Mechanical and tribological properties of AlCuFe quasicrystal and Al(Si)CuFe approximant thin films
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ltilayered thin films of Al/Cu/Fe have been prepared by magnetron sputtering and annealed into quasicrystalline and approximant phases on Al2O3 and Si substrates, respectively. The nanomechanical and nanotribological properties, such as hardness, elastic modulus, friction, and toughness, have been measured using a triboindenter and analytical methods. The approximant phase was proved to be slightly harder than the quasicrystalline phase with a hardness of about 15.6 GPa, and with a similar elastic modulus of about 258 GPa. These values however decreased rapidly with an increasing amount of Si in the approximant. The indentation toughness of the approximant, ,0.1 MPa/m½, was however inferior to that of the quasicrystals with 1.5 MPa/m½. Friction coefficients were measured in a range of 0.10–0.14 for both the quasicrystalline and approximant thin films.
I. INTRODUCTION 1
Ever since the discovery of quasicrystals, research has been underway to determine the properties of these aperiodic phases. It did not take long to realize that these phases behaved in an unusual manner for intermetallic phases.2–4 Several of their properties were more reminiscent of ceramic compounds rather than metallic ones e.g. resistivity and indentation toughness. The reasons for this behavior were investigated and several properties were found to be inherent from the aperiodic structure of quasicrystals. In relation to quasicrystals, a group of phases called approximants was discovered.5 The approximant phases are complex structures that are locally similar to the structure of quasicrystals, but lacking the long-range aperiodicity. Instead they are periodic with large unit cells. The investigation of the physical properties of these phases is important for the understanding of the properties due to aperiodicity. An important factor in determining the physical properties is the preparation and crystalline state of the samples. Depending on the crystal size and quality, large variations in e.g. hardness and resistivity have been observed within the same phase.6 This becomes important when considering possible applications of the material, and consequently, comparing sample preparation techniques becomes a matter of importance. The room-temperature brittleness of quasicrystals can be overcome by the deposition of thin films onto different substrates.4 This provides a combination of a high
Contributing Editor: Sam Zhang a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.384 232
J. Mater. Res., Vol. 31, No. 2, Jan 28, 2016
toughness of the substrate with advantageous properties of the quasicrystalline phases. Therefore, the trend toward using quasicrystalline materials as thin films has increased. Various physical vapor deposition (PVD) methods have been used to deposit quasicrystalline coatings including co-sputtering from individual elemental targets, deposition of elemental multilayers, evaporation, and sputtering from cast composite quasicrystalline targets.7–9 In most cases deposition is followed by ann
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