Tribological properties of a B2-type Al-Pd-Mn quasicrystal approximant
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Cynthia J. Jenks and Patricia A. Thiel Department of Chemistry and Ames Laboratory, Iowa State University, Ames, Iowa 50011
Andrew J. Gellmana) Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 (Received 17 February 2003; accepted 24 March 2003)
The tribological properties of a B2-type Al–Pd–Mn quasicrystal approximant were investigated and compared with those of an Al–Pd–Mn icosahedral quasicrystal. The approximant was of the  phase, having a crystalline CsCl-type structure and nominal composition Al48Pd42Mn10. Friction coefficients measured in ultrahigh vacuum between a pair of Al48Pd42Mn10 samples having truly clean surfaces were found to be twice as high as those reported for the Al70Pd21Mn9 quasicrystal. When the surfaces were oxidized by exposure to O2 or H2O, the friction coefficients decreased by roughly a factor of two for both materials but the friction coefficient for the approximant remained roughly twice that of the quasicrystal. The rate of oxidation of the approximant was found to be one order of magnitude higher than that of the quasicrystal. This corroborates findings that suggest that quasicrystals exhibit an inherent resistance to oxidation and corrosion. Vickers hardness measurements show that the quasicrystal is roughly three times as hard as the approximant.
I. INTRODUCTION
Quasicrystalline materials have been studied extensively since their discovery over 20 years ago, yet much debate remains as to the origins of their desirable materials properties. These desirable properties reportedly include low friction, high hardness, high wear and corrosion resistance, and low surface energy.1–14 Quasicrystals were also found to have low thermal conductivity and moderate electrical conductivity, making them suitable for use in thermoelectric devices.1,15–18 There are, however, some limits to their potential for commercial use due, in part, to their brittle nature. As a result, the properties of quasicrystalline thin films are also being thoroughly examined since it seems that this is the first form in which they are likely to achieve widespread commercial application. Despite this extensive work, the fundamental question of whether or not the desirable properties of quasicrystals are a direct result of quasicrystallinity remains unclear.
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 6, Jun 2003
Quasicrystals are metallic alloys that typically exist over a narrow region of the compositional phase diagram. The quasicrystalline phases are surrounded by related crystalline phases called approximants. The approximant phases, though crystalline, are related to the parent quasicrystal both structurally and electronically. Thus, one would expect the approximants to have materials properties similar to those of the quasicrystals. These approximant phases indeed play a role in the tribology of the quasicrystals as demonstrated by Dong et al., who showed that one of the deformation mechanisms during scrat
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