H. Traxler, R. Jörg, M. Zabernig, L.S. Sigl
June 11, 2013
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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 Mo 2 B and MoB followed by Mo 5 Si 3 and MoSi 2 , and that the other phases of the Mo-Si-B ternary system, i.e. Mo 3 Si and Mo 5 SiB 2 (T 2 ), do not occur. Notably, the two borides and the Mo 5 Si 3 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 MoSi 2 -layer has a polycrystalline structure both in the lateral and in the normal directions. Furthermore, Mo 5 Si 3 and MoSi 2 both exhibit marked textures.