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Langmuir 2000, 16, 8352-8359
Interactions at the Ni3Al(111)-S-Al2O3 Interface at Elevated Temperatures: Ordering of Al2O3 on an S-Modified Substrate S. Addepalli, N. P. Magtoto, and J. A. Kelber* Department of Chemistry, University of North Texas, Denton, Texas 76203 Received February 14, 2000. In Final Form: July 3, 2000 Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM) have been used to investigate the growth, morphology, and thermal stability of the oxide prepared on a (2 × 2)-S-covered Ni3Al(111) surface. The results demonstrate that although sulfur significantly alters the oxidation rate and oxide morphology on the Ni3Al(111) substrate at room temperature, it does not inhibit the formation of the ordered γ′-Al2O3 at elevated temperatures. The oxide formed on the S-modified Ni3Al(111) surface at room temperature is stable up to at least 1100 K. Sulfur remains at the oxideNi3Al(111) interface during oxygen exposure at ∼300 K. During annealing, however, sulfur is removed from the oxide-alloy interface. Annealing from 300 to 1100 K results in the segregation of aluminum to the oxide-alloy interface, as evidenced by an enhancement in the Al(1396)/Ni(848) atomic ratio, and the appearance of a metallic aluminum peak in the Auger spectra.
I. Introduction Aluminides and other ternary alloys containing aluminum are frequently used in aggressive, corrosive environments because of their ability to form hightemperature, corrosion-resistant aluminum oxide scales.1 The presence of impurities, especially sulfur, at oxidesubstrate interfaces has been associated with oxide spallation from various substrates at elevated temperatures2,3 and can critically impact technological applications such as aerospace, power plant operation, composite materials fabrication, catalysis, and microelectronics. Studies on single-crystal alumina/Ni(poly)4 and polycrystalline alloy/alumina2,3,5,6 substrates have demonstrated that sulfur segregates to the oxide-substrate interface at elevated temperatures and weakens the metal-oxide bond. The deleterious effects of sulfur have also been observed in chromia-6 and iron oxide-7-9 forming systems. Because of its implications for the performance of various alloys and steels, sulfur interface chemistry is a topic of technological as well as scientific interest. The exact mechanism of sulfur-induced oxide spallation, however, is still unclear, and has been the subject of long-standing debate.6,10,11 In an effort to elucidate sulfur interactions with metaloxide bonds, several investigators have conducted studies * Corresponding author. Tel, (940)-565-3265; FAX, (940)-3698295; e-mail,
[email protected]. (1) Meier, G. H. Mat. Corros. 1996, 47, 595. (2) Hou, P. Y.; Wang, Z.; Pru¨ssner, K.; Alexander, K. B.; Brown, I. G. In Proceedings of the 3rd International Conference on Microscopy of Oxidation; Hou, P. Y., Wang, Z., Pru¨ssner, K., Alexander, K. B., Brown, I. G., Eds.; The Institute of Materials, London: Trinity Hall, Cambridge, U.K., 1996; p 1. (3) Smialek, J. L.; Jayne, D. T.; Schaeffer, J. C.; Murphy, W. H. Thin Solid Films 1994, 253, 285. (4) Kiely, J. D.; Yeh, T.; Bonnell, D. A. Surf. Sci. 1997, 393, L126. (5) Smialek, J. L. Metall. Trans. A 1991, 22A, 739. (6) Hou, P. Y.; Stringer, J. Oxid. Met. 1992, 38, 323. (7) Lin, J.-S.; Ekstrom, B.; Addepalli, S. G.; Cabibil, H.; Kelber, J. A. Langmuir 1998, 14, 4843. (8) Cabibil, H.; Kelber, J. A. Surf. Sci. 1995, 329, 101. (9) Cabibil, H.; Kelber, J. A. Surf. Sci. 1997, 373, 257. (10) Stott, F. H. Rep. Prog. Phys. 1987, 50, 861 and references therein. (11) Grabke, H. J.; Wiemer, D.; Viefhaus, H. Appl. Surf. Sci. 1991, 47, 243.
on corresponding model interfaces prepared on Fe7-9,12 under ultrahigh vacuum (UHV) conditions. These studies have demonstrated that chemical reactions that are endothermic based on bulk bond dissociation enthalpies nevertheless occur at surfaces or interfaces, for two reasons: (1) interfacial Fe-S bonds have electronic structures and reactivities quite different from those found in corresponding bulk phases.13 (2) The migration of reaction products into the bulk, with subsequent alloying, may provide a powerful driving force for the reaction.12 To ascertain a microscopic mechanism for the destabilization of alumina scales on aluminides and other aluminumcontaining alloys, we prepared alumina films on clean and sulfur-modified Ni3Al(111) substrates in UHV. Ni3Al(111) and other single-crystal aluminides (Ni-Al alloys) can form highly ordered, ultrathin (
