Frictional behavior of C60 monolayer films on graphite (HOPG)
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Y8.30.1
Frictional behavior of C60 monolayer films on graphite (HOPG) S. Okita1*, A. Matsumuro1 and K. Miura2 Department of Microsystem Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan 2 Department of Physics, Aichi University of Education, Hirosawa 1, Igaya-cho, Kariya 448-8542, Japan 1
Abstract The frictional behavior of a C60 monolayer film between graphite substrates is studied using a homebuilt surface force apparatus. The mean frictional force from the C60 monolayers is estimated to be approximately 2mN, which is one-fifth that of C60 thin films. The C60 monolayer films exhibit a low frictional force of 2mN up to one hundred scans under a normal stress of 8MPa, which indicates that the C60 monolayer film is highly promising for use as a lubricant. Key words, C60 monolayer film , Lubrication, Mechanical properties, Micromachines Introduction A C60 thin film is expected to be a good lubricant because of the nearly spherical shape of C60 molecules, low surface strength and high robustness [1,2]. However, a pin-on-disk experiment for the C60 thin film has a large frictional coefficient of 0.8, which suggests that the C60 thin film is not a good lubricator [3]. Furthermore, a few studies in air and in ultrahigh vacuum (UHV) have cast doubt on the possibility of low friction. In UHV experiments [4,5], molecular-resolution images have been presented using both an atomic force microscope (AFM) and a frictional force microscope (FFM), but it has, unfortunately, been concluded that the frictional forces of C60 films up to 18nN load are larger than that at a NaCl(001) surface. In an experiment conducted in air [6], the behavior of frictional force up to a load of 20nN has been investigated using the FFM, but it was concluded that the frictional force acting between the probe tip and the first C60 layer is larger than that acting between C60 layers.
Y8.30.2
In a recent study by Miura et al. [7], it was found that a graphite flake placed on a C60 monolayer on graphite stacks in the same way as the C60 molecules on graphite. The C60 molecules act as molecular bearings, assisted by the nanogears of the six-membered carbon ring between C60 molecules and graphite, in which the mean dynamical frictional forces are zero up to a load of one hundred nanonewtons. However, it is not always evident that the superlubricity of the C60 monolayer film proposed by Miura et al. [7] arises over a larger area because the contact between the C60 monolayer and the graphite flake is on the order of nanometers. For the application of the C60 monolayer film in a microsystem, the occurrence of superlubricity over a larger area is crucial. In this study, the frictional behavior of a C60 monolayer film of micrometer size is investigated using a homebuilt surface force apparatus. Furthermore, we discuss the usefulness of the C60 monolayer as a lubricant film. Experiments C60 films on highly oriented pyrolytic graphite (HOPG), KCl(001), mica and Si(001) were prepared by evaporation from a BN crucible. The temperatures of the substr
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