Influence of Hydrogen Bonding and Ionicity of Protic Ionic Liquids on Lubricating Steel–Steel and Steel–Aluminum Contacts: Potential Ecofriendly Lubricants and Additives

Abstract

Three ecofriendly protic ionic liquids (PILs), 2-hydroxyethylammonium 2-ethylhexanoate, 2-hydroxymethylammonium 2-ethylhexanoate, and 2-hydroxydimethylammonium 2-ethylhexanoate, with different extents of hydrogen bonding and ionicities, were synthesized and examined as neat lubricants and lubricant additives to a mineral oil (MO) under steel/steel and steel/aluminum contacts. The stability of each PIL-MO blend was observed and correlated to their individual structural features; their physicochemical and tribological properties were investigated and compared to a commercial mineral oil-based lubricant. The addition of any PIL to the mineral oil improved lubricity in steel/steel contact. In aluminum/steel contact, the PILs were studied as neat lubricants and additives showing different effects. The PIL with lower extent of hydrogen bonding but higher ionicity presented poor friction and led to higher wear as neat lubricant. However, as additive, this PIL prevented the intense tribo-corrosion reactions and generated a stable lubrication film that reduced friction; and was capable to form a protective layer on aluminum surface to avoid severe wear.

A review of advances in tribology in 2020–2021

“…ILs can be added into other lubricants as additives to reduce coefficient of friction (COF) and wear rate [12], such as bio-based lubricant [13], water-based lubricants [14], and oil-based lubricants [15], even at relatively high temperature [16]. ILs can increase the stability of lubricant additives [16], such as NiAl particles and trihexyl(tetradecyl)phosphonium cation [17], and improve their tribological performance.…”

Ionic Liquids as High-Performance Lubricants and Lubricant Additives

“…Recently, Guo et al [58,59] examined the tribological properties of three hydroxylammonium hexanoate PIL additives to a nonpolar mineral oil and a polar biodegradable oil. The impact of PILs ionicity and hydrogen bonding on the friction and wear performance was discussed.…”

Influence of alkyl chain length on the tribological performance of choline ionic liquids as water-based lubricating additives under electric field conditions

“…[10,11] The large-scale use of ionic liquids may cause serious environmental pollution. With the green chemical voices, the boat is high, people also put forward increasingly high requirements for lubricants and lubrication additives [12][13][14]. Developing green lubricants are imminent.…”