Oxidation behavior of intermetallic Al-Cr and Al-Cr-Fe macroparticles

• Published in: Journal of vacuum science & technology. A, Vacuum, surfaces, and films Vacuum, surfaces, and films, 2017-11, Vol.35 (6)
• Main Authors: Koller, Christian Martin, Kirnbauer, Alexander, Hahn, Rainer, Widrig, Beno, Kolozsvári, Szilárd, Ramm, Jürgen, Mayrhofer, Paul Heinz
• Format: Article
• Language: English
• ISSN: 0734-2101; 1520-8559
• DOI: 10.1116/1.4986928

The incorporation of macroparticles into arc evaporated thin films is generally unfavorable as they, in most cases, induce growth defects. However, in recent studies, the authors found that nucleation of hexagonal (α-phase) crystallites occurs on top of specific macroparticle types and is especially pronounced when using Fe-alloyed Al0.7Cr0.3 cathodes. In order to further investigate this interesting finding, intermetallic Al-Cr and Al-Cr-Fe macroparticles, generated from powder-metallurgically produced and nonreactively arc evaporated Al0.7Cr0.3 and Al0.675Cr0.275Fe0.05 cathodes, were collected and annealed in ambient air. Detailed structural and chemical analyses were carried out by transmission electron microscopy studies of focused ion beam-prepared samples. The predominately γ2-type Al8Cr5 and Al8(Cr,Fe)5 droplets oxidized at 900 °C for 3 and 6 h form a layered oxide structure consisting of a thin amorphous a-Al2O3 layer, on top of which transient γ-/θ-Al2O3 facets develop. The presence of Fe promotes the formation of slightly denser oxide scales with a thinner a-Al2O3 layer and, more important, occasional α-Al2O3 crystallites which develop at the interface between the oxidized intermetallic droplet and the a-Al2O3 scale. The underneath-lying Al-depleted Al8Cr5 and Al8(Cr,Fe)5-droplets are subject to pronounced microstructural modifications including elemental separation and the generation of holes and cavities. Their progressing oxidation leads to stoichiometric sesquioxides of α-Cr2O3 or θ-Al2O3 structure.