Effect of Lubricant Additives on Tribochemical Decomposition of Hydrocarbon Oil on Nascent Steel Surfaces

A nascent surface has high activity to catalyze the decomposition of lubricants under boundary lubrication conditions. The effects of sulfur-containing, nitrogen-containing, phosphorus-containing additives and phosphate-containing ionic liquid were investigated on the decomposition of synthetic hydrocarbon oil (multialkylated cyclopentane, MAC). The decomposition processes of the lubricants on the nascent surface of bearing steel AISI 52100 were investigated using a ball-on-disk friction tester in a vacuum chamber with a quadrupole mass spectrometer. Three parameters related to the decomposition process were observed: the induction period for the decomposition, the desorption rate of gaseous products, and the critical load for the activation of the decomposition. The order of efficiency of additives in extending the induction period was: sulfur-containing additive (S)<nitrogen-containing additive (N)<phosphorus-containing additive (P)<phosphate-containing ionic liquid (P-IL). The order of efficiency in increasing the critical load was: N<S<P<P-IL, and the order of efficiency in decreasing the decomposition rate was: N<S<P<P-IL. These results suggest that additives which can form iron salts (such as iron phosphate and iron sulfide) will deactivate the nascent surface, decreasing the decomposition rate and increasing the critical load.