A Chemical Potential Equation for Modeling Triboelectrochemical Reactions on Solid–Liquid Interfaces

Abstract

Triboelectrochemical reactions occur on solid–liquid interfaces in wide range of applications when an electric field strong enough and a frictional stress high enough are simultaneously imposed on the interfaces. A characteristic of triboelectrochemical reactions is that not only the thermal energy but also the electrical and mechanical energies can activate, assist, or mitigate the solid–liquid interface chemical reactions, the products of which affect electrical and tribological behavior of the interfaces inversely. In previous studies, we have found that the coupling of frictional and electric effects could physically change the migration, adsorption, and desorption behaviors of the polar molecules, ions, or charged particles included in aqueous or nonaqueous base lubricant toward or away from the interfaces and thus control the boundary lubrication. Recently, we have found that the friction coefficient and surface appearance of some kinds of metals could also be modulated to some extent even in pure water or pure base oils under external electric stimulations. We attribute these changes to the triboelectrochemical reactions occurred when a strong external electric field is imposed on. Based on the effective collision model of chemical reactions, a chemical potential equation, which includes both electrical and mechanical contributions, has been derived. The proposed chemical potential equation can be used to explain the observed triboelectrochemical phenomenon in experiments. Based on the model, a novel method for oxidation coloring of the selected areas in metal surfaces is proposed. Together with the physical adsorption and desorption model of lubricant additives, the triboelectrochemical reaction model can well explain the phenomena of potential-controlled boundary lubrication in different lubrication systems and also provides a theoretical basis for other solid–liquid interface processes under the effects of electromechanical coupling.

Intermolecular and surface forces in atomic-scale manufacturing

“…Liu et al [119] designed a three-electrode friction system and verified the feasibility of theory. The important discovery is that the lubricated friction can be controlled by the applied elasticity.…”

“…Gutman [117] and Chen [118] suggested that concept of chemical potential, instead of Gibbs free energy, should be used in material science. Based on this concept, Liu et al [119] proposed their model for explaining the process of tribo-electrochemistry, which can be used to analyze the electricity-assisted manufacturing process.…”

Mitigation of tribocorrosion of metals in aqueous solutions by potential-enhanced adsorption of surfactants

“…As we know, the traditional electrochemical processes generally include adsorption/desorption processes when the surface potential of electrode is within the electrochemical potential window, reduction process at negative and oxidation process at positive potentials. For the electrode processes with the influence of external forces, triboelectrochemical processes including frictioninduced reduction, friction-inhibited reduction, friction-induced oxidation, and friction-inhibited oxidation should be involved [22]. Friction could affect the electrode processes, as well as the electrochemical potential window.…”

“…The types and structures of the oxides affect the tribological behaviors mostly. For example, CuO or CuO 2 can reduce the friction coefficient [22,29]. While, iron oxides increase the friction coefficient [7].…”

“…The response time of the COF to increase from 0.10 to 0.45 was less than 1.5 s when the potential was changed from the open circuit potential (OCP) to a negative value. Most recently, we investigated the effect of normal loads on the electrochemical window and found a difference in electrochemical reactions between the friction contact and non-contact areas [22]. It had been predicted that if the local external pressure was 1 GPa and the activation volume of metal atoms was 1 mol, the potential change caused by external force was around 20−40 mV.…”