Environmental Engineering Reference
In-Depth Information
interfacial stresses between the TBC and the bond coating. Even though ZrO
2
is
polymorphic in nature having crystal structures as monoclinic (room temperature
to 1373 K), tetragonal (1373-2543 K), and cubic (2543-2953 K), the high-tem-
perature cubic phase can be stabilized by the addition of Y
2
O
3
, MgO, CaO, CeO
2
,
etc. With the addition of sufficient stabilizer, the ZrO
2
structure can be fully
stabilized and retained in cubic form even at ambient temperatures. However, by
maintaining the addition of stabilizer at a sufficiently low value it is possible to
obtain partial stabilization of ZrO
2
and all three phases (cubic, tetragonal, and
monoclinic) can be retained on cooling to room temperature. Such partially stabi-
lized ZrO
2
(PSZ) is a superior TBC material compared with a fully stabilized
ZrO
2
. The superiority of PSZ is due to its better thermal shock resistance and
lower linear thermal expansion coefficient compared with that of fully stabilized
ZrO
2
. Accordingly, ZrO
2
partially stabilized by addition of 6-8 wt% Y
2
O
3
is
increasingly being used as a TBC due to its superior mechanical stability under
thermal cycling conditions prevalent in gas turbine environments. Even though
the porous nature of TBCs enhances the thermal shock resistance, which is very
much desired, the thermal expansion mismatch between the TBC and the sub-
strate results in the development of interfacial residual stresses. Moreover, the
porous nature of TBC allows the corroding gases to penetrate, resulting in a high
corrosion rate of the substrate alloy. Hence, to reduce these effects, a bond coating
with high corrosion resistance is employed as an intermediate layer between the
TBC and the substrate. Bond coating also minimizes the thermal expansion mis-
match. In general, TBCs are deposited on superalloys by air plasma spraying on
top of a vacuum plasma-sprayed M-Cr-Al-Y bond coating. The microstructure
of such a coating is demonstrated in Fig. 6.53. Performance tests of various com-
positions of Ni-Cr-Al-Y bond coating suggest that the optimum chromium and
yttrium contents should be 14-18 wt% and 0.3 wt%, respectively, for reducing
the tendency for spallation at the TBC-bond coating interface, which is a com-
mon mode of TBC failure. So the major factors affecting thermal cycling surviv-
ability and high-temperature performance of plasma-sprayed TBC (ceramic)
coatings include thermal expansion mismatch between the oxide coating and the
alloy substrate, phase transformations of the coating material, interfacial interac-
tions during thermal treatment, compositional effects, bond coating corrosion,
and oxidation resistance. These factors will, of course, interact and determine the
coating behavior.
6.8.4 Degradation of Coatings
The efficacy of a metal/alloy coating system in service environments is judged
in terms of coating-environment and coating-substrate reactions and interactions
along with accompanied vaporization processes (if any). The vaporization pro-
cesses may include (1) vaporization of the protective external oxide film, (2)
