Superlubricity and Corrosion Inhibition Properties of Solvate Ionic Liquids in Hybrid Carbon Fiber Reinforced Plastic–Steel Interfaces

Abstract

This study demonstrates the dual functionality of solvate ionic liquids (SILs) comprising lithium bis(trifluoromethanesulfonyl)imide ([LiTFSI]) coordinated with triethylene glycol dimethyl ether ([Li(G3)][TFSI]) or tetraethylene glycol dimethyl ether ([Li(G4)][TFSI]) as both high-performance lubricants and effective corrosion inhibitors at polymer–steel interfaces. Tribological testing of polyamide 6 (PA6), polyphenylene sulfide (PPS), and their carbon–fiber (CF) composites revealed that SIL lubrication reduced the coefficient of friction by ∼60% compared to dry sliding, reaching steady-state values as low as 0.04–0.05 for PPS lubricated with [Li(G4)][TFSI]. Dry PPS/CF composites exhibited elevated CoF up to ∼0.60, underscoring the benefit of SIL retention and tribofilm formation. SEM examination of stainless-steel surfaces following 24-h immersion confirmed that both [Li(G3)][TFSI] and [Li(G4)][TFSI] were noncorrosive, in contrast to severe pitting observed in 1 M HCl. Corrosion weight-loss measurements in 1 M HCl further demonstrated clear inhibition behavior, with only 100 ppm of [Li(G3)][TFSI] (∼68.5 μL/L) reducing corrosion-product weight loss by 63% compared to 54% for 100 ppm [Li(G4)][TFSI] (∼71.4 μL/L). Complementary electroimpedance spectroscopy analysis revealed distinct interfacial responses for the two SILs, [Li(G3)][TFSI] (R_ct = 366 Ω, interface inhibition effect of ∼40%) and [Li(G4)][TFSI] (R_ct = 308 Ω, interface inhibition effect of ∼28%), indicating a more pronounced modification of the steel–electrolyte interface.