The sulfidation and oxidation behavior of sputter-deposited Al-Ta-Si alloys at high temperatures

The effect of silicon addition on the sulfidation and oxidation behavior of sputter-deposited AI-Ta alloys was investigated as a function of alloy composition and temperature. The sulfidation kinetics of Al-Ta-Si alloys generally followed a parabolic rate law, and the sulfidation resistance of Al-Ta alloys was improved by the addition of adequate amounts of silicon. The excess addition of silicon to Al-Ta alloys decreased the resistance to high temperature sulfidation. In the case of high tantalum alloys containing about 80 at. % Ta, the dense and columnar sulfide scale of tantalum disulfide was formed on the alloys, and the formation of this protective scale led to the excellent resistance to high temperature sulfidation. The effect of silicon addition on the oxidation behavior of Al-Ta alloys was rather complicated. In general, the oxidation rate of Al-Ta alloys was also decreased by the addition of adequate amounts of silicon, although the excess addition was harmful to the oxidation resistance of these alloys. In the case of high aluminum alloys containing about 30 at % Ta, continuous aluminum-rich oxide scale and the porous underlying alloy were formed after oxidation, and these voids in the underlying alloy were assumed to accelerate oxidation. However, at 1073 K, the rapid weight loss, which was observed in binary Al-Ta alloys during oxidation, was suppressed by silicon addition. (C) 1997 Elsevier Science Ltd.