Ionic Liquids as Lubricants of Titanium–Steel Contact. Part 3. Ti6Al4V Lubricated with Imidazolium Ionic Liquids with Different Alkyl Chain Lengths

The tribological behaviour and surface interactions of Ti6Al4V sliding against AISI 52100 steel have been studied in the presence of three commercial methylimidazolium (mim) room-temperature ionic liquids (ILs) containing the same anion, bis(trifluoromethylsulfonyl)amide, [(CF₃SO₂)₂N] (Tf₂N), and cations with increasing alkyl chain length, 1-ethyl-3-methylimidazolium [C₂mim], 1-butyl-3-methylimidazolium [C₄mim] and 1-octyl-3-methylimidazolium [C₈mim]. Increasing alkyl chain length increases viscosity whilst reducing the onset temperature for thermal degradation in air, the surface tension and the molecular polarity of the ILs. At room temperature, the tribological performance of the three ILs has been compared with that of a mineral oil (MO). The results show the reduction of the running-in period for the ILs with respect to the MO. In contrast with previously described results for IL lubrication, wear rates for Ti6Al4V at room temperature increase as the alkyl chain length of the ILs increases. The maximum wear reduction, of a 39%, with respect to MO is obtained for the [C₂mim] cation, with only two carbon atoms on the lateral chain. This was the IL selected for the tests at 100 °C. At this temperature, the reduction of the mean friction coefficient with respect to the MO is higher than 50%, whilst the wear rate of Ti6Al4V is reduced by 78%. The friction-sliding distance records for the IL at 100 °C show sharp transitions, related to formation of wear debris and surface interactions between the Tf₂N anion and the aluminium present in the Ti6Al4V alloy. Surface tribolayers and wear debris have been studied by SEM–EDX observations and XPS analysis.