Atomistic insights into anti-wear mechanisms and protective tribofilm formation in polytetrafluoroethylene composites

Abstract

Abstract Polytetrafluoroethylene (PTFE) has a low friction coefficient but poor wear resistance (k ~ 10−3 mm3/Nm) against various surfaces. Mechanical modeling suggests that the enhanced anti-wear performance of PTFE composites (k ~ 10−5 mm3/Nm) relies on load support by filler in the matrix. Recent studies found that tribochemical polarization of PTFE polymers triggered the formation of highly protective transfer film, thus resulting in an exceptionally low wear (k ~ 10−7 mm3/Nm) in certain composites. However, atomistic interaction was believed to play an important role in the known anti-wear mechanisms, which has yet to be fully described. Here, environmental and computational experiments allowed detailed mechanistic studies for representative PTFE composites, including metal-, ceramic-, carbon-, and polymer-filled composites. Experimental results found that the protective and polarized transfer film formed only in environmental water/oxygen, which could also reduce the composite wear by 10-fold or more. Density-functional-theory (DFT) calculations revealed that the electrophilic atom at solid surface tends to defluorinate PTFE molecule, which enables the tribochemical products of polarized PTFE accumulated near the sliding surfaces. Molecular dynamics simulations suggested that the strengthening of nonbonding interactions resulted from polar polymers improved polymer composites' adhesion and cohesion strengths against steel counterface, which was responsible for the achievement of macro-scale ultralow wear in PTFE composites. The relation between the atomistic interactions and the macroscopic wear behavior of composites was systematically discussed.