Influence of cation valence states on electrically controlled friction behavior

The control of interfacial sliding friction is crucial for the performance and longevity of sophisticated mechanical systems. This study systematically investigates how cation valence states influence electrically regulated friction by employing seven inorganic salts (LiCl, NaCl, KCl, CsCl, NH₄Cl, MgCl₂, LaCl₃) as lubricant additives. Friction tests demonstrate that monovalent cations exhibit significant potential-dependent behavior, with the coefficient of friction decreasing under negative potentials. In contrast, divalent (Mg²⁺) and trivalent (La³⁺) cations show no such electro-response. Post-test analyses of X-ray photoelectron spectroscopy and energy dispersive spectrometer reveal that negative potentials promote the adsorption of monovalent cations onto the negatively charged steel ball, forming a protective hydration layer that reduces friction. Conversely, the adsorption capacities and tribochemical reactions of higher-valence cations remain potential-insensitive, resulting in negligible changes in their friction-reduction properties. These findings provide insights into the design of electro-responsive lubricants by highlighting the critical role of cation valence.