High Temperature Sintering and Oxidation Behavior in Plasma Sprayed TBCs [Single Splat Studies]
Paper 1—Role of Heat Treatment Variations 115
markable impact. Even a short intermediate vacuum
treatment changes the high temperature behavior ob-
served otherwise during long air treatment and causes
sintering of most fine microcracks. Thus sintering of
microcracks is directly influenced by bond coat oxi-
dation.
Microcracks in as-sprayed splats coincide with inter-
nal oxide positions within bond coat. A path is thus
provided for oxygen to reach a bond coat area that is
locally depleted of aluminum. As such, other oxides
such as NiO form locally at these microcracks. Che-
mical changes occur in the bond coat oxidation due to
presence of the YSZ splats on top.
Outward oxide growth into microcracks increases
with heat treatment temperature as well as time.
Longer thermal treatment durations cause further Al
depletion within bond coat leading to formation of
oxides other than α-alumina in the TGO, thereby cre-
ating thickness imperfections in the TGO at some lo-
cations. These TGO undulations are observed to push
grains in the splats upward and induce splat lift-
ing/spalling. Interaction between evolving TGO and
YSZ top coat is thus apparent in the form of splat
surface roughening.
The distinctions of TGO formation and interactions
with YSZ as a function of initial YSZ splat layer (mor-
phology, purity) and initial bond coat microstructure
(processing condition s) will be addressed in a subseq uent
paper.
5. Acknowledgements
We would like to thank Glenn Bancke, AnirudhaVaidya
and Li Li (CTSR) for preparation of the specimens and
spraying diagnostics.
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