Environmental Engineering Reference
In-Depth Information
ence of alloying elements. Chromium and aluminum increase the strength of the
nickel matrix by solid solution strengthening and also form protective Cr
2
O
3
-
and Al
2
O
3
-rich films by reaction with the environment. Aluminum further helps
in the formation of
phase, resulting in antiphase boundary strengthening. Spall-
ation of the protective film is reduced by chromium in the superalloy-coating
matrix. Minor amounts of Y are added for further improvement in scale adherence
[84].
Most failures of the TBC-bond coating system often occur at the ceramic-
bond coating interface because of high compressive stresses in the TBC. The
extent of these stresses are determined by the amount of strain relieved, e.g., by
plastic deformation or by microcracking. The transformation strains, which are
inversely dependent on the degree of stabilization in ZrO
2
, may or may not coun-
teract some of the thermal expansion strain depending on the degree and nature
of texturing. Furthermore, microcracking associated with phase transformation
or porosity (as obtained in plasma-sprayed coating) is a potential mechanism of
relieving mismatch strains.
During performance of the TBC-bond coating system at high temperatures,
the interfacial bonding oxide, which is predominantly Al
2
O
3
-orCr
2
O
3
-rich,
grows in thickness. Once the oxide layer reaches sufficient thickness, its own
thermal shock resistance property comes into play. It has been suggested [85]
that this oxidation is the single most time-dependent factor that limits the service
life of TBCs. Moreover, it has been further stressed that the role of oxidation-
induced strains that combine with cyclic strains to promote slow crack growth
in the ceramic layer should not be ignored. Hence, the bond-coating material
should be such that during service it forms an impervious, tenacious oxide bond-
ing layer that doesn't allow the oxide to grow in thickness with time at its opera-
tional temperature. It is well known that Al
2
O
3
has a poor thermal shock resis-
tance for which the compositions of currently preferred bond coatings [84] are
adjusted to low aluminum with higher chromium in order to utilize the superior
thermal shock resistance properties of Cr
2
O
3
.
γ′
6.9 CONCLUSION
In general, the reaction behavior of protective coatings in environments of their
use and their interactions with the substrates during high-temperature perfor-
mance is not well understood. This is basically due to the limited research under-
taken in this area in comparison with the investigations on bulk homogeneous
metals/alloys. The demand for better performance of components has led to de-
velopment of new coating compositions and appropriate methods of their forma-
tion. Such development has encompassed an interaction between the physical
metallurgy of the coating and its processing. A fundamental understanding of
the mechanism of formation of different coatings and their mode of degradation
