Comparative Study of the Tribological and Oxidative Properties of AlPdMn Quasicrystals and Their Cubic Approximants
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Comparative Study of the Tribological and Oxidative Properties of AlPdMn Quasicrystals and Their Cubic Approximants Chris Mancinelli, Jeff S. Ko1, Cynthia J. Jenks2 , Patricia A. Thiel2 Amy R. Ross2 and Thomas A. Lograsso2, and Andrew J. Gellman Department of Chemical Engineering, Carnegie Mellon University Pittsburgh, PA 15213, U.S.A. 1 Merck & Co., Inc., West Point, PA 19486 2 Department of Chemistry and Ames Laboratory, Iowa State University Ames, IA 50011, U.S.A.
ABSTRACT An experimental comparison has been made between the properties of the surfaces of an Al70Pd21Mn9 quasicrystal and its Al48Pd42Mn10 approximant. The Al70Pd21Mn9 sample was a single grain icosahedral quasicrystal cut to expose its five-fold symmetric (000001) surface. The approximant was polycrystalline β-phase Al48Pd42Mn10, which has a CsCl-type cubic structure. Surfaces of both were prepared under ultra-high vacuum (UHV) conditions and then used for comparative measurements of their frictional properties and oxidation rates. Both materials are oxidized by reaction with O2 to form a thin film of aluminum oxide that ultimately passivates their surfaces. The interesting difference between the two is that the rate of oxidation of the approximant is significantly higher than that of the quasicrystal in spite of the fact that the bulk Al concentration of the approximant is lower than that of the quasicrystal. Friction measurements were made under UHV conditions between pairs of quasicrystals and pairs of approximants whose surfaces were either clean or oxidized to varying degrees. The friction between pairs of the approximant surfaces is significantly higher than that measured between the quasicrystal surfaces under all conditions of surface oxidation.
INTRODUCTION The extraordinary structural properties of quasicrystals have motivated numerous measurements of materials properties that might exhibit anomalous behavior that is a direct result of quasicrystallinity. Two such properties that might, in principle, lead to important commercial and technological applications of these materials are their tribological and corrosion properties. There have been a number of reports of apparently low friction measured on the quasicrystal surfaces and there have also been a number of studies of their oxidation behavior [113]. In addition there are other properties of quasicrystals such as high hardness, low thermal conductivity, and interesting electronic properties that might be of practical interest and importance [1, 14-17]. From a materials science perspective, however, the interesting question is whether such materials properties are a direct consequence of quasicrystallinity. There are several possible origins of the low frictional properties of quasicrystals. One of the most intellectually titillating ideas is that because of their inherent lack of periodicity quasicrystalline surfaces can never come into commensurate contact with one another or with any material having a periodic structure. While the connection between commensurability and K8.2.1
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