Effect of Cr + Implantation on the Thermal Oxidation of Ta
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EFFECT OF Cr+ IMPLANTATION ON THE THERMAL OXIDATION OF Ta K.S. GRABOWSKI AND C.R. GOSSETT
Naval Research Laboratory, Washington, D.C. 20375 ABSTRACT Cr+ implantation of Ta was undertaken in an attempt to improve oxidation resistance at temperatures between 500 and 750 0C, and for oxidation times up to lOOh. Samples were implanted with 1.5x10 17 Cr+/cm2 at 150 keV, 17and compared to samples implanted with ixlO16 or Ix1O Ta+/cm2 at 145 keV to evaluate the role of physical effects from ion implantation. Following oxidation, samples were examined using helium and proton backscattering, electron and optical imaging techniques, and auger electron spectroscopy. Improved resistance to oxidation was observed in Cr+-implanted samples oxidized at 5000 C for up to lOOh, and at 6000C for about lh. However, some local breakdown occurred in these samples and no protection at all was observed at 750 0C. Reasons for this breakdown are discussed and alternate approaches for improving oxidation resistance using ion implantation are proposed. INTRODUCTION Ta is one of the refractory metals which suffers from poor resistance to thermal oxidation. Coatings and alloys have been developed to improve its oxidation resistance, but these approaches degrade bulk mechanical properties, are expensive, and provide poor resistance to thermal shock, as reviews have discussed [1,2]. Recent work by Kaufmann et al. [3] attempted to overcome these difficulties by implanting various ions into Ta, thereby maintaining bulk mechanical properties while producing, hopefully, protective well-integrated coatings. This approach was unsuccessful for 1000 0C oxidation, but Cr+ implantation was shown to improve the oxidation resistance of Ta at 500 0C for lh. Analysis suggested that a thin Cr-rich oxide at the surface provided the protection. This work expands that 0earlier investigation of Cr+ implantation to longer times at 500 C and to the intermediate temperatures of 600 and 750 0C. The oxidation of pure Ta between 500 and 750 0C has been well studied and reviewed and yet is not completely understood [1,2,4]. The oxidation mechanisms are complex and time dependent. Sub-oxide platelet formation and their incursion into the metal has been observed, and considerable dissolution of 0 into the metal occurs. In the temperature regime of interest, diffusion controlled oxide growth undergoes a transition to a more rapid oxide growth process. It was hoped that Cr+ implantation might prevent, or at least delay, this transition. EXPERIMENTAL Specimens were prepared from 99.9% pure Ta sheet material having a grain size of 20 pm. Large coupons 375 pm thick were cleaned and implanted in the central re ion with 1.5 x 1017 Cr+/cm2 at 150 keV, or with ixlO1 6 or Ix10 17 Ta /cm2 at 145 keV. In each case a current density of about 4 pA/cm 2 was used, and the target-chamber Mat.
Res. Soc. Symp. Proc.
Vol. 27 (1984)(Elsevier Science Publishing Co., Inc.
742
pressure was about 1.5x10- 4 Pa during the implantation. Twelve smaller coupons (6.4x12.7 mm) were cut from each of th
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