Fundamental studies of tribological properties of quasicrystalline and crystalline metal surfaces modified with boundary lubricants.

摘要

The combined use of an ultrahigh vacuum (UHV) tribometer and a variety of surface science techniques has enabled us to explore the tribological properties of interfaces between single crystalline or quasicrystalline metal surfaces.;A UHV tribometer was used to make friction measurements between two single-grain Al70Pd21Mn9(00001) quasicrystal surfaces. The friction coefficient of the perfectly clean quasicrystal surfaces is lower than those reported for the perfectly clean surfaces of many pure metals but is higher than those measured on quasicrystal surfaces exposed to air. Under shear the quasicrystal surfaces slide over one another without exhibiting stick-slip behavior or adhesion. Studies were performed to determine the effects of surface oxidation, surface structure, and surface composition on quasicrystal friction.;One of the hypotheses of the low friction measured between the quasicrystal surfaces is the lack of commensurate contact at the interface. Friction measurements have been made between a pair of Ni(100) surfaces which were prepared to be truly clean, modified by the presence of adsorbed atomic sulfur, or modified by the presence of films of adsorbed ethanol. Measurements made with systematic variation of the relative crystallographic orientations of the two Ni(100) surfaces have revealed that the friction coefficient is anisotropic with respect to lattice orientation. The friction anisotropy observed between Ni(100) surfaces suggests that surface lattice commensurability is not the cause of friction anisotropy in this system.;The effect of hardness was investigated by making friction measurement between clean surfaces of pairs of Cu(111), Ni(100), W(110), and Al70 Pd21Mn9 quasicrystals in order to identify the reasons for low friction coefficients reported for quasicrystalline materials. These measurements have shown that the friction between the clean Al 70Pd21Mn9 quasicrystal surfaces is not significantly different from that measured between the surfaces of a hard metal such as W.;The tribological properties of Ni(100) surfaces modified by atomic or molecular species were investigated as a function of coverage. Friction measurements made with ethanol coverages ranging from submonolayer to multilayers reveal that the friction coefficient is discontinuous in coverage and can be correlated to discontinuities in heat of adsorption. Sliding never commences between clean Ni(100) surfaces with or without atomic sulfur or ethanol adsorbed. In the submonolayer coverage regime of either atomic sulfur or adsorbed ethanol, adhesion is the primary cause of high friction. An abrupt decrease in both the static friction coefficient and adhesion coefficient occurs at coverages of one monolayer of ethanol on each surface. At a total coverage between two and five monolayers of ethanol, static friction coefficient decreased in a step-wise manner and is correlated to the heat of adsorption. This step-wise decrease in both the friction coefficient and the heat of adsorption may be due to molecular layering of the ethanol. A transition from boundary to the mixed lubrication regime can be seen between 5--10 ML of total ethanol coverage. Frictional behavior in this coverage regime is a mix of both stick-slip and slip sliding behavior. (Abstract shortened by UMI.)