Structurally complex metallic coatings in the Al-Cu system and their orientation relationships with an icosahedral quasicrystal

Abstract

Quasicrystals have been identified as alloys possessing unusually low surface energy. This results in poor adhesion properties of quasicrystalline coatings when deposited on metallic substrates, hindering the development of these new materials for technological applications. Here we investigate the possible use of complex Al-Cu metallic phases as interface layers to accommodate the structural and electronic mismatch between a quasicrystalline coating and a metallic substrate and improve adhesion. First, we show that all stable low-temperature phases of the Al-Cu system can be grown as thin films using DC magnetron sputtering. Among the various possible phases, we select the g-brass g-Al 4 Cu 9 as a promising candidate for the interface layer. Then the g-Al 4 Cu 9 phase is grown on the fivefold surface of an icosahedral (i-) Al-Pd-Mn quasicrystal. The interface is investigated by transmission electron microscopy and shows a clear texturing of the film. The grains exhibit rotational epitaxy with the substrate. We find that the interface is mainly composed of a b-phase of unknown chemical composition and sometimes exhibits g grains in direct contact with the quasicrystalline substrate. Occasionally, we observe a fourth phase at the b/g interface, identified as b 1 , possessing a lattice parameter a b1 equal to 2a b and 2/3a g .

Friction and solid-solid adhesion on complex metallic alloys

“…Among all compounds, the icosahedral material manifests itself at the lowest friction coefficient, but the difference compared to two Al–Cu binaries (dot for Al 3 Cu 2 and square for h-AlCu close to the star in figure 13) falls within the error range of the measurement. This similarity was already exploited to design bonding layers specific to quasicrystalline coatings [35]. Another interesting piece of evidence is the regular decay observed on μ when it is plotted as a function of the number of valence electrons in the compound, which may be directly inferred from composition (figure 14).…”

“…Thin enough coatings of hard and brittle materials are well-known technological solutions, but they face a supplementary drawback in the case of quasicrystals that comes from their low surface energy and therefore their intrinsically poor adherence to the substrate [51]. Adaptation bond coats are then necessary [35]. To the best of our knowledge, no successful utilisation of a CMA coating has been achieved so far in contact mechanics.…”

Self-lubricating, low-friction, wear-resistant Al-based quasicrystalline coatings

“…The Al 4 Cu 9 material was prepared by alloying the raw elements in a levitation furnace under a protective argon atmosphere, then crushing pieces of the ingot in a planetary ball mill to produce a powder of adequate mesh size. This powder forms a γ-brass structure with 54 atoms per unit cell that was identified [11] as an efficient material to bond a quasicrystal to a metallic substrate, thus balancing the reduced adhesion towards metals characteristic of quasicrystalline alloys [3, 4]. …”

Thin Film Growth on Quasicrystalline Surfaces

“…The surface alloy obtained is identified as an epitaxial layer of fivefold twinned γ‐Al 4 Cu 9 (110) by analyzing XPS, XPD, and LEED data. The orientation relationship between the γ‐Al 4 Cu 9 (110) film and the fivefold surface of i ‐Al‐Pd‐M,n as well as the structure at the film–substrate interface, were investigated by T. Duguet et al 45. They prepared a γ‐Al 4 Cu 9 (110) film with a thickness of 1 μm by annealing an Al/Cu bilayer which is grown by deposition of both Al and Cu.…”