Tribological characteristics of aluminum alloys sliding against steel lubricated by ammonium and imidazolium ionic liquids

Sliding friction and wear characteristics of aluminum alloys against AISI 52100 steel lubricated by ionic liquids (ILs) were investigated at both room and elevated temperatures. The tested aluminum alloys include a commercially pure aluminum Al 1100-O, a wrought alloy Al 6061-T6511, and a cast alloy Al 319-T6. The lubricating performance of two ILs with the same anion, one ammonium-based [C₈H₁₇]₃NH.Tf₂N and one imidazolium-based C₁₀mim.Tf₂N, were compared to each other and benchmarked against that of a conventional fully-formulated engine oil. Significant friction (up to 35%) and wear (up to 55%) reductions were achieved by the ammonium IL when lubricating the three aluminum alloys compared to the engine oil. The imidazolium IL performed better than the oil but not as well as the ammonium IL for Al 1100 and 319 alloys. However, accelerated wear was unexpectedly observed for the Al 6061 alloy when lubricated by C₁₀mim.Tf₂N. Surface chemical analyses implied complex tribochemical reactions between the aluminum surfaces and ILs during the wear testing, which have been demonstrated either beneficial by forming a protective boundary film or detrimental by causing severe tribo-corrosion. The effects of the IL cation structure, aluminum alloy composition, and tribo-testing condition on the friction and wear results have been discussed.