The tribological properties of trifluorotris(pentafluoroethyl) phosphate [(C 2 F 5 ) 3 PF 3 -, FAP]-derived ionic liquids were evaluated under boundary conditions. The anion is hydrophobic in comparison with bis(trifluoromethylsulfonyl)imide [(CF 3 SO 2 ) 2 N -, TFSI]. 1,3-Dialkyli midazolium salts of FAP provided much lower friction than 1,3-dialkylimidazolium salts of TFSI. In addition, the FAP salts exhibit better anti-wear properties than the TFSI salts. Another advantage of the FAP anion is availability of several cations to prepare ionic liquids. For example, tetraalkyl phosphonium, N,N-dialkylpyrrolidium, and tetramethylisouronium salts of FAP provided friction coefficient of approximately 0.1. Straight-chain carboxylic acids as model friction-reducing additives improved the tribological properties of the FAP salts. Surface analyses were conducted to study the boundary film formed by rubbing. It was found that the boundary film is composed of adsorbed anion on uppermost surfaces and reacted anion on sub-surfaces. The model friction-reducing additives were found on the rubbed surfaces.

The tribological properties of room temperature ionic liquids containing tetraalkylphosphonium cations were evaluated on the basis of the chemical structure of their salts. The tribochemistry of these ionic liquids was discussed on the basis of the results of tribo-tests and surface analyses. The tribological properties of the tetraalkylphosphonium salts examined in this work were observed to be better than those of 1,3-alkylimidazolium salts. The structure of the alkyl group in the phosphonium cation also has a slight effect on the tribological properties of the salts. During a friction test carried out under lowload conditions, the phosphonium cation was oxidized to phosphate to form a boundary film. This film inhibited the reaction of the bis(trifluoromethanesulfonyl)amide anion that yielded metal fluoride on the rubbed surfaces. The combination of the phosphonium cation with a phosphate anion or thiophosphate anion resulted in a better lubricant than 1,3-alkylimidazolium bis(trifluoromethanesulfonyl)amide. The reactions of the phosphate anion and thiophosphate anion yielded a phosphate boundary film that exhibited better tribological properties than those of the fluoride boundary film.

Hydrocarbon oil with low vapor pressure has been used as a lubricant in high-vacuum conditions. Decomposition of the oil under boundary lubrication conditions was studied by investigating desorption of hydrogen and hydrocarbons, generated by tribochemical reaction occurred on nascent surface of 52100 steel during the sliding process in a ball-on-disk type sliding tester. Gaseous products by tribochemical decomposition were monitored by a quadrupole mass spectrometer (Q-MS), which would reveal the decomposition mechanism of hydrocarbon oil. It is found that tribochemical reactions of hydrocarbon oil occurred on active sites on steel generated by sliding, the desorption amount of hydrogen and gaseous hydrocarbons increased linearly with sliding velocity, and parabolically with load. A critical load for the activation of decomposition of the hydrocarbon oil on bearing steel was found to be about 1.1 N. In this paper, the decomposition mechanism of hydrocarbon oil was also explored.

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 phosphatecontaining 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.

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