Vail et al. 2008
Multifunctionality of Single-Walled Carbon NanotubePolytetrafluoroethylene Nanocomposites
J.R. Vail1, D.L. Burris2 and W.G. Sawyer1
1
Department of Mechanical and Aerospace Engineering
University of Florida
2
Department of Mechanical Engineering
University of Delaware
Abstract
Multifunctional nanocomposites are increasingly used in applications requiring a
prescribed set of physical properties. For example, Polytetrafluoroethylene (PTFE) is a
popular solid lubricant due to its low friction coefficient and high chemical inertness, but
its use is limited by high rates of wear. Nanoparticles have been shown to successfully
reduce the wear of PTFE by 3-4 orders of magnitude, but these systems are often
inadequate for high performance applications requiring excellent mechanical, electrical
and thermal properties. In this study, single-walled carbon nanotubes (SWCNT) are
investigated as a route to improve wear resistance, toughness, electrical and thermal
conductivity of PTFE without sacrificing low friction, high temperature capability and
chemical inertness. Tribological, tensile and surface electrical measurements were made
on 0, 2, 5, 10 and 15 wt % SWCNT filled PTFE nanocomposites. Surface resistance
measurements demonstrated percolation of 2 wt% SWCNTs, and in general,
nanocomposites had ~50% improved engineering ultimate stress (~200% increase in true
ultimate stress), two orders of magnitude (~10,000%) improved engineering strain to
failure, two orders of magnitude (~10,000%) improved wear resistance and ~50%
increased friction coefficient.
keywords: nanocomposite, dispersion, interface, Polytetrafluoroethylene, PTFE, space,
solid lubrication
corresponding author
David L. Burris
Dept. of Mechanical Engineering
University of Delaware
Newark, DE 19716
burrisdl@gmail.com
(941) 716-8331
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Vail et al. 2008
Introduction
Polymeric composites are attractive engineering materials, offering designers a
wide range of tunable physical properties. Carbon fillers have excellent strength,
stiffness, thermal and electrical conductivity and carbon reinforced polymer composites
find widespread use in high performance applications where prescribed sets of properties
(multifunctionalities) are needed. Carbon nanotubes (CNTs) have an unrivalled
combination of high strength, stiffness, conductivity, aspect ratio and specific (per mass
or volume) surface area for stress transfer, and as a result, they have enormous potential
as fillers in multifunctional polymer nanocomposites. A large number of theoretical and
experimental studies have been conducted on these unique materials, and while
impressive practical improvements in wear, friction, electrical conductivity, strength and
toughness have been observed [1-3], theoretical studies suggest that the full potential of
carbon nanotubes has yet to be extracted [4-6].
One area in particular need of novel multifunctional materials is tribology.
Tribological components must reliably provide low friction with minimal wear and
deformation, conduct frictional heat, carry large normal stresses, and in some cases,
conduct electrons. Moving mechanical assemblies in space rely upon a variety of gears,
bearings, gimbals, latches, motors and slip-rings for remote operation in hard vacuum and
thermal extremes that preclude traditional lubrication strategies. The challenges of space
operation have motivated significant research efforts aimed at the development of
multifunctional solid lubricants for extended use in a broad range of extreme
environments [7-18].
Polytetrafluoroethylene has a unique combination of low friction coefficient, low
outgassing, high chemical inertness and high temperature capability that makes it an
attractive candidate material for extreme environment tribology. However, its use has
been limited by high wear rates. Nanoscale fillers have been found to promote high wear
resistance [8, 19-22], but these systems do not have the multifunctional potential of
single-walled carbon nanotubes (SWCNTs). Chen et al. [23] filled PTFE with SWCNTs,
but wear improvements were relatively modest and the cause remains unknown. In this
study, SWCNT-PTFE nanocomposites are studied as candidate multifunctional solid
lubricants for extreme environments.
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Vail et al. 2008
Experimental Methods
The matrix material used in this study was virgin Polytetrafluoroethylene (PTFE,
Teflon 7C from DuPont) compression molding resin with an average particle size of
approximately 30μm. The filler material was Elicarb Single Walled Carbon Nanotubes
(SWCNTs) from Thomas Swan & Co; the nanotubes have average reported diameter and
length of 2 nm and 5 μm, respectively. Figure 1a shows a transmission electron
microscopic (TEM) image of the as-received nanotubes following 1 hour of
ultrasonication in isopropyl alcohol. Surveying revealed diameters between 2 and 15
nanometers. Despite efforts to disband agglomerations using ultrasonication, the
nanotubes were found to be highly agglomerated with individual tubes only being
observable at the edges of agglomerates. Prior to processing, the SWCNTs were
classified using a 40 mesh sieve; only powders that easily passed were processed further.
The sieve classified nanotubes, glass beads and ethanol were combined and mixed
mechanically in a Hauschild Speed Mixer as described in McElwain et al. [21] at high
speed (3,000 RPM) for 10 minutes. Following mechanical mixing, the container was
ultrasonically mixed for 30 minutes and the beads were removed. Appropriate masses of
the SWCNTs and PTFE Teflon™ 7C were combined to achieve 0, 2, 5, 10 and 15 wt. %
SWCNT-PTFE mixtures. A single 5 wt. % sample was jet-milled as described in Sawyer
et al. [22]. Samples of 2, 5, 10 and 15 wt. % SWCNT, combined with ethanol, were
mechanically mixed at high speed for five minutes, then ultrasonicated for ten minutes
and allowed to dry overnight at 60°C in rough vacuum. The dried powders were
compression molded at 363°C and machined into test specimens. A control sample of 5
wt % C60 was created using the same mechanical and ultrasonication processing to test
the effects of aspect ratio of the sp2-bonded carbon nanofillers.
Relative reductions in surface resistance of 13mm diameter by 6.5 mm thickness
samples were measured using a Keithley 2400 SourceMeter. Two electrodes of 100µm
contact radius were randomly placed on the sample with a separation no less than 6 mm
and a potential difference of 200V. The resistance to current flow was measured at least
40 times per sample.
Bulk mechanical properties of unfilled PTFE and 2 wt. % SWCNT were tested
using an MTS 858 Mini Bionix II. A dog-bone shaped specimen was necessary for
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Vail et al. 2008
static gripping of the low friction samples; the final CNC samples had a thickness of 2.5
mm. One eighth inch radii provided a transition from the 15.5 mm long by 4.4 mm wide
test area to the 8mm long by 10 mm wide grip area without introducing significant stress
concentration. Tests were performed in open laboratory air at an extension rate of
1mm/min.
The linear reciprocating tribometer described in Schmitz et al. [24, 25] was used
to quantify wear and friction. The apparatus is contained in a soft-walled clean room and
exposed to air of ~ 25% relative humidity. Each test used a new rectangular flat
counterface of lapped (150 nm Ra) 304 stainless steel which was mounted to the linear
reciprocating stage. The 6.3 x 6.3 x 12.7 mm3 tribology samples were mounted directly
to a 6-channel load cell. A normal force of 250 N was applied using a pneumatic
cylinder/linear thruster assembly. The sliding speed was nearly constant at 50.8 mm/s
over a sliding distance of 25.4 mm. Interrupted mass measurements were made
periodically during each test and wear rates and uncertainties were quantified using the
Monte Carlo methods described in Burris [26].
The effects of the large nanoparticle interface area on the polymer matrix are
widely discussed as potential mechanisms for property enhancement in the
nanocomposites literature. A TA Instruments Q20 differential scanning calorimeter
(DSC) was used to study potential effects of the carbon fillers on the crystallinity and of
crystallinity on the wear of the PTFE matrix. The samples were heated at 10°C/min to
400°C, equilibrated for 10 minutes and cooled at a rate of 10°C/min. Heats of fusion
were calculated from cooling and recrystallization measurements.
Results and Discussion
The averages and standard deviations of surface resistance (R) measurements of
the composites are shown in Fig. 2 as a function of filler wt. %. PTFE is an outstanding
electrical insulator and the true resistance is likely many orders of magnitude greater than
the detection limit of the technique used here. At 2 wt%, detectable electrical currents
were observed in 16 of 45 measurements suggesting effective percolation of the
nanotubes over extended areas. From 5 to 15 wt% SWCNT, the resistance continued to
decrease and was far less sensitive to probe locations and changes in SWCNT loading. A
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Vail et al. 2008
5 wt% jet-milled sample had a consistent mean resistance, but lower variance suggests
improved dispersion.
Quantitatively, these results are consistent with previous studies of nanotube
percolation in the polymer nanocomposites literature and provide evidence of effective
dispersion [3, 27, 28]. C60 fillers are composed of the same sp2-bonded carbon, but
because they are spherical in shape, the probability for inter-particle networking is only
appreciable at high loadings; Ltaief et al. [29] demonstrated percolation with C60 at
approximately 40 vol% filler loading. Experimental results found here support intuition
with no measureable currents detected for a 5 wt% C60-PTFE nanocomposite.
The results of uniaxial tensile experiments in Figure 3 show that 2 wt% SWCNTs
had a dramatic effect on the mechanical properties of PTFE. The unfilled PTFE had a
maximum engineering stress of   7.67MPa and an engineering strain at failure of
  4.81% . With 2wt% SWCNTs, the maximum engineering stress was increased by
more than 50% to   12.13MPa and engineering stain strain to failure increased by
nearly two orders of magnitude to   356.96% .
Friction coefficients are plotted versus sliding distance for unfilled PTFE and
ultrasonically dispersed SWCNT-PTFE nanocomposites on the left of Figure 4. Unfilled
PTFE had a friction coefficient of approximately  = 0.12. In all cases, the friction
coefficients of the nanocomposites were significantly higher on the very first cycle of
sliding. While friction coefficients varied with sliding distance due to wear and transfer
film development, they were stronger functions of SWCNT loading.
Volume loss is plotted versus sliding distance on the right of Figure 4. Unfilled
PTFE lost 29.5 mm3 within the first 0.25 km of sliding; the nanocomposites required well
over an order of magnitude longer distances for comparable wear losses. The 2 and 5
wt% samples had brief periods of higher wear rates followed by lower steady-state wear
rates while the 10 and 15 wt% samples did not have measureable wear transients.
Despite suggestions of improved dispersion, jet-milling had no effect on either
friction or wear. The dispersed C60 was far less effective in reducing wear, suggesting
that the morphology of the filler has a far more important wear reducing role that
dispersion. This result is consistent with traditional rules of mixtures models that involve
load support and crack arresting by high aspect ratio fillers. In general, the wear
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Vail et al. 2008
resistance of the composite correlated with the ability of the filler to reduce debris size;
reduced debris size promoted transfer film uniformity and stability. Chen et al.
[23]hypothesized that wear improvements were the result of SWCNT entanglement with
the polymer and super-strong mechanical properties of the nanocomposites. Improved
mechanical properties were confirmed in this study and while there is correlation between
wear resistance and electrical conductivity, nanotubes/polymer entanglement likely
dominated inter-nanotube networking.
Friction coefficient and wear rate are plotted versus filler wt% in Figure 5. The
data from Chen et al. [23] have been reproduced for comparison. The values of friction
coefficients obtained for unfilled PTFE are different due to the known sensitivity of
PTFE friction to sliding speed [30]. At low speeds, low friction intra-crystalline slip
occurs through shear of the amorphous regions that separate crystalline striae [31, 32].
At higher rates, these regions are less mobile and friction coefficients increase; this is
well illustrated by the higher unfilled friction coefficient at the higher speed of Chen’s
experiments. The nanocomposites, on the other hand, behaved similarly, suggesting that,
unlike PTFE, the nanocomposites are velocity insensitive. This hypothesis was tested
here and it was found that the friction coefficient of 5 wt% SWCNT-PTFE increased by
less than 10% when sliding speed increased from 13 to 130 mm/s.
Based on the observation of monotonically decreased friction with SWCNT
loading, Chen et al. [23] concluded that the nanotubes were self-lubricating. The current
study refutes this conclusion by demonstrating that friction increases at lower speeds
when SWCNTs are added. This result suggests that the nanotubes immobilize the
amorphous regions of the polymer, thus disabling the low friction mechanism of PTFE at
lower speed. As filler loading increases, damage is localized, debris size is reduced,
debris particle density is increased, transfer films are improved and friction is reduced
due to effective third body lubrication.
There is a large body of evidence of nanoparticles immobilizing amorphous
polymers in the polymer nanocomposites literature kfgjkdfjgkj. The interactions at the
filler-matrix interface are widely regarded to dominate behavior [1, 33-41]; strong
interactions can have long-range effects on polymer mobility, morphology, phase and
crystallinity. These regions of ‘bound’ polymer chains are referred to as interphases or
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Vail et al. 2008
interfacial regions. Interfacial regions can have thicknesses larger than filler diameter
and as a result, they can comprise a large portion of the matrix. These regions and their
effects on the properties of the polymer have been detected using a variety of
experimental techniques including AFM nanomechanical measurements [42], Raman
spectroscopy [1], and Differential Scanning Calorimetry (DSC) [33, 34, 36]. DSC
experiments were conducted on several systems here in an attempt to relate the observed
properties to interactions at the interface. The DSC results in Figure 5 show that different
nanofillers and dispersion techniques have unique effects on the crystallinity of the
PTFE; increased crystallinity suggests that the nanoparticles initiate crystallization in
otherwise amorphous regions. Ultrosonically dispersed SWCNTs had virtually no effect
on the crystallinity of the PTFE which suggests little or no interaction between filler and
matrix. Jet-milled SWCNTs and ultrasonically dispersed C60 showed subtle increases in
crystallinity, but in general, the carbon fillers had poor interaction with the matrix. The
mechanism of reinforcement in this case appears to be purely mechanical, and the most
likely immobilization mechanism is the physical tethering of individual crystallites
described by Chen et al. [23] and depicted in Figure 6.
Dramatically increased crystallinity of PTFE with two phases of alumina
nanoparticles suggests strong interactions with the amorphous regions (these powders
were jet-milled, but the compression molding procedures were identical). Interestingly,
the  nanocomposites consistently show poor wear performance [22, 43] (comparable
to the C60 nanocomposite), while the  alumina filled PTFE nanocomposites have
consistently shown wear resistance that is far superior to the carbon nanofillers [21, 26,
43]. These findings demonstrate that interfacial activity is insufficient for low wear
PTFE. AFM imaging from Burris et al. [19] showed that effective fillers (fluorinated
and neat  phase alumina) promoted a unique crystalline structure while ineffective
fillers ( phase alumina) did not.
SWCNTs were able to provide PTFE with impressive gains in conductivity,
toughness and wear resistance, despite the apparent lack of interaction of carbon fillers
with the matrix. A fluorinated silane surface treatment of alumina nanofiller was
successful in improving dispersion and retaining very high wear resistance of PTFE. If
the interactions are driven by the surface chemistries as is one of the prevailing
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Vail et al. 2008
hypotheses, a similar treatment of the nanotubes should make an enormous impact on
wear resistance and other mechanical properties while retaining other attractive
functionalities.
Conclusions
(1) Surface resistance measurements showed percolation near 2 wt% loading of
SWCNT.
(2) The use of SWCNT fillers in PTFE increased strength by ~50%, increased tensile
strain by ~8,000%, increased wear resistance by ~2,000% and increased friction
coefficients by ~50%.
(3) Wear properties of the C60-PTFE nanocomposite was inferior to that of SWCNTPTFE nanocomposites; differences were consistent with traditional rules-ofmixture models of load support and crack deflection.
(4) The percolation threshold was insensitive to the dispersion technique, but the
variance in resistance was smaller using jet-milling over the ultrasonication
technique.
Acknowledgements
This material is based upon an AFOSR-MURI grant FA9550-04-1-0367. Any opinions,
findings, and conclusions or recommendations expressed in this material are those of the
authors and do not necessarily reflect the views of the Air Force Office of Scientific
Research.
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Vail et al. 2008
Figures and Figure Captions
Figure 1. Materials used in the study. Left: Elicarb single-walled carbon nanotubes
(SWCNTs). A typical nanotube agglomerate is located at the top of the image and
individual nanotubes are found at the edge toward the bottom. Right: Ultrasonic
dispersion of 5 wt% SWCNT-PTFE. The nanotubes can be seen decorating the ~15-30
m PTFE particles.
Figure 2. Surface resistance plotted versus filler loading for PTFE composites. The
black squares represent sonicated SWCNT samples. Jet-milling had no effect on the
average resistance, but the dispersion in resistance measurements was substantially lower
and is suggestive of the dispersion differences. Not surprisingly, C60 did not enable
electron conduction to a measureable extent. Confidence intervals represent the standard
deviation of averaged data collected during the test
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Vail et al. 2008
Figure 3. Engineering stress plotted versus engineering strain for neat PTFE and 2 wt%
SWCNT-PTFE. Although substantial improvements in stress and strain were found with
the high aspect ratio nanotubes, the benefits from alumina nanoparticles were even larger
suggesting that the high aspect ratio and excellent mechanical properties of the nanotubes
may be dominated by the effects of the nanoscale interfaces on the polymer itself.
Figure 4. Effect of sliding distance on tribological properties. Left: friction coefficient
plotted versus sliding distance. Right: volume loss plotted versus sliding distance;
confidence intervals representing experimental uncertainty are smaller than the data
markers. Normal load was 250 N and sliding speed was 50.8 mm/s.
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Vail et al. 2008
Figure 5. Effect of filler loading on friction coefficient (left) and wear rate (right) of
SWCNT-PTFE nanocomposites. Confidence intervals on friction coefficient data
represent the standard deviation during the test. Confidence intervals on wear rate data
represent the experimental uncertainties in both worn volume and sliding distance and are
smaller than data markers in all cases. Normal load was 250 N and sliding speed was 50.8
mm/s.
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Vail et al. 2008
Figure 6. Heat of fusion measurements from differential scanning calorimetry (DSC) of
PTFE nanocomposites plotted versus filler loading. Confidence intervals represent the
experimental uncertainty in the calculation of heat of fusion as described in Burris [26].
Since the filler mass is involved in the calculation of heat of fusion without providing any
signal input, lines of constant PTFE crystallinity are also functions of filler loading. Five
samples of PTFE were processed and tested for repeatability. Above is an illustration of
the semi-crystalline banded structure of the PTFE with the hypothesized immobilization
mechanisms of SWCNTs and effective nanoparticles.
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Vail et al. 2008
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