Tailoring Surface Roughness for Enhanced Superlubricity of Ni-Based 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.