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Abstract Although lubricants play an essential role in reducing wear and friction in mechanical systems, environmental issues persist. In the past decades, Ionic Liquids (ILs) have arisen as environmentally friendly alternatives to conventional lubricants and additives. ILs are low-volatile and non-flammable salts that possess low melting points (below 100 °C). Their tunable properties, achieved by selecting the appropriate cation and anion, make them ideal candidates for different applications, including lubricants. In recent times, Protic Ionic Liquids (PILs) have attracted attention in the tribological community as a cost-effective alternative to conventional aprotic counterparts. In this work, a choline-amino acid ionic liquid, derived only from renewable, biodegradable, and biocompatible products, was synthesized, and investigated as both neat lubricant and additive to non-polar oil. The lubricating properties of [CHO][GLY] were studied both as a neat lubricant and as a 1 wt. % additive to a polyalphaolefin (PAO) oil using a ball-on-flat reciprocating friction tester. AISI 52100 steel disks were tested against AISI 52100 steel balls using either [CHO][GLY] or the mixture of PAO+[CHO][GLY]. For comparison purposes, the commercially available base oil, PAO, was also tested. Preliminary results showed no major differences in friction between the lubricants used. Nevertheless, the addition of 1 wt.% to the PAO demonstrated a remarkable 30% reduction in wear on the steel disk. This encouraging improvement in anti-wear characteristics raises the potential advancement of lubrication technology with the choline-amino acid ionic liquid, coupled with its environmentally friendly nature. Energy-dispersive X-ray (EDX) spectroscopy, non-contact profilometry, and scanning electron microscopy (SEM) were used to study the worn steel surfaces and elucidate the wear mechanisms. 

The development of high-performance and environmentally-compatible lubricants is crucial for minimizing energy losses in mechanical systems and increasing the lifetime of moving mechanical components, thus preserving our environment. While ionic liquids (ILs) have emerged as promising next-generation materials for lubrication purposes owing to their attractive physico-chemical properties, several challenges currently limit their use in engineering applications, including their high cost and corrosivity. Recently, eco-friendly, protic ILs (PILs) have been synthesized and showed great advantages compared to tradition (aprotic) ILs, such as low cost, ease of preparation, and good lubricating properties. Despite these advancements, remarkably little is known about the interrelationship between PIL molecular structure and lubrication mechanisms. In this work, the physico-chemical and lubricating properties of a family of PILs synthesized by using only renewable, biodegradable, and biocompatible products and constituted by the same choline cation and amino-acid anions with different side chains, were investigated. The molecular structures of the choline amino acid-based ionic liquids (AAILs) were confirmed through magnetic resonance and Fourier transform infrared spectroscopy, while their thermal behavior was evaluated by differential scanning calorimetry and thermogravimetric analysis. The antiwear and friction-reducing performance of the choline AAILs when used as neat lubricants was studied as a function of normal load by reciprocating ball-on-flat tribometry using steel-steel contact. Surface analytical measurements (Raman and XPS) performed on the worn steel surfaces confirmed that the excellent lubricating performance of choline AAILs originates from the formation of oxygen- and carbon-rich tribolayers. The formation of these protective layers are influenced by the applied normal load and the molecular structure of the amino acid. The results of this work open the path for the rational design of environmentally-friendly PILs for tribological applications.