Environmental Engineering Reference
In-Depth Information
Figure 6.56
Schematic illustration of the degradation of disilicide coatings to lower
silicides by coating-substrate interaction during oxidation: (a) MoSi
2
on Mo after 30 h
at 1948 K; (b) WSi
2
on W after 13 h at 1923 K [80].
protective behavior, but due to their low silicon content, a longer period of oxida-
tion is needed.
During diffusion-controlled interaction of the silicide coating with the sub-
strate, the thickness of the different layers is governed by the relative rates of
diffusion in the respective layers and the chemical potential gradient of the diffus-
ing species. If diffusion through one layer is rapid, the corresponding layer will
be thick. On the contrary, if the rate of diffusion through a layer is low compared
with the neighboring layers, the thickness of the grown layer will automatically be
thin, provided the temperature dependencies of growth of the layers are identical.
Diffusion of the coating constituent into the substrate metal/alloy may be reduced
by predepositing a high-melting-point metal prior to application of the coating
[79].
Oxidation of molybdenum and tungsten silicides leads, on the one hand, to
