An Investigation of Water-Based Cutting Fluids: Choline Amino Acid Protic Ionic Liquids as a Potential Sustainable Additive

This comprehensive investigation examines the use of choline amino acid protic ionic liquids (CHAAPILs) as environmentally friendly additives for water-based cutting fluids (WBCFs) and showed a significant improvement in both friction and wear compared to traditional cutting fluids. Notably, water alone is ineffective as a boundary lubricant for aluminum machining promoting cyclic hydrolysis that generates brittle Al(OH)3 films while generating friction above dry sliding. As traditional petroleum-based lubricants are increasingly recognized for their detrimental environmental impact, the development of sustainable alternatives has become essential in the tribology community. CHAAPILs derived from renewable amino acids and choline sources were specifically formulated to transform water into high performing machining fluids without compromising performance. Integrating these green additives into WBCFs aligns industrial practices with sustainable manufacturing goals, addressing ecological and operational demands. CHAAPIL analyses included but were not limited to, the following: the synthesis and characterization of CHAAPILs, using FTIR, NMR, TGA, and DSC, and confirming their purity and stability as lubricant additives. The three CHAAPILs (choline aspartate, choline leucine, and choline isoleucine) exhibited > 99 % purity and remained stable up to 200-220 °C. Their contact angles on polished AISI 52100 steel and 6061 aluminum fell below 60°C, and their viscosities decreased from tens of pascal-seconds at 25 °C to under 0.15 Pa·s at 100 °C, ensuring both wetting and flow under metal-cutting conditions. The performance of CHAAPILs with WBCFs is benchmarked to the results of a traditional additive. The evaluation on the friction-reducing properties of CHAAPILs as additives was performed using tribological testing on a custom ball-on-flat reciprocating tribometer. Testing from the ball-on-flat reciprocating tribometer quantified and generated the coefficients of friction (COF) data via LABVIEW. The wear volumes calculation used an optical microscope to measure the worn track width. The results revealed CHAAPIL-enhanced water reduced the steady-state coefficient of friction by roughly one-third compared to a leading commercial cutting fluid. Friction and wear volume evaluations after testing with CHAAPILs in the lubrication system showed a reduction in wear volume of approximately 90%. Advanced analyses of the surface morphological changes included electron microscopy and 3-D profilometry. SEM and 3-D profilometry confirmed the elimination of deep plowing grooves and built‐up edge. Meanwhile, EDS mapping revealed carbon‐rich, oxygen‐depleted tribo‐layers in CHAAPIL cases. Raman spectroscopy further corroborated the suppression of hydrolytic aluminum‐hydroxide formation. Additionally, the spectroscopy also validated the chemisorption of ionic‐liquid species onto metal oxides with carboxylate and C-O vibrational modes not occurring in neat water. Collectively, these findings demonstrated that CHAAPILs reconcile water’s environmental advantages with the high‐performance demands of machining.