Tribological Behavior of Two 1-Ethyl-3-methylimidazolium Alkyl Sulfates as Neat Lubricants for a Steel–Steel Contact

This work analyzes the friction coefficients and the wear volumes obtained in probes made of AISI 420 stainless steel worn with AISI 52100 chrome steel balls under loads of 8, 18, and 30 N; the contact was lubricated with 1-ethyl-3-methylimidazolium ethyl sulfate or 1-ethyl-3-methylimidazolium hexyl sulfate. Under the two lowest loads, the results show that the higher the alkyl chain length in the sulfate anion, the greater the friction coefficient. In addition, an increase in the anion alkyl chain side length produces a decrease in the wear volume. However, under 30 N load the opposite tendencies are obtained for both properties. Both ionic liquids (ILs) show better tribological properties than two high-performance commercial oils of similar viscosities tested as a reference under the same conditions. X-ray photoelectron spectroscopy (XPS) analyses reveal the presence of tribofilms mainly composed of sulfates and oxides.