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 Al 48 Pd 42 Mn 10 . Friction coefficients measured in ultrahigh vacuum between a pair of Al 48 Pd 42 Mn 10 samples having truly clean surfaces were found to be twice as high as those reported for the Al 70 Pd 21 Mn 9 quasicrystal. When the surfaces were oxidized by exposure to O 2 or H 2 O, 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. O, 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. Keywords Ames Laboratory

An experimental comparison has been made between the properties of the surfaces of an Al 70 Pd 21 Mn 9 quasicrystal and its Al 48 Pd 42 Mn 10 approximant. The Al 70 Pd 21 Mn 9 sample was a single grain icosahedral quasicrystal cut to expose its five-fold symmetric (000001) surface. The approximant was polycrystalline β-phase Al 48 Pd 42 Mn 10 , which has a CsCl-type cubic structure. Surfaces of both were prepared under ultra-high vacuum (UHV) conditions and then used for comparative measurements of their frictional properties and oxidation rates. Both materials are oxidized by reaction with O 2 to form a thin film of aluminum oxide that ultimately passivates their surfaces. The interesting difference between the two is that the rate of oxidation of the approximant is significantly higher than that of the quasicrystal in spite of the fact that the bulk Al concentration of the approximant is lower than that of the quasicrystal. Friction measurements were made under UHV conditions between pairs of quasicrystals and pairs of approximants whose surfaces were either clean or oxidized to varying degrees. The friction between pairs of the approximant surfaces is significantly higher than that measured between the quasicrystal surfaces under all conditions of surface oxidation. sample was a single grain icosahedral quasicrystal cut to expose its five-fold symmetric (000001) surface. The approximant was polycrystalline β-phase Al 48 Pd 42 Mn 10 , which has a CsCl-type cubic structure. Surfaces of both were prepared under ultra-high vacuum (UHV) conditions and then used for comparative measurements of their frictional properties and oxidation rates. Both materials are oxidized by reaction with O 2 to form a thin film of aluminum oxide that ultimately passivates their surfaces. The interesting difference between the two is that the rate of oxidation of the approximant is significantly higher than that of the quasicrystal in spite of the fact that the bulk Al concentration of the approximant is lower than that of the quasicrystal. Friction measurements were made under UHV conditions between pairs of quasicrystals and pairs of approximants whose surfaces were either clean or oxidized to varying degrees. The friction between pairs of the approximant surfaces is significantly higher than that measured between the quasicrystal surfaces under all conditions of surface oxidation. Keywords