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C. Nano-Tibology of A
Polytetrafluoroethylene Transfer Films
 (a) |
 (b) |
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Figure 1. (a) Interface of two PTFE films before sliding,
(b) Interface of two PTFE films after 20 Å sliding at 10 m/s.
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Mechanical devices
for space applications need to be able
to operate reliably over an extreme range of environments. They
are produced and tested on earth, launched, and then operated
in space. The reliability of the moving mechanical assemblies is
one of the most critical issues in attempting to prolong service
life. These components experience an extreme environmental
range, and nanostructured polymer composite materials are
candidate materials for improved performance. In particular,
PTFE either as filler or a matrix is being explored. The C-F
bond in PTFE provides both thermal and oxidative stability,
and the strong interchain interactions in PTFE confer resistance
to almost all solvents [1]. The intrinsically poor wear resistance
of PTFE is improved by incorporation with nanoscopic fillers.
For example, Sawyer et al. created a nanocomposite of PTFE
with alumina that has a reduction in wear rate of over 2 orders
of magnitude compared with unfilled PTFE [2].
Classical MD
simulations numerically integrate Newton's
equations of motion with a third-order Nordsieck predictor
corrector [3] using a timestep of 0.2 fs. Short-range interatomic
forces are calculated using the C-H-F reactive empirical bond
order (REBO) potential [4] based on Brenner's second
generation REBO potential for hydrocarbon systems [5]. Long range
van der Waals interactions are also included in the form
of a Lennard-Jones (LJ) potential to calculate interchain
interaction [3] The LJ potential is only active at distances
greater than the covalent bond lengths.
In Fig. 1, we can
see the interface deformation and it is thought that the pattern of
this deformation has a strong influence on the frictional force. It is
expected that different chain configurations will exhibit different
frictional behavior.
Figure
2. Compressive and shear forces during the sliding process.
The rate of sliding is 10 m/s.
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The measured
frictional coefficient is about 0.25 from Fig. 2. This is higher than
the macroscopic value (μ < 0.2), but the simulation results are
obtained from a well-contacted nanoscale region which make it difficult
to carry out direct comparisons between experimental values and this
simulation results.
References
[1] H. R. Allcock, F. W. Lampe, and J. E. Mark,
Contemporary polymer chemistry, 3rd ed. (Pearson/Prentice Hall, Upper
Saddle River, N.J., 2003).
[2] W. G. Sawyer, K. D. Freudenberg, P. Bhimaraj, and
L. S. Schadler, Wear 254, 573 (2003).
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