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Tribological Behaviour of Electroless NiP Coatings with an Extreme Pressure Additive: Synergistic or Antagonistic?


Electroless nickel–phosphorus (ENiP) coatings are widely used in many applications because of their excellent corrosion and wear resistance properties. However, their behaviour under lubricated conditions is less explored. This study examines the friction and wear properties using an Schwing–Reib–Verschleiss tribometer and the film-forming properties using scanning electron microscopy with energy-dispersive X-ray spectroscopy, atomic force microscopy and X-ray photoelectron spectroscopy of an extreme pressure (EP) additive for ENiP/steel and steel/steel contacts. The results show that the ENiP coating forms EP additive-derived tribofilms and these tribofilms consist of pad-like structures. These pad-like films have superior wear resistance properties compared to those formed on steel surfaces. The EP additive interacts synergistically with the ENiP coating and forms a tribofilm of about 130 nm thickness. The EP additive-derived tribofilms formed on the ENiP coating have a layered-structure, comprised of phosphates in the outermost layer followed, in order, by sulphides and phosphides. The results indicate that the ENiP coating could be a potential candidate for applications operating under elevated contact conditions lubricated with, for example, gear oils.