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.

The tribological properties of halogen-free ionic liquids, tricyanomethanide [[Formula: see text]] salt, tetracyanoborate [[Formula: see text]] salts, and N-alkylimidazole-trialkylborane complexes were evaluated by laboratory tribo-testing of steel–steel contact under boundary conditions. Tricyanomethanide salt is composed of hydrogen, carbon, and nitrogen. The other two types of liquids are composed of hydrogen, boron, carbon, and nitrogen. They are free of halogens and heavier elements that are components of common ionic liquids, such as fluorine, phosphorus, and sulphur. As expected, the halogen-free ionic liquids exhibited low corrosion properties to steel. When evaluated as neat liquid, these halogen-free ionic liquids provided less tribological properties in comparison with a reference, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide. Tributylmethylphosphonium dimethylphosphate was examined as a prototype tribo-improving additive. It improved the wear-preventing properties and friction reducing properties of tetracyanoborate salts by 10–25% and 20–30% at a concentration of 10 mM (620 ppm of phosphorus), respectively. The additive performances for tricyanomethanide salt and the imidazole-trialkylborane complexes were not uniform under these conditions. Boron oxide and iron oxides were found by surface analysis of rubbed surfaces with tetracyanoborate salts.