Schematic of the molecular dynamics simulations. (a) The repeating structure of polytetrafluoroethylene is helical with repeating units. (b) The atomistic simulation is periodic on the and faces. The simulation cell is comprised of two cross-linked aligned films of PTFE. The hexagonal structure of the PTFE crystal is illustrated along with the dimensions of the simulation. (c) The simulation is comprised of opposing regions of rigid atoms, thermostatic atoms, and active atoms of layer thickness 4, 8, and , respectively. After equilibration, the upper block of rigid atoms are moved in the direction (parallel) or the direction (perpendicular), where parallel and perpendicular refer to the sliding direction relative to the chain alignment.
The simulations are initially compressed to a load of 5 nN before sliding is initiated. Friction coefficients are computed by the corresponding normal force. (left) For parallel sliding, the average normal force decays with sliding distance due to the relaxation of compressive stresses, whereas the lateral forces remain low and steady. (right) For perpendicular sliding, an increase in average normal force is observed due to dilation of the system with sliding; lateral forces for this system are relatively higher and erratic.
(Color online) A sequence of snapshots of the upper 25 PTFE chains from the lower stationary PTFE crystal (left). The five surface chains are highlighted with blue (carbon) and orange (fluorine) atoms. The structure and alignment of these chains appears to be intact for the parallel sliding configuration (bottom), whereas the perpendicular sliding configuration produces gross chain motions and mixing within the highlighted region. The snapshots are taken at approximately 2, 5, 10, and 40 nm of sliding. The top view of the carbon atoms for the five surface chains are shown at the same times for perpendicular sliding. Chain scission and realignment in the sliding direction is apparent in the 40 nm view.
A histogram of the displacements along the sliding direction for the carbon atoms in the surface PTFE chains highlighted in Fig. 3. The carbon atoms in the parallel configuration move very little during the 40 nm of simulation; the distribution suggests that the chains are moving in a discrete fashion. In contrast the perpendicular configuration has substantial chain motion over the first 10 nm of sliding (moving over 3 nm on average during the first 10 nm of sliding). The sliding distances, means, and standard deviations of the parallel and perpendicular configuration are tabulated and plotted in the inset.
(Color online) (a) An intermittent contact AFM image of the transfer film produced through reciprocal sliding of PTFE on a polished steel substrate. The observed oriented features are highly correlated with the direction of sliding during the generation of the PTFE transfer film. (b) The single line profile orthogonal to the sliding direction portrays surface features on the order of 10 nm in width and 2–3 nm in height; such features within the image strongly suggest the fibrillated and oriented nature of the transfer film.
Microtribometry friction results for the crossed-cylinder foils with oriented PTFE transfer films. The friction coefficients are displayed versus reciprocation cycle for (a) parallel and (b) perpendicular configurations. The evolution of friction coefficient along the reciprocation track is also plotted for both the parallel and perpendicular configurations. The perpendicular alignment of the films leads to rapid failure of the films and high friction forces during the first 10 reciprocating cycles, whereas over 150 cycles are needed to realize similar levels in the parallel configuration. The low coefficient of friction for parallel sliding is consistent with the behavior of bulk PTFE samples; the transition to higher coefficients of friction is consistent with the film thickness and a wear rate of .
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