Deposition of Al 65 Cu 23 Fe 12 and Composite Quasicrystalline PVD Thin Film Coatings
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DEPOSITION OF Al65Cu23Fe12 AND COMPOSITE QUASICRYSTALLINE PVD THIN FILM COATINGS DANIEL J. SORDELET1,2 MATTHEW F. BESSER1and FRANK M. KUSTUS3 1 Ames Laboratory, Ames, IA 50011, 2Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011 3 Engineered Coatings Inc, Parker, CO 80134 ABSTRACT Thin film coatings of Al-based quasicrystals were deposited by magnetron sputtering. Sputtering targets of Al65Cu23Fe12 and Al65Cu23Fe12+5v/o Fe-Al were prepared with plasma arc spraying by forming thick (~5mm) coatings onto Cu substrates. By incorporating a controlled fraction of porosity and micro-cracks within the plasma sprayed target, cracking or delamination of the target during magnetron sputtering could be avoided. Compositions of the as-deposited PVD films were close to the sputtering target composition when the bias voltage was kept around –40V; higher bias voltages (e.g., -100 to -200V) lead to coatings that were deficient in Al. As-deposited coatings prepared with the lower bias voltage could be subsequently annealed at 700°C for two hours to yield a nearly single-phase icosahedral structure. After annealing, composite coatings indicate the presence of an Fe-Al phase along with the icosahedral phase. INTRODUCTION The unusual atomic structure and electronic configuration of quasicrystalline alloys are well known to be associated with atypical mechanical behavior [1], particularly at elevated temperatures [2,3], and surface-related response[4]. The aperiodic atomic ordering within quasicrystalline alloys interrupts dislocation motion, which results in high hardness at ambient temperatures; moreover, the aperiodic structure leads to anomalous work softening behavior at elevated temperatures [5]. Regarding the surface behavior of Al-based quasicrystals, X-ray spectroscopy studies have shown that they have a reduced density of states at the Fermi level for Al3p-type electrons, as compared to chemically similar, yet crystalline alloys [4]. Recent studies of Al-Cu-Fe alloys have demonstrated a clear correlation between the Al3p density of states at the Fermi energy and the apparent surface energy. The term ‘apparent’ is used because surface energy was deduced through a series of wetting angle measurements using liquids with differing polar and dispersive components. The results illustrate that the reduced density of states is associated with an apparent surface energy that is lower than crystalline intermetallic structures in the same alloy system (e.g., cubic Al50Cu35Fe15 or tetragonal Al70Cu20Fe10) or oxides (e.g., Al2O3). In addition, these wetting experiments show that quasicrystals behave more as covalent materials than free electron structures. The above characteristics of quasicrystals have some important practical implications. Combining the high hardness and reduced surface energy offers a material that could be used in applications requiring both wear resistance and weak adhesion against liquid, semi-liquid or polymeric substances. Examples of these applications might include die cavity su
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