Tailoring Surface Roughness for Enhanced Superlubricity of NiBased Coatings in Neutral Aqueous Sodium Hypophosphite Lubricant

Superlubricity is typically attained on sliding counterpart surfaces with relatively smooth morphologies. This study explores the lubrication efficacy of NaH2PO2 aqueous lubricant on Ni−P coatings with a surface roughness in the range of 70−400 nm. The results demonstrate that superlubrication can be achieved on the Ni−P coatings with roughness values of ∼95 and ∼200 nm. Throughout a wide range of normal loads and sliding speeds, the durable superlubricity behaviors were maintained for an extended period of up to 8 h. The underlying mechanism of superlubricity was elucidated through atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analysis. Notably, it was found that the beneficial tribochemical polishing effect took place between the friction pairs, resulting in a relatively smooth surface morphology and forming the reaction film containing Si−O and P−O on the wear track. In addition, the initial surface morphology of the coatings significantly influenced the content of Si−O, P−O, and Ni−O compounds and the topography of the wear surface, thereby directly affecting the feasibility of achieving superlubricity. This study demonstrates the superlubricity behaviors of Ni−P coatings with high surface roughness that hold substantial significance for advancing industrial applications.