Deposition and Applications of Quasicrystalline Coatings
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properties.8 The shortcoming of quasicrystals generally manifests itself as extreme brittleness in bulk form. It is not trivial to fabricate a quasicrystalline body using bulk-forming and machining technologies. This handicap has led to efforts by several groups throughout the world—particularly in France, Japan, the United States, and Germany—to develop coatings of quasicrystalline materials that could be applied to robust substrates. A universal goal of these efforts is the desire to mitigate the brittle behavior of the quasicrystals while still maintaining their desirable mechanical, chemical, or optical properties. This move from science to practice includes opportunities to improve the release characteristics of plastic molding equipment, increase the wear resistance of aluminum or steel parts in automotive systems, extend the life of components exposed to elevated temperature and
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Quasicrystalline Coatings Research with quasicrystalline materials has demonstrated many desirable characteristics such as high hardness,1'2 low friction,3 oxidation and corrosion resistance,45 low thermal and electrical conductivity, 67 and unusual optical MRS BULLETIN/NOVEMBER 1997
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Figure 1. Isothermal section (800°C) of Al-Cu-Fe phase diagram, after Reference 11.
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oxidizing environments, or provide valuable optical properties for solar absorbers. Even with the broad range of materials that have been found to form quasicrystalline phases, it is difficult—using typical methods—to deposit a coating that has the desired stoichiometry and quasicrystalline phase structure. This problem is not unique to quasicrystalline coatings. Chemistry and phase changes are seen during the deposition of many materials such as tungsten carbide9 or ceramic superconductors.10 Many of the quasicrystalline phases are not thermodynamically stable at room temperature and must therefore be prepared by rapid quenching. Other phases may be stable up to their melting temperature but favor a conversion from the quasicrystalline structure at room temperature. These materials are discussed in more detail by A.P. Tsai in this issue of MRS Bulletin. Quasicrystals in the Al-Cu-Fe system have been widely studied as coating materials due to (1) their stability at temperatures near their melting point and (2) their favorable cost, availability, and lack of toxicity. Since the quasicrystalline phase is stable in this system, it can be depicted by traditional phase diagrams,11 as shown in Figure 1. Note the rather small range of composition over which the quasicrystalline phase (i/0 exists. This affects the ability to obtain the ip phase during surface coating, as will be discussed later in more detail. Coatings in general are commonly applied as either thick or thin films. The differentiation in nomenclature between thick and thin films relies mainly on physical dimensions or method of deposition.12 For example, thermal-spray coatings are regarded as thick films
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