Tribological properties of crystalline and quasicrystalline surfaces

The tribological properties of a B2-type Al-Pd-Mn quasicrystal approximant were investigated and compared with those of a Al-Pd-Mn icosahedral quasicrystal. The approximant was of the β-phase, having a crystalline CsCl type structure and nominal composition Al₄₈Pd₄₂Mn₁₀. 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 an 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.; A study of the tribological and oxidative performance of Al_xCu _yFe_z films was performed to determine how these properties are affected by changes in structure. The amorphous Al_xCu _yFe_z films were found to have lower oxidation rates and film thicknesses than both the icosahedral and orthorhombic Al_xCu _yFe_z films. For all Al_xCu_yFe_z film structures, however, oxidation by H₂O resulted in higher oxidation rates as well as higher film thicknesses than oxidation by O ₂ alone. Friction measurements made between pairs of identical Al _xCu_yFe_z films were found to be low for clean and oxidized surfaces. Friction was not, however, significantly structure sensitive.; Measurements have revealed that friction is anisotropic with respect to Pd(100) surface lattice orientation. When the surfaces were aligned, corresponding to a commensurate interface, and sliding along the <110> direction with 4 ML of octane at the interface, the static friction coefficients was μs > 8.0 ± 2.0. A minimum μs was obtained when the two Pd(100) surfaces with 4 ML of octane at the interface were misoriented by &thetas;∼45°, for which μs = 4.0 ± 2.0. Higher coverages of octane decreased the friction, but friction anisotropy was observed with as much as 20 ML of octane at the sliding interface between the two Pd(100) surfaces. Plastic deformation was observed on both of the surfaces which suggests that surface lattice commensurability is not the primary cause of friction anisotropy in this system. The fact that friction anisotropy is observed for surfaces undergoing plastic deformation and in the presence of adsorbed layers with no well-defined periodicity suggests that friction anisotropy originates from mechanical anisotropy of the subsurface bulk crystal lattice. (Abstract shortened by UMI.)...