Synergistic Enhancement Mechanism of Electric Double Layer Structure and Interfacial Tribochemical Reactions Regulated by pH in Water-Based Lubrication Systems

Precise regulation of interfacial electrochemistry through pH control represents an emerging strategy for enhancing water-based lubrication. However, the synergistic effects between pH-modulated electric double layer (EDL) structures and tribochemical reactions remain inadequately quantified. In this paper, we systematically investigate the tribological behavior of GCr15 steel-SiC friction pairs lubricated with water-based lubricants of Na₂SiO₃ and NaHCO₃ at concentrations of 1, 5, and 10 wt %. Friction test and triboelectric test, coupled with scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) analyses, were employed to reveal how pH influences interfacial processes on multiple scales. The results showed that 5 wt % Na₂SiO₃ achieves the low coefficient of friction (COF) and reduces wear by 67.43%, owing to the formation of a dense and protective silicate tribofilm. In contrast, 10 wt % NaHCO₃ achieved a low COF but provided limited wear protection due to the development of a porous and unstable carbonate layer. Furthermore, we identified a closed-loop feedback mechanism of the friction-polarization electric field-EDL-ion migration-tribochemical reaction, where pH alters the OH^- concentration, thereby compressing or expanding the EDL and regulating anion adsorption kinetics as well as tribofilm stability. In order to verify the above mechanism, a quantitative multivariable model incorporating pH and short-circuit current was constructed to capture system-specific sensitivities. The model revealed that Na₂SiO₃ systems display higher pH responsiveness due to compression of the EDL and interfacial passivation, whereas NaHCO₃ systems were dominated by tribofilm rupture-regeneration dynamics. This work provides a new framework for the rational design of high-performance water-based lubricants.