Tribological Behaviour of Electroless NiP Coatings with an Extreme Pressure Additive: Synergistic or Antagonistic?
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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.
KeywordsENiP coatings NiP EP additive Wear Pad-like tribofilms XPS
This work was funded by the Austrian COMET Programme (Project K2 XTribology. Nos. 824187 and 849109) and carried out at the ‘‘Excellence Centre of Tribology’’. The authors wish to thank Collini Applied Surface Intelligence, Evonik Industries AG Oil Additives, High Tech Coatings GmbH, Magna Powertrain AG & Co KG, OMV Refining & Marketing GmbH for their financial support and active research cooperation. The authors would also like to thank Michael Schweitzer, Markus Premauer and Christian Tomastik of AC2T research GmbH, Austria for conducting SRV experiments, carrying out optical profiler and XPS measurements, respectively.
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