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The Phase and Microstructural Analysis of Protective Oxide Coatings on Molybdenum

  • H. Traxler , R. Jörg , M. Zabernig and L.S. Sigl
Published/Copyright: June 11, 2013
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Practical Metallography
From the journal Practical Metallography Volume 48 Issue 9

Abstract

Due to their high sensitivity to oxygen, the use of refractory metals requires an effective protection against oxidation. In the case of molybdenum this is achieved by using a silicon and boron based coating commercially marketed under the trade name SIBOR®. In the moduction of a SIBOR®-coating, a mixture of Si, B and C is plasma sprayed in air onto the Mo-surface to be protected and subsequently annealed in hydrogen. Previous investigations have shown that in doing so a stationary coating of Mo-borides and Mo-silicides is formed. However, the exact phase arrangement and composition has until now remained unclear.

Energy dispersive X-ray spectroscopy (EDS) in the scanning electron microscope (SEM) is able to analyse the silicides in SIBOR®, although due to the overlapping of the Mo- and B-spectral partial a clear identification of the borides was not possible. Using a combination of electron back scatter diffraction (EBSD) and wavelength dispersive X-ray spectroscopy (WDS) it is, however, shown that SIBOR® is made up of a series of sub-layers of Mo2B and MoB followed by Mo5Si3 and MoSi2, and that the other phases of the Mo-Si-B ternary system, i.e. Mo3Si and Mo5SiB2(T2), do not occur.

Notably, the two borides and the Mo5Si3 exhibit a structure which is polycrystalline in the lateral direction yet normal to the surface forms of only a single layer of crystallites. In contrast, the final MoSi2-layer has a polycrystalline structure both in the lateral and in the normal directions. Furthermore, Mo5Si3 and MoSi2 both exhibit marked textures.

Kurzfassung

Wegen Ihrer hohen Empfindlichkeit gegen Sauerstoff erfordert der Einsatz von Refraktärmetallen an Luft einen wirksamen Schutz gegen Oxidation. Im Falle von Molybdän wird dies durch eine Beschichtung auf der Basis von Silizium und Bor erzielt, die unter dem Handelsnamen SIBOR® kommerziell vertrieben wird. Als Vorläuferschicht für SIBOR® wird ein Gemenge aus Si, B und C durch atmosphärisches Plasmaspritzen auf die zu schützende Mo-Oberfläche aufgetragen und anschließend in Wasserstoff geglüht. Ältere Untersuchungen haben gezeigt, dass sich dabei eine stationäre Schicht aus Mo-Boriden und Mo-Siliziden bildet, deren genaue Phasenanordnung und -zusammensetzung bisher unklar war.

Mittels energiedispersiver Röntgenspektrometrie (EDS) im Rasterelektronenmikroskop können die Silizide in der SIBOR®-Schicht nachgewiesen werden. Allerdings ist wegen der Überlagerung der Mo- und B-Spektren keine eindeutige Identifizierung der Boride möglich. Durch Kombination von Elektronenbeugung (EBSD) und wellenlängendispersiver Röntgenanalyse (WDS) wird gezeigt, dass SIBOR® aus einer Abfolge von Sub-Schichten aus Mo2B und MoB gefolgt von Mo5Si3 und MoSi2 besteht. Die aus dem Dreistoffsystem Mo-Si-B bekannten Phasen Mo3Si und Mo5SiB2 (T2) treten dagegen nicht auf.

Bemerkenswert ist, dass die beide Boride und das Mo5Si3 eine Schichtstruktur aufweisen, die in lateraler Richtung polykristallin ist, während sich in Richtung senkrecht zur Oberfläche jeweils nur eine Einzellage von Kristalliten ausbildet. Im Gegensatz dazu ist die abschließende MoSi2-Lage sowohl in lateraler als auch in normaler Richtung polykristallin strukturiert. Mo5Si3 und MoSi2 weisen außerdem eine ausgeprägte kristallographische Vorzugsorientierung auf.

Translation: P. Tate

Hannes Traxler Diploma in technical physics from the Technical University of Vienna, Austria. Ph.D. in engineering science from the University of Saarbrücken about acoustic emission. Since 2002 development of testing methods in the materials testing laboratories of PLANSEE with focus on non destructive testing and electron microscopy.

Roland Jörg Vocational training in material testing at PLANSEE followed by education at the engineering college for materials technology in Eisenstadt, Austria. Rejoined PLANSEE in 1996 with a focus on product development for the automotive and aerospace industry. Since 2009 responsible for electron microscopy in the materials testing labs.

References/Literatur

[1] H.-P.Martinz, J.Matej, M.Sulik: SIBOR® – An Effective Oxidation Protective Coating for Molybdenum Parts, Proc. 12th Conf. on Electric and Other Highly Efficient Ways of Glass Melting, 71–81 (2001)Search in Google Scholar

[2] H.-P.Martinz, B.Nigg, J.Matej, M.Sulik, H.Larcher, A.Hoffmann: Properties of the SIBOR® Oxidation Protective Coating on Refractory Metal Alloys, Proc. 16th Int. Plansee Seminar Vol. 1, Eds. G. Kneringer, P. Rödhammer, H. Wildner, 67–84, (2005)Search in Google Scholar

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Received: 2011-5-4
Accepted: 2011-6-21
Published Online: 2013-06-11
Published in Print: 2011-09-01

© 2011, Carl Hanser Verlag, München

Articles in the same Issue

  1. Contents/Inhalt
  2. Contents
  3. Editorial
  4. Editorial
  5. Technical Contributions/Fachbeiträge
  6. The Phase and Microstructural Analysis of Protective Oxide Coatings on Molybdenum
  7. Defects and Fracture Statistics in Low-Pressure Injection Moulded Alumina Components
  8. 10.3139/147.110151
  9. The Metallographer's Role in Damage Analysis
  10. Meeting Diary/Veranstaltungskalender
  11. Meeting Diary
https://doi.org/10.3139/147.110156

Articles in the same Issue

  1. Contents/Inhalt
  2. Contents
  3. Editorial
  4. Editorial
  5. Technical Contributions/Fachbeiträge
  6. The Phase and Microstructural Analysis of Protective Oxide Coatings on Molybdenum
  7. Defects and Fracture Statistics in Low-Pressure Injection Moulded Alumina Components
  8. 10.3139/147.110151
  9. The Metallographer's Role in Damage Analysis
  10. Meeting Diary/Veranstaltungskalender
  11. Meeting Diary
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