Friction anisotropy: A unique and intrinsic property of decagonal quasicrystals
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C.J. Jenks and P.A. Thiel Ames Laboratory & Department of Chemistry, Iowa State University, Ames, Iowa 50011
J. Brenner Austrian Centre of Competence for Tribology Research GmbH, A-2700 Wiener Neustadt, Austria; and Austrian Research Centre-Seibersdorf Research GmbH, A-2444 Seibersdorf, Austria
J.M. Dubois Laboratoire de Science et Génie des Matériaux et de Métallurgie (LSG2M), Institut Jean Lamour [FR2797 Centre National de la Recherche Scientifique (CNRS)-Institut National Polytechnique de Lorraine (INPL)-Université Henri Poincaré (UHP)], Nancy-Université Ecole des Mines, Parc de Saurupt, F-54042 Nancy cedex, France (Received 10 January 2008; accepted 26 February 2008)
We show that friction anisotropy is an intrinsic property of the atomic structure of Al–Ni–Co decagonal quasicrystals and not only of clean and well-ordered surfaces that can be prepared in vacuum [J.Y. Park et al., Science 309, 1354 (2005)]. Friction anisotropy is manifested in both nanometer-size contacts obtained with sharp atomic force microscope tips and macroscopic contacts produced in pin-on-disk tribometers. We show that the friction anisotropy, which is not observed when an amorphous oxide film covers the surface, is recovered when the film is removed due to wear. Equally important is the loss of the friction anisotropy when the quasicrystalline order is destroyed due to cumulative wear. These results reveal the intimate connection between the mechanical properties of these materials and their peculiar atomic structure. I. INTRODUCTION 1
Among metallic alloys, quasicrystals are interesting because they exhibit both unusual atomic structure (order without periodicity) and unusual physical properties. The tribological properties of quasicrystals—including low friction, high hardness, low surface energy, and high wear resistance—have attracted much interest during the last 15 years.2–7 Anomalously low coefficients of friction between quasicrystalline materials and diamond or steel under ambient condition were first reported in 1991 by Dubois et al.4 In the original observations, the experiments had two noteworthy features.2,3 First, the environment was air and so the surface of the aluminum-rich quasicrystals was covered by a layer of oxide 2–5 nm thick. Second, irreversible deformation and wear of the sample occurred during sliding.8–11 In addition to low friction, a new and remarkable property was discovered recently on clean, oxide-free quasicrystal surfaces. Using an atomic force microscope (AFM) in ultra-
II. EXPERIMENTAL
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0187 1488
http://journals.cambridge.org
high vacuum, Park et al. reported a large directional anisotropy of the friction force12: friction was found to be eight times larger when sliding along the periodic direction of the surface than when sliding along the aperiodic one. These experiments were performed under low loads, such that the contact was elastic and reversible (no wear). The anisotropy was attributed to the pe
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