Combined Effect of Phosphate Ester and OBCS on Tribochemical Decomposition of Hydrocarbon Oil on Nascent Steel Surfaces

Combined effect of additives, over-based calcium sulfonate (OBCS) and phosphate ester were investigated on the decomposition of multi-alkylated cyclopentane (MAC). The decomposition processes of the lubricants on the nascent surface of bearing steel AISI52100 were investigated using a ball-on-disk friction tester in a vacuum chamber with a quadrupole mass spectrometer. According to our previous report, the decomposition reaction can be deactivated by the effect of lubricant additives such as organic sulfides and organic phosphates. In this report, the order of efficiency in decreasing the decomposition rate was: OBCS < phosphate ester < phosphate ester + OBCS. The mixed additive was more effective and stable for suppression of the tribochemical decomposition under severe contact conditions of high load and high sliding speed. The critical load for the decomposition which is the lowest load to detect the gas evolution due to the decomposition increased with the mixed additive. XPS and TOF–SIMS analysis revealed that additive molecules competitively chemisorbed on the steel surface and reaction occurred by the formation of calcium phosphate, which covered the steel surface and deactivated the catalytic activity of active sites leading to deactivation. As a result, decomposition of MAC decreased significantly by the combined effect of the additives.