Improving tantalum's oxidation resistance by Al + ion implantation
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INTRODUCTION
T A N T A L U M and its alloys possess many properties which make them prime candidates for use in aerospace and other industries. They exhibit high melting temperatures and, as a result, retain several desirable mechanical properties over a wide temperature range that extends to at least 2200 ~ In addition, tantalum is easily formed or worked at room temperature. Such attributes make these materials excellent choices for fabrication of parts intended for use at elevated temperatures. Unfortunately, tantalum and many of its alloys oxidize rapidly at temperatures above 300 ~ t2[ Protective coatings are usually required to achieve useful service lives and dependability under such circumstances. Conventional coatings often suffer from exfoliation or lack of adherence at the coating/substrate interface. Such problems can be circumvented if the coating can be made an integral part of the substrate, so that no abrupt coating/ substrate interface is present. As demonstrated in this paper, ion implantation is useful in creating such coatings, and aluminum ion implantation is particularly helpful in retarding oxidation of tantalum, at least in the intermediate temperature range up to - 8 0 0 ~ Ion implantation has the further advantage of altering surface properties such as oxidation, while leaving desirable bulk properties unaltered. Kaufman e t a l . pj have investigated the response of tantalum to oxidation after implanting it with selected elements, including aluminum. Their findings, in the case of aluminum implantation, were that implantation reduced oxidation by about a factor of 2 at 500 ~ but did not cause a reduction at 1000 ~ the second of the two temperatures investigated. It was suggested that the implantation fluence of 2 • 1017 A l + / c m 2 used in that work, M. SAQIB, formerly Graduate Student, Metallurgy Department, University of Connecticut, is Research Associate with the Department of Mechanical Systems and Engineering, Wright State University, Dayton, OH 45435, and Visiting Scientist, Wright-Patterson Air Force Base, Dayton, OH 45433. J.M. HAMPIKIAN, Graduate Student, and D.I. POTTER, Professor of Metallurgy, are with the Metallurgy Department, School of Engineering and Institute of Materials Science, University of Connecticut, Room 111,97 North Eagleville Road, Storrs, CT 06269-3136. Manuscript submitted December 20, 1988. METALLURGICAL TRANSACTIONS A
and the resulting implanted concentrations that approached 20 at. pct A1, was near that where saturation due to sputtering occurs. However, our previous work [41 and that presented here shows that much higher aluminum concentrations are obtained at higher fluences. These higher fluences, and the resulting oxidation barriers they produce, have effectively stopped oxidation at temperatures where the layers are stable, as will be described later in this paper. The work that follows was based on the premise that high fluence A1 § implantation of tantalum, and other refractory metals as well, would yield intermetallic compounds of aluminum at the
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