Takeru Omiya1,2, Albano Cavaleiro1,2, Fabio Ferreira1,2
1University of Coimbra, Coimbra, Portugal
2Instituto Pedro Nunes, Coimbra, Portugal
Improved approaches for friction and wear management could not only result in enormous energy savings but also lead to drastic reductions in greenhouse gas emissions. Among the materials, diamond-like carbon (DLC) coatings for their wear resistance and ionic liquids (ILs) for their friction reduction properties have attracted considerable attention. Phosphorus-containing ILs have shown particular promise in reducing friction and wear by forming a protective film on steel and aluminum surfaces under harsh contact conditions. Despite the importance of these studies, very few studies have focused on the improvements achieved by DLCs when lubricated by ILs. Recently, our group performed tribological characterization of the phosphorus-containing IL, thitexyltetradecylphosphonium bis(2-ethylhexyl) phosphate [P6,6,6,14][DEHP], on gadolinium- and europium-doped DLC. In the previous research, gadolinium and europium, which have lanthanide ions in the trivalent oxidation state known to have a high affinity for IL, were compared as dopants for DLC and it was found that IL exhibits wear properties mainly on gadolinium-doped DLC (Gd-DLC) coatings. However, the interactions and mechanisms between ILs and the doped DLC coating are not clear. In this study, DLC coatings deposited by high-power impulse magnetron sputtering were doped with the rare earth metal gadolinium and their sliding lubricity was evaluated in the presence of three different phosphorus-containing ILs. The test results showed a significant reduction in wear rate of approximately 80% when two different ILs were used. In addition, post-test wear tracks were observed using advanced surface observation equipment such as XPS and ToF-SIMS, which revealed details of the interaction between the IL and the Gd-DLC coating. This comprehensive study is expected to clarify the interaction between Gd and ILs and lead to further optimization of the lubrication system design by comparing the anionic groups of ILs.