Environmental Engineering Reference
In-Depth Information
It can be concluded that silica scales offer better protection than chromia
scales, which are susceptible to vaporization loss via CrO 3(g) at temperatures
above 1273 K. There is no doubt about the superiority of alumina scales over
silica scales at gas turbine operating temperatures. However, above 1773 K, silica
scales are more effective barriers in inhibiting degradation for which silicide
coatings are used to protect refractory metals at superhigh temperatures (approx.
1973 K).
Durability of TBCs. The use of ceramic thermal barrier coatings in the protec-
tion of components of gas turbine engines to significantly improve the service
life of components and engine performance has already been discussed (Sec.
6.8.3). TBCs are attractive because they can resist corrosion and oxidation (due
to inertness) of the high-temperature components and thereby extend their service
life through lowering of functional temperature and thermal stresses (strains) in-
duced during engine transients. They simultaneously provide an opportunity to
increase the turbine inlet temperatures or reduce cooling airflow in turbine air-
foils, contributing to higher engine efficiency and power output.
The preferred material for TBC is partially stabilized zirconia (ZrO 2 -22 wt%
MgO or ZrO 2 -6 to 8 wt% Y 2 O 3 ), for its inertness, insulating property with low
thermal conductivity, high resistance to corrosive and erosive environments, high
coefficient of thermal expansion to enhance compatibility with metallic sub-
strates, and thermal shock resistance properties. Although such material proper-
ties of the partially stabilized zirconia (PSZ) are of importance in performance,
other properties, such as adhesion strength, residual stresses, porosity, etc., mark-
edly influence the integrity of the coating system. The thermal expansion mis-
match between the TBC and the alloy substrate results in interfacial residual
stresses, leading potentially to coating delamination. The residual stresses depend
on a variety of mismatch strains and on the extent to which these strains result
in mismatch stresses. It is well known that thermal gradients and the transition
from molten to solid state may generate stresses in plasma-sprayed ceramic coat-
ings. The liquid-solid volume shrinkage, which may often be as high as 10%
(for ZrO 2 ), results in large strains in all melt-coating processes. Such shrinkage
is not only an important source of porosity but also of stress concentration [74].
While the porous nature of the TBC enhances its thermal shock resistance, it
also allows easy penetration of corrodents through the coating, resulting in fast
degradation of the substrate alloy. Hence, to reduce such effects, a material hav-
ing high oxidation and corrosion resistance property is employed as an intermedi-
ate bond coat between the TBC and substrate, which also minimizes the thermal
expansion mismatch.
Ni-Cr-Al-Y coatings are commonly used for such bond coats. The FCC matrix
of the Ni-based alloy is favored for high-temperature applications because of its
nearly filled third electron shell, which results in phase stability even in the pres-
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