Comprehensive damage evaluation of localized spallation of thermal barrier coatings
Journal of Advanced Ceramics
2226-4108
Comprehensive damage evaluation of localized spallation of thermal barrier coatings
Wei-Wei ZHANG 1 2 3
Guang-Rong LI 3
Qiang ZHANG 0 3
Guan-Jun YANG 3
0 AECC Beijing Institute of Aeronautical Material , Beijing 100095 , China
1 Institute of Publication Science, Chang'an University , Xi'an 710064 , China
2 School of Materials Science and Engineering, Chang'an University , Xi'an 710064 , China
3 State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University , Xi'an 710049 , China
Thermal barrier coatings (TBCs) enable the hot section part to work at high temperatures owing to their thermal barrier effect on the base metal components. However, localized spallation in the ceramic top-coat might occur after long duration of thermal exposure or thermal cycling. To comprehensively understand the damage of the top-coat on the overall hot section part, effects of diameter and tilt angle of the spallation on the temperature redistribution of the substrate and the top-coat were investigated. The results show that the spallation diameter and tilt angle both have a significant effect on the temperature redistribution of the top-coat and the substrate. In the case of the substrate, the maximum temperature increment is located at the spallation center. Meanwhile, the surface (depth) maximum temperature increment, having nothing to do with the tilt angle, increases with the increase of the spallation diameter. In contrast, in the case of the top-coat, the maximum temperature increment was located at the sharp corner of the spallation area, and the surface (depth) maximum temperature increment increases with the increase of both the spallation diameter and the tilt angle. Based on the temperature redistribution of the substrate and the top-coat affected by the partial spallation, it is possible to evaluate the damage effect of spalled areas on the thermal capability of TBCs.
thermal barrier coatings (TBCs); thermal property; localized spallation; damage evaluation
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During past decades, the operating temperature of gas
turbine engines has been elevated significantly, with the
aim to increase their efficiency. Correspondingly, high
temperature durability of the engine components has
increased as well [
1–4
]. Significant advances in the high
temperature capability have been achieved through the
* Corresponding author.
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application of nickel-based and cobalt-based
superalloys [
5–7
]. Nonetheless, these
monolithicformed alloys are often susceptible to damage by
oxidation and hot corrosion, so it is impossible to retain
adequate mechanical strength. Moreover, these alloys
are unable to bear high service temperature (e.g., above
1000 ℃ [
8,9
]). Therefore, thermal barrier coatings
(TBCs) are deposited on the hot section components to
enhance the temperature capability of the underlying
metal substrate. For instance, in the turbine components
with suitable internal cooling, temperature drops of
more than 200 K can be realized by the TBCs with
thicknesses from 200 to 500 µm [
8,10
].
In order to work effectively, the TBCs show excellent
thermal barrier effect and high resistance to spallation
when exposing to high temperature environment. A
typical TBC system often exhibits a multi-layer
structure: a metallic bond layer deposited preferentially
on the component surface, followed by an adherent
ceramic layer providing the thermal insulation effect.
Commonly, the ceramic top layer of a TBC deposited
by plasma spraying (PS), makes for a lamellar
microstructure composing of splats lying parallel to the
substrate surface [
11
]. Moreover, the quantity of
intersplat pores is connected with intrasplat cracks,
making for a continuous pore network. Consequently,
the unique porous structure of the PS top-coat
contributes to a low thermal conductivity in the
through-thickness direction, as well as high strain
tolerance in the in-plane direction.
However, during the service, the TBC system may
fail by spallation of the ceramic top-coat, which
originates from the extension of those existing
microcracks. On one hand, during thermal exposure at
high temperatures, a thermally grown oxide (TGO)
layer is predominantly formed by alumina [
12
]. The
formation of the TGO plays a crucial role on the failure
of the TBCs. The associated failure mechanism often
results from the spallation at or close to the TGO layer
within either the yttria stabilized zirconia (YSZ) or the
bond-coat [
13–15
]. To begin with, small cracks and
separations nucleate at imperfections near the TGO.
Once nucleated, the small cracks extend and coalesce,
while the TBC may remain attached by remnant
ligaments. Finally, spallation occurs when the ligaments
are detached over a sufficient area and eventually spalls
from the substrate. On the other hand, sintering may
lead to the stiffening of the top-coat. Consequently, the
strain energy r (...truncated)