Multiscale Friction Simulation of Dry Polymer Contacts: Reaching Experimental Length Scales by Coupling Molecular Dynamics and Contact Mechanics

Abstract

This work elucidates friction in Poly-Ether-Ether-Ketone (PEEK) sliding contacts through multiscale simulations. At the nanoscale, non-reactive classical molecular dynamics (MD) simulations of dry and water-lubricated amorphous PEEK-PEEK interfaces are performed. During a short running-in phase, we observe structural transformations at the sliding interface that result in flattening of the initial nanotopographies accompanied by strong polymer chain alignment in the shearing direction. The MD simulations also reveal a linear pressure -shear stress dependence and large adhesive friction in dry conditions. This dependence, summarized in a nanoscale friction law, is of central importance for our multiscale approach, since it forms a link between MD and elastoplastic contact mechanics calculations. An integration of the nanoscale friction law over the real area of contact yields a macroscopic friction coefficient that allows for a meaningful comparison with measurements from macroscopic tribometer experiments. Severe normal loading conditions result in significant wear and high experimental friction coefficients µ≈0.5-0.7, which are in good agreement with the calculated values from the multiscale approach in dry conditions. For milder experimental loads, our multiscale model suggests that lower friction states with µ≈0.2 originate in the presence of physisorbed molecules (e.g., water), which significantly reduce interfacial adhesion.

A Combined Experimental and Atomistic Investigation of PTFE Double Transfer Film Formation and Lubrication in Rolling Point Contacts

“…The force field parameters are fitted to DFT results to accurately describe the elastic response of Fe 2 O 3 and Fig. 4 WLI images of ball/disk contact at a ball load of 20 N and a slide-roll ratio of 10%: a reference of an unlubricated contact at v Roll = 0 mm/s captured before measurement; b-e the film formation at v Roll = 400 mm/s and F Pin = 80 N is inhomogeneous within the contact cross section and varies within the measurement intervals of 500 s; f-h the film formation at v Roll = 400 mm/s and F Pin = 0 N is more homogeneous and shows PTFE residues even without relubrication Upon shearing at T = 300 K, with P n = 100 MPa and v x = 100 m/s, for both the Fe-and the OH-terminated systems, we observe a shear-induced alignment of the molecules, which is consistent with the results of other MD studies on similar systems [22,30], and that results in pseudo-crystalline arrangements reminiscent of the hexagonal Phase II [33] (see snapshots in Fig. 6b and the Supplemental Videos).…”

“…2.2) between two α-Fe 2 O 3 (0001) slabs. Even though PTFE is typically semicrystalline a purely amorphous sample is considered as a limit case to explore whether shear can result in ordering of the molecules [30].…”

“…The PTFE in our tribosystems is represented by an allatom oligomer model. This choice is motivated by a previous study with polyether ether ketone [30] showing that the mechanical and tribological response of a thermoplastic polymer can be reasonably described by finite chains. To create an amorphous PTFE sample, we initially randomize 400 C 40 F 82 PTFE oligomer molecules at 1500 K using a Langevin thermostat with time constant = 1 ps for 1 ns within a simulation box with fixed volume corresponding to a density of 0.1 g/cm 3 .…”

A Combined Experimental and Atomistic Investigation of PTFE Double Transfer Film Formation and Lubrication in Rolling Point Contacts

“…The PTFE-lubricated model system is obtained by inserting the 21-nm-thick, amorphous PTFE slab (modelled with 400 C40F82 oligomer; see Section 2.2) between two α-Fe2O3 (0001) slabs. Even though PTFE is typically semicrystalline a purely amorphous sample is considered as a limit case to explore whether shear can result in ordering of the molecules [25].…”

“…The PTFE in our tribosystems is represented by an all-atom oligomer model. This choice is motivated by a previous study with polyether ether ketone [25] showing that the mechanical and tribological response of a thermoplastic polymer can be reasonably described by finite chains. To create an amorphous PTFE sample we initially randomize 400 C40F82 PTFE oligomer molecules at 1500 K using a Langevin thermostat with time constant = 1 ps for 1 ns within a simulation box with fixed volume corresponding to a density of 0.1 g/cm 3 .…”