Voltage-Driven Growth of Phosphorus Tribofilms

Abstract

Ashless phosphorus-based lubricant additives, which are increasingly deployed in next-generation formulations, often suffer from slow tribofilm formation and poor film stability, limiting their effectiveness under demanding operating conditions. As mechanical systems become increasingly electrified, understanding how lubricants respond to electrical stimuli and developing strategies that exploit such stimuli have become critical for ensuring reliable operation. Here, we investigate the tribological performance of bis(2-ethylhexyl) phosphite (BEPite) in polyalphaolefin (PAO2) lubricated, electrified steel/steel contacts. Current is kept low to limit current-induced effects. Compared to an unbiased surface, BEPite produces thicker and denser tribofilms on anodic rubbing surfaces as voltage increases. This enhancement, however, is only observed when sliding is present. Chemical analysis reveals the presence of oxidised PO₃²⁻-related species and abundant iron in the tribofilm. Static electrochemical cell experiments show surface oxidation is enhanced on anodic surfaces. This suggests voltage-driven triboelectrochemical oxidation and reaction promote tribofilm formation. Either iron oxides or released Fe ions may alter the tribofilm structure. These phenomena appear general for phosphorus-based ashless additives, as both phosphites and phosphates with different structures have seen increased tribofilm growth at anodic rubbing surface. This work demonstrates that an applied voltage, coupled with sliding, can intensify triboelectrochemical reactions, providing new insights for the design of next-generation lubricants.