Voltage-Driven Growth of Phosphorus Tribofilms

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 steel/steel contacts under controlled voltage and current. BEPite produces thicker and denser tribofilms on anodic rubbing surfaces when a voltage is applied. Increasing the current between lubricated contacts from < 0.01 mA to 0.5 mA has little effect on tribofilm formation, whereas increasing voltage enhances it. This enhancement is only observed in rubbing contacts. Chemical analysis reveals the presence of oxidized PO ₃ ²⁻ -related species and abundant iron in the tribofilm. This suggests voltage-driven triboelectrochemical oxidation, as well as enhanced additive adsorption 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 voltage alone can intensify triboelectrochemical reactions, providing new insights for the design of next-generation lubricants.