Tunable friction behavior of oriented carbon nanotube films

Abstract

Measured friction coefficients of carbon nanotubes vary widely from l < 0.1-l > 1.0 [1][2][3][4][5][6], while theoretical studies suggest intrinsically high friction coefficients, approaching unity [7]. Here we report that measured friction coefficients of MWNT films are strong functions of surface chemistry and temperature, but are not dependent on the presence of water vapor. We hypothesize that the origin of the temperature dependence arises from the interaction of the surface chemical groups on the nanotubes [8][9][10][11][12] and rubbing counterface. The friction coefficient of individual films can be easily tuned by changing the surface temperature and chemistry of either the countersurface or the nanotubes, we have demonstrated the ability to create and control high and low friction pairs through plasma treatments of the nanotube films with argon, hydrogen, nitrogen, and oxygen. This behavior is completely reversible, and when coupled with the superior strength, thermal, and electrical properties of nanotubes, provides a versatile tunable, multifunctional tribological system. KEY WORDS: carbon nanotubes, coefficient of friction, micro-tribology, engineered surfaces A schematic of the MWNT film grown with a vertical orientation is shown in figure 1 (a). Scanning electron microscopy images of a free edge of the vertical film are shown in figures 1(b)-1(d). The vertically aligned film was grown by a chemical vapor deposition (CVD) process using ferrocene and xylene precursors [13]. Following CVD growth, the MWNT films are cleaned using an oxygen plasma treatment. The vertically aligned MWNT films are approximately 65 lm thick and 5% dense. The MWNTs were vertically aligned, with the last few micrometers from the top surface the films entangled and intertwined as shown in figures 1(b)-1(d). A sample of transversely orientated nanotubes was also prepared by mechanically removing the vertical MWNTs, sonicating in acetone and dispersing onto an identical quartz substrate. After drying, this transversely oriented nanotube film was found to be approximately 5 lm thick and was comprised of a distributed ensemble of entangled nanotubes oriented in plane with the quartz substrate (figure 1(e) and 1(f)).The mechanical, electrical, and thermal properties of individual nanotubes are highly anisotropic [14][15][16][17][18], and the frictional behavior was also recently found to be anisotropic [6]. These friction experiments used a countersurface made from a borosilicate glass pin (schematically shown in figure 2 (a)) and were run in a regime where wear was not observed. Under both inert gas and ambient conditions, the transversely distributed films were repeatedly found to have friction coefficients l $ 0.1, while the vertically aligned sample had l $ 0.9. Molecular dynamics studies report nanotube films can accommodate relative motions or slip by rolling, sliding, or a combination of rolling and sliding at the interface [19]. However, this mobility related hypothesis seems unlikely considering the degree of nanotube ent...

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