Lubricating Ability of Protic Ionic Liquids as Additives to a Biodegradable Oil for Aluminum-Steel Contact: Effect of Alkyl Chain Length and Propensity to Hydrogen Bonding

Although aluminum alloys are widely used in the automotive and aerospace industries due to their excellent strength-to-weight ratio and good corrosion resistance, the poor tribological performance and low compatibility of these materials with lubricant anti-wear and anti-friction additives in conventional mineral oils are major limitations. In addition, environmental awareness has increased the need for more environmentally friendly lubricants. Ionic Liquids (ILs) have exhibited significant potential as lubricants and lubricant additives. One of the more interesting properties of ILs is that they can form physically-adsorbed or chemically-reacted layers that reduce friction and wear of the surfaces in contact. Among ILs, Protic Ionic Liquids (PILs) have received more attention recently because of their simple and economic synthesis route. Furthermore, the anions and cations of PILs can be selected to be considered environmentally benign. In this article, the tribological behavior of a family of six PILs are studied as additives to a biodegradable oil (BO), under aluminum-steel contact. Al2024 disks slid against AISI52100 steel balls under a normal load of 3 N and a frequency of 5 Hz at room temperature and using a ball-on-flat reciprocating tribometer. PILs used in this study, were synthesized using two strong acids, with short and long hydrocarbon chains, and three weak bases with different propensities to hydrogen bonds. Results show that, although adding just 1 wt.% of any PIL to BO reduced friction and wear, the alkyl chain length influenced the lubricating ability of these ordered fluids. Wear mechanisms and surface interaction are discussed on the basis of 3D profilometry, SEM-EDX and RAMAN spectroscopy.