Purpose The structure and chemical composition of boundary layers built under tribological stress affect the friction and wear of solid-state surfaces in a major way. Therefore, information about the chemical composition of the outermost surface and boundary layer are of great importance. Preliminary time of flight secondary ion mass spectrometry (ToF-SIMS) investigations have shown that metal surfaces that have been immersed at high temperatures in phosphonium phosphate-containing oils contain at least some characteristic signals for phosphate containing anti-wear layers. The purpose of this work is to investigate the influence of additive concentration and oil temperature on the formation of phosphate containing layers. Design/methodology/approach To investigate the formation of phosphate containing layers as a function of temperature, samples of rolling bearing steel 100Cr6 were first heated in a furnace to selected temperatures of 200, 300, 400 and 500 °C, respectively. Then, they were immersed in a model fluid containing ionic liquids as additive in PAO-2 and analysed by ToF-SIMS. Findings 100Cr6 surfaces immersed in trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate additive oil show characteristic signals of phosphate-like layers at temperatures of 400-500 °C. In addition, characteristic surface signals show a decrease in these ionic liquids at these temperatures. Originality/value Ionic liquids could be an alternative to zinc dialkyldithiophosphates as an oil additive. Targeted investigations under friction load could provide information on whether wear-reducing layers are formed. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-10-2019-0436

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