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 like PTFE and 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.