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MATERIALS RESEARCH

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Tribological properties of fullerenes C60 and C70 microparticles Wei Zhao,a) Jinke Tang, and Ashok Puri Department of Physics, University of New Orleans, New Orleans, Louisiana 70148

Ray L. Sweany Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148

Yuxin Li and Liquan Chen Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China (Received 7 August 1995; accepted 6 June 1996)

The frictional behaviors of fullerenes C60 and C70 were studied because they were speculated to be solid lubricants. For the sublimated pure C60 films on Si(001), a high friction coefficient (0.55–0.8) was observed under different loads and pin materials. For the C70 film, the friction coefficient showed a pin dependence, which changed from 0.5 with an Al2 O3 pin to about 0.9 with a 440 stainless steel pin. The relatively high friction coefficients of C60 and C70 films were due to the tendency of the C60 and C70 particles to clump and compress into high shear strength layers rather than due to the impurities in the fullerenes. The benzene-solvated C60 ? 4C6 H6 and C70 ? xC6 H6 showed a lowered friction coefficient (0.25 for C60 ? 4C6 H6 and 0.3 for C70 ? xC6 H6 ), which might result from the lowered shear strength of the hcp structure of C60 ? 4C6 H6 and C70 ? xC6 H6 molecular crystals in which the benzene molecules were intercalated.

I. INTRODUCTION

Since soccer-like fullerene C60 was discovered, its halogenated derivatives like C60 F60 were speculated to be lubricants.1 Later it was realized that unlike Teflon these fluorinated species were chemically unstable. Therefore, they failed to be lubricants.2 For C60 pristine, its lubricating properties were anticipated because of its spherical shape, strong intramolecular and weak intermolecular bonding, and high chemical stability. It was expected to act as tiny ball bearings.3,4 Pristine C60 solid is a molecular crystal which is of well-known face-centered cubic (fcc) structure at room temperature.5 It can readily sublimate at temperatures about 450 ±C in a vacuum, and dissolves in a wide range of nonpolar solvents such as benzene and hexane, which allows C60 molecules to easily attach to the surface of a substrate in the form of films. So far, the tribological properties of C60 were studied only by a few laboratories whose results showed some contradictions. Blau and Haberlin3 reported a high friction coefficient (,0.6) of C60 powder with 90% purity (10% C70 ) on aluminum. Bhattacharya et al.6 obtained similar results for sublimed pure C60 film on Si3 N4 substrate. In contrast, Bhushan et al.4,7 found a low friction coefficient (,0.12) of sublimed C60 film on Si. They also observed an increased friction coefficient of C60 film as the amount of C70 and impurities in the film increased. We noticed that in their experiments, the measurement conditions including pin materials, loads, a)

Author to whom correspondence should be addressed. J. Mater. Res., Vol. 11, No. 11, Nov 1996

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