Synergistic interfacial strengthening and ordered transfer film enable ultralow friction of PEEK in ethylene glycol

Achieving ultralow friction in polymer–ceramic contacts typically requires complex material modifications or interface engineering. This study demonstrates that exceptional tribological performance can be unlocked through rational lubricant–polymer pairing without altering the polymer. Specifically, polyetheretherketone (PEEK) lubricated by ethylene glycol (EG) against Si₃N₄ achieves an ultralow, stable coefficient of friction (COF = 0.0087) and exceptional wear resistance (specific wear rate = 0.99×10⁻⁶ mm³·N⁻¹·m⁻¹), outperforming polymers such as polytetrafluoroethylene (PTFE) and ultra-high-molecular-weight polyethylene (UHMWPE). A comparative investigation of five polymers reveals that PEEK’s superiority originates from a unique interfacial synergy with EG, manifesting as concurrent surface strengthening (~34% increase in nanohardness) and in situ formation of a continuous, highly ordered graphitic carbon transfer film on the counterface (lowest I_D/I_G = 0.48). Density functional theory calculations attribute this synergy to a geometrically matched, dual-site chemisorption between the diols of EG and the carbonyl/ether oxygens on the PEEK backbone (binding energy = −0.01157 Ha), in contrast to the weak physisorption or suboptimal polar interactions observed for other polymers. This work establishes a clear structure–affinity–interface–performance relationship and proposes a paradigm-shifting design strategy for polymer tribosystems based on molecularly synergistic lubricant selection, paving the way for green, sustainable lubrication in precision engineering and biomedical applications.