Original
Paper(Vol.58 No.9 pp.759-766)
Delamination Behavior of Thermal Barrier Coatings
under Thermo-Mechanical Fatigue
Shigeru YOSHITAKE, Tetsuji IIO, Atsuki FUNAO, Masaru SEKIHARA,
Hideyuki ARIKAWA and Kunihiro ICHIKAWA
Abstract:It has become common
to apply thermal barrier coatings (TBC) to hot gas path parts in gas turbines to
increase thermal efficiency. However, some uncertainty about the delamination
life prediction method for TBC still remains. The influences of mechanical
parameters on delamination behavior were investigated through thermo-mechanical
fatigue (TMF) tests to improve TBC delamination life prediction. The parameters
investigated were loading pattern, strain range, ceramic coating thickness, and
high temperature exposure processing prior to TMF tests.
Delamination
behavior with a simulated loading pattern seemed to better correspond to
behavior under out-of-phase (OP) loading than that with in-phase (IP) loading.
The OP loading pattern was better for evaluating delamination lives because of
its shorter test period. The delamination lives became shorter under larger
strain ranges. Of the specimens tested, the specimen with a 0.9-mm-thick ceramic
coating had the shortest delamination life. The specimen with a 0.6-mm-thick
coating had almost the same or longer life than the specimen with a 0.3-mm-thick
coating. These results strongly suggest that an optimum ceramic coating
thickness keeps the cooling performance consistent with delamination strength.
Thermally grown oxides (TGO) are generally thought to be the main cause of
TBC degradation in long-term operated gas turbine blades. The growth behavior of
TGO was observed by exposing specimens to a high-temperature environment and it
was suggested that the Larson-Miller parameter (LMP) could predict TGO thickness
well. In addition, some specimens were exposed to high-temperature environments
prior to TMF tests to evaluate the effect of TGO thickness on delamination
lives. This high temperature pre-exposure seemed to accelerate the delamination
growth rate due to sintering and TGO.
Key Words:Gas Turbine, Thermal Barrier Coatings, Thermo-Mechanical
Fatigue, Thermally Grown Oxides, Delamination