Wetting and friction on quasicrystals and related compounds
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Wetting and friction on quasicrystals and related compounds Jean Marie Dubois£, Vincent Fournée£ and Esther Belin-Ferré§ £ Laboratoire de Science et Génie des Matériaux et de Métallurgie, UMR CNRS INPL 7584, Ecole des Mines de Nancy, F-54042 Nancy. § Laboratoire de Chimie Physique Matière et Rayonnement (UMR 7614), 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05.
ABSTRACT Reduced wetting and friction are two essential surface properties that to a large extent currently embody the technological potential of quasicrystalline coatings. By quasicrystalline compounds, one considers here the whole family of complex Al-(Cu or Pd)-(Fe, Cr or Mn) intermetallics, which comprises true quasicrystals, their approximants and some crystalline materials of related composition. Although covered by a layer of native Al2O3 oxide, wetting by water on these materials exhibits a clear correlation between the reversible adhesion energy of water and the bulk density of states at the Fermi energy. Similarly, in high vacuum, the friction coefficient measured in contact against a hard-steel rider is characteristically smaller than the one measured against conventional metallic alloys (including steel). Observing that wear is nearly non existent under such friction conditions, experiment allows us for the first time to derive a fair estimate of the true surface energy of quasicrystals and related complex metallic alloys. Similarly to wetting, the electronic density of states seems to determine the friction characteristics of these compounds.
INTRODUCTION According to literature, it would certainly be a great surprise to observe in a same series of specimens a correlation between, on the one hand, the macroscopic friction coefficient measured in conventional pin-on-disk tests, using a large spherical indenter under a load of typically 2 N, and on the other hand the reversible adhesion energy deduced from contact angle measurements, using ultra-pure water as a liquid. This is nevertheless clear experimental evidence (Figure 1) that is pointed out when studying the surface of a series of Al-based complex metallic alloys (binary and ternary compounds of the Al-Cu(-Fe), Al-Fe and Al-Cr-Fe systems). The sample surface was prepared as described in the next section and experiments were performed in vacuum for the pin-on-disk tests and in ambient atmosphere for contact angle measurements. In the former case, the indenter comes in close contact with the naked surface of the material since the native oxide, always present on top of such Al-based materials immediately after being brought to ambient atmosphere (G. Bonhomme et al., this conference), is removed from the bottom of the friction track after only a few turns of the disk. In the later case, water wets the top surface of the oxide layer and not the intermetallic substrate. One should therefore expect that friction is dominated by the plowing component induced by the elasto-plastic deformation of the disk under the effect of Hertz contact to the loaded indenter. Yet, an adhesion com
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