TOF-SIMS study on the adsorption behavior of mixtures of a phosphite and a friction modifier onto ferrous material

The adsorption behavior of a mixture of lubricant additives onto ferrous materials has been investigated by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Tri- n-butylphosphite (TBPi) and di- n-butylphosphite (DBPi) were selected as model compounds of phosphate-type additives and palmitic acid (PA), oleyl amine (OA) and glycerol mono-oleate (GM) were selected as model compounds of friction modifiers. The mixtures of one of the phosphites and one of the friction modifiers were dissolved in a paraffinic mineral oil for the lubrication tests. By TOF-SIMS analysis of the friction surfaces, the following conclusions were obtained. Adsorption of PA and GM was inhibited in the presence of phosphites, but the adsorption of OA was not inhibited by the phosphites. On the other hand, the reaction of TBPis was inhibited in the presence of OA, but for other mixed systems no inhibition of the reaction of phosphites by a friction modifier was observed. Except for OA mixed systems, the amount of reaction product of the phosphites was found to be correlated to the lubrication properties for all other mixture systems.