Oxidative and tribological properties of amorphous and quasicrystalline approximant Al-Cu-Fe thin films

The origins of the tribological properties and corrosion resistance of amorphous and quasicrystalline approximant alloys have been studied by comparing their properties in thin Al-Cu-Fe alloy films with compositions lying near the quasicrystalline region of the ternary compositional phase diagram. Six sputtered thin films of an Al-Cu-Fe alloy were studied using X-ray diffraction, X-ray photoemission spectroscopy (XPS), and an in situ ultrahigh vacuum (UHV) tribometer. The films were annealed in UHV to induce the formation of orthorhombic, rhombohedral, and amorphous bulk structures. The properties of these thin films were then determined in the same UHV apparatus without exposing the films to air. The rates of surface oxidation by H2O and O2 were measured using XPS. Although the oxidation rates and oxide thicknesses were dependent on the oxidant, they were not sensitive to the structures of the films. Friction was measured between identical samples in sliding contact. The friction coefficients (micros = 0.36 +/- 0.11 to 0.56 +/- 0.08) were comparable to those observed in other experiments using quasicrystals and approximants in UHV; however, there was no strong correlation between the friction coefficients and either the film structure or the degree of surface oxidation. These results suggest that the tribological and corrosion resistance properties of these quasicrystalline approximant alloys are not directly connected to crystalline structure.