Volume 48, Issue 9

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.

The strength and the defects responsible for failure occurring in tubular alumina components were investigated. Strength testing was performed on machined components in 3-point and 4-point bending. Weibull statistics were applied to describe the strength distributions. Fracture surfaces were investigated using stereo-microscopy and SEM. Fractography revealed that three different types of defects were responsible for failure of the 3-point as well as the 4-point bend specimens. Using an iterative procedure, a separate Weibull distribution for each defect type was evaluated using the data of the two different specimen types.

The metallographer's role is of extraordinary importance when clarifying cases of damage. This statement is documented by means of an example of corrosion damage on a brewery pipe. Several different methods have been applied in order to provide for a final result, such as emission spectroscopy analysis, examination of damage areas under the scanning electron microscope including an examination of coatings by means of EDX analysis and examination of the metallographic section. The comprehensive formation of a metallographer allows for a responsible examination and a summary of results that provides for an overall picture to clarify beyond doubt the cause of the damage.