Choline amino acid protic ionic liquids as potential sustainable additives for water-based cutting fluids

Friction and wear in metal cutting reduce efficiency and increase energy use. Conventional mineral-oil-based cutting fluids are effective in metal cutting but pose environmental and health risk, driving the need for safer alternatives. Air-stable protic ionic liquids (PILs) show promise as lubricants due to their unique properties, tribofilm formation, and tunable chemistry, particularly offering benefits for aluminum alloys, which are prone to seizure and adhesion during machining. In this work, three amino acid-based PILs, synthesized entirely from renewable, biodegradable, and biocompatible materials, are investigated as additives to water and compared with a commercially available cutting fluid in aluminum–tungsten carbide contacts. Using a ball-on-flat tribometer, results show that the addition of choline amino acid PILs to water, particularly 4 wt% CHOILEU, significantly reduces friction and wear in aluminum–tungsten carbide contacts at both room and high temperatures by forming protective tribofilms that mitigates adhesive and fatigue wear, whereas conventional cutting fluids provide inferior protection. • Three renewable, biodegradable PILs were tested as additives to water-based cutting fluids. • 1 wt% and 4 wt% CHOILEU in water reduce friction and wear in Al–WC contacts. • 4 wt% CHOILEU lowers friction by 41% and wear by 82% at room temperature. • At 90 °C, 4 wt% choline amino acid PILs reduce friction by 76% and wear by 90%. • Surface analysis shows protective tribofilms that reduce adhesive and fatigue wear.