Characterization and Control of Aluminum Oxide Thin Films Formed on Surfaces of FeCo-V Alloys

Abstract

X-ray photoelectron spectroscopy (XPS), ellipsometry, and cathode luminescence were utilized for characterizing oxide films formed on the surfaces of FeCo-V alloys (Permendur) that contained small amounts of aluminum via annealing under a low partial pressure of oxygen. XPS spectra showed that the oxide films were composed primarily of aluminum oxide, which was a result of preferential oxidation of aluminum on the surfaces of the alloys during annealing. The thicknesses of the oxide films were estimated from XPS depth profiles coupled with the sputtering rate of a SO2/Si film, and the thicknesses were compared with those of the oxide films measured via non-destructive ellipsometry. The thicknesses obtained from the depth profiles may have been overestimated because of a difference in sputtering rate between silicon oxide and aluminum oxide. As measured via ellipsometry, the thickness of the aluminum oxide thin film, which was less than 100 nm, increases with the aluminum content in the alloys. Wide XPS spectra from the sample surfaces with different roughness suggested that the aluminum oxide thin films were homogenously formed on the smooth surfaces, whereas metallic elements in the alloy substrates were detected on the rough surfaces. Furthermore, the electrical resistivity of the alloys with the smooth surfaces increases significantly with the aluminum content, which may be attributable to the formation of the aluminum oxide thin film. These results indicate that the addition of a small amount of aluminum and surface smoothness are important for controlling formation of the aluminum oxide thin films with high resistivity in FeCo-V alloys by annealing under a low partial pressure of oxygen.

Characterization of Aluminum Oxide Thin Films Formed on Surfaces of FeCo-V Alloys by Annealing under a Low Partial Pressure of Oxygen

“…Figure 2 shows the thickness of the aluminum oxide films on the FeCo-V alloys, with Al mass % of 0.25, 0.50, and 0.75, measured by ellipsometry. In a previous study [10], non-destructive ellipsometry provided reasonable results rather than sputtered depth profiles; the thickness of those samples annealed for 1173 K for 0.5 h are also shown in Figure 2. The thickness of the aluminum oxide thin films increased with increasing the aluminum content in the alloys, annealing temperature, and annealing time.…”

“…These results indicate that aluminum, one of the less noble elements, is preferentially oxidized in the Fe-based alloy, containing such elements annealed under a low partial pressure of oxygen. This annealing process is effective for fabricating aluminum oxide thin films on the surface of the FeCo-V alloys, as bulk aluminum oxide is fundamentally insulated [10]. To understand the formation conditions of aluminum oxide thin films, the mechanism of the oxide film formation has been discussed based on the thermodynamic properties of the relevant elements.…”

“…To understand the formation conditions of aluminum oxide thin films, the mechanism of the oxide film formation has been discussed based on the thermodynamic properties of the relevant elements. It was assumed that the preferential oxidation of aluminum in the present alloys with a small amount of oxygen in the atmosphere occurred on the alloy surface, since a very small amount of oxygen remained in the hydrogen atmosphere [10], in which the partial pressure of oxygen 𝑝𝑝 O 2 in the present annealing conditions was estimated to be 1 × 10 −13 Pa. For instance, an Ellingham diagram of Co, Fe, V, and Al and schematic illustrations of layered structure of iron-based alloys annealed under different annealing conditions are shown in Figure 1 [10]. To understand the oxide films formed on the surfaces of FeCo-V alloys containing aluminum, an extensive study is necessary.…”

Characterization of <i>α</i>-Al₂O₃ in Structural Isomers of Alumina Formed by Oxidation of Fe–Cr–Al Alloys

“…The formation of Al 2 O 3 in iron-based alloys and heatresistant steels has also been investigated. [10][11][12][13][14][15][16][17][18][19] It is often necessary to understand and analyze the structural changes of Al 2 O 3 on the surfaces of iron-based alloys at high tem-peratures because of the practical applications of these alloys. For example, X-ray diffraction, which is practically applied to on-line analysis of galvannealed coating layers, 20) would be applicable to the on-site identification of a specific structural isomer of Al 2 O 3 .…”

“…A previous study demonstrated that the oxidation of a less-noble metal (such as aluminum) at high temperatures often results in the formation of a non-conductive thin layer on alloy surfaces under controlled annealing. 19) In this study, X-ray photoelectron spectroscopy (XPS) and ellipsometry were used to investigate the growth of Al 2 O 3 thin layers on Fe-based alloys. The results show that ~100 nm thick Al 2 O 3 layers are formed on the surface of the samples after annealing at high temperatures and under oxygen at low partial pressure.…”