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Ultrathin metallic coatings for silver surfaces: Function and utilization in low Earth orbit

Schaefer, Glen Allen
http://rave.ohiolink.edu/etdc/view?acc_num=case1060351586

Abstract Details

1993, Doctor of Philosophy, Case Western Reserve University, Materials Science and Engineering.
Silver is a material for space-based mirror technologies operating within the visible spectrum of solar radiation and for electrical contacts utilized in space power transmission applications. However, in spacecraft operating in the low Earth orbit environment, structures utilizing silver components are susceptible to attack from incident atomic oxygen. The structures require protective coatings against atomic oxygen to shield silver surfaces from the environment in order to provide durability. The focus of this research centers on modifying thin film silver surfaces with ultrathin protective coatings of gold and aluminum to provide resistance to a low Earth orbit, atomic oxygen environment. A physical vapor deposition process involving electron beam evaporation produced the thin film coatings in a high vacuum at 3 × 10-7 torr. The coatings were characterized for reflectance with a Perkin Elmer UV-VIS-NIR Spectrophotometer, for thickness with a quartz crystal monitor and for microstructural features with SEM, TEM, AES and ESCA. The samples were oxidized in a modified barrel asher which utilized quartz crystal technology to monitor the plasma process and in the University of Toronto Institute for Aerospace Studies atomic oxygen directed beam facility. A low Earth orbit exposure aboard Atlantis on STS-46 in the Center for Materials for Space Structures LDCE-1 experiment concluded the environmental testing. The silver layer was highly susceptible to plasma oxidation at the thin film grain boundaries. The grain boundary oxidation rate was ten times faster than the bulk silver oxidation process. Aluminum coatings with thicknesses of 80-100 A (30-40 monolayers) wet these silver surfaces and grain boundary regions, thus providing protection from the atomic oxygen. The gold coatings did not protect these sensitive regions and failed in the high energy atomic oxygen environments. The aluminum-silver coatings equalled pure aluminum coatings in the total solar reflectance criteria and the oxidation resistance criteria. The 80 A aluminum layer underwent complete conversion to aluminum oxide in the low Earth orbit exposure. This conversion process was modelled by the inverse logarithmic solution to the Mott-Cabrera rate equation originally proposed by M. J. Dignam. The particular 450°C solution to this rate equation resulted in a linear relationship between oxide thickness and the environmental potential. The aluminum oxide thickness was verified in the UTIAS beam facility at 2 eV and in the low Earth orbit at 5 eV atomic oxygen species. These thin boundary layer aluminum coatings offer protection for silver surfaces which must operate in harsh oxygen environments and present a new candidate material system worthy of additional study
John Wallace (Advisor)
221 p.
Engineering, Metallurgy
Silver surfaces, ultrathin metallic coatings; Low earth orbit

Recommended Citations

Citations

  • Schaefer, G. A. (1993). Ultrathin metallic coatings for silver surfaces: Function and utilization in low Earth orbit [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1060351586

    APA Style (7th edition)

  • Schaefer, Glen. Ultrathin metallic coatings for silver surfaces: Function and utilization in low Earth orbit. 1993. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1060351586.

    MLA Style (8th edition)

  • Schaefer, Glen. "Ultrathin metallic coatings for silver surfaces: Function and utilization in low Earth orbit." Doctoral dissertation, Case Western Reserve University, 1993. http://rave.ohiolink.edu/etdc/view?acc_num=case1060351586

    Chicago Manual of Style (17th edition)

case1060351586
720
© 1993, all rights reserved.
This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.