Tribological properties and the decomposition process of ionic liquids (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide) on a nascent surface of bearing steel 52100 were investigated by a ball-on-disk friction tester in a vacuum chamber equipped with a quadrupole mass spectrometer (Q-MS). Ionic liquids exhibited better tribological properties than synthetic hydrocarbon oil (multialkylated cyclopentane (MAC)) in high vacuum conditions. The induction period for decomposition of MAC was about 10 km, while no obvious gaseous products were observed for ionic liquids even after a sliding distance of 22 km under the same mechanical conditions. The mass spectra indicated that both the anionic and cationic moieties of ionic liquids decomposed on the nascent steel surface during friction processes. The cationic moiety with a longer alkyl chain was more difficult to decompose on the nascent steel surface than that with a shorter alkyl chain. XPS analysis revealed that the tribofilm formed by ionic liquid was mainly composed of FeF2 and FeS, which deactivated the nascent surface. As a result, desorption rate of gaseous products decreased appreciably comparing with MAC. The critical load for the mechanical activation of the decomposition correspondingly increased from 1.1 N of MAC to 8 N of ionic liquids. (C) 2009 Elsevier B.V. All rights reserved.