Vail et al. 2008
Multifunctionality of Single-Walled Carbon Nanotube-Polytetrafluoroethylene Nanocomposites
J.R. Vail1, D.L. Burris2 and W.G. Sawyer1
1Department of Mechanical and Aerospace Engineering
University of Florida
2Department 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
(941) 716-8331
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.
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 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 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 and an engineering strain at failure of . With 2wt% SWCNTs, the maximum engineering stress was increased by more than 50% to and engineering stain strain to failure increased by nearly two orders of magnitude to.
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 m = 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 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 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 D:G nanocomposites consistently show poor wear performance [22, 43] (comparable to the C60 nanocomposite), while the a 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 a phase alumina) promoted a unique crystalline structure while ineffective fillers (D:G phase alumina) did not.