceeding the Occupational Safety and Health Administration (OSHA) limits. The nickel–tungsten alloy coating has been developed for engineering application. It is of nanocrystalline structure (~ 2 nm crystallite size) and demonstrates excellent hardness and wear resistance. The coating is 60% nickel, 40% tungsten by weight. Due to high tungsten content in the alloy, it is thermally stable. Moderately elevated temperatures (200–500 °C) do not cause grain size growth and softening as the case of most other hard coatings (such as hard chromium, nickel phosphorus). Actually, the nickel–tungsten alloy coating can be further hardened by exposure to high temperature of 200–500 °C for a short period of time. Beyond the hardness and wear properties, the coating has been further characterized by XRD, electronic and optical microscopy, hydrogen embrittlement, salt spray corrosion, tribology, axial fatigue, and other testing.