Environmental Engineering Reference
In-Depth Information
ties of the coating because SiO
2
can again spontaneously grow during subsequent
heating as a self-healing layer. As a result, during subsequent heating cracks are
again healed partly by thermal expansion and sintering, and partly by formation
of new oxide. It has been suggested [83] that the deteriorating effects of thermal
cyclings can be minimized by the deposit of a layer of molybdenum boride be-
tween the disilicide and the metal. The positive effect of such a layer is due to
better filling and sealing of the cracks resulting from fluxing action of boron
oxides.
It is also important to note that the structure of the SiO
2
layer changes with
temperature, which has a strong bearing on its protective properties. At tempera-
tures above 1473 K it is glassy, whereas at lower temperatures it becomes increas-
ingly crystalline. At 1073 K, the scale on MoSi
2
consists of crystalline SiO
2
(crystobalite) associated with a complex MoO
2
-SiO
2
phase. The glassy layer
formed above 1473 K offers the best protective properties. During cooling in
the intermediate temperature range (1273-1473 K), the SiO
2
layer undergoes
transformation from glassy to crystalline structure. Thus, it appears advantageous
to preoxidize MoSi
2
coatings at 1673-1773 K before using the coated component
at lower temperatures [78].
Another factor that affects the protective properties of MoSi
2
adversely is the
reduced oxygen partial pressure in the environment. At reduced pressures, the
degradation process gets localized, resulting in numerous pinholes. Under such
conditions, the oxidizing power of the gas phase is not sufficient to allow forma-
tion of a continuous protective film of SiO
2
. Moreover, SiO
(g)
evaporation also
becomes an important phenomenon at low oxygen pressures. Therefore, if sili-
cide-coated molybdenum is to be employed in atmospheres having low oxygen
activity, preoxidation of the coating in an environment having high oxygen activ-
ity ought to be carried out to develop a thin protective SiO
2
layer on the coating
surface for its subsequent satisfactory performance.
In all practical coating systems containing silicon as a major protective ele-
ment, there is a pronounced gradient in the physical, mechanical, and chemical
properties of the coating-metal/alloy system. Brittleness and thermal expansion
mismatch give rise to cracking and spallation during thermal cyclings. The inher-
ent brittleness of silicide intermetallics is one of the most important reasons for
their early failures, although the ductile-brittle transition temperatures of silicide
coatings are higher than those for other coating systems [79]. To overcome this
disadvantage and maintain the silicon content in the surface layer of the coating
unaffected from loss due to oxidation over longer periods of exposure, it is often
suggested that tough matrices and dispersed silicide reservoir phases in the Ni-
Cr-Si and Ni-Cr-Si-Ta overlay coating systems be used. The problems associated
with interdiffusion in silicide coating-substrate systems can be minimized by the
addition of refractory metals, such as Ta, Ti, Nb, or Mo, to reduce the diffusion
rate, Ta being the most effective.
