2009 Volume 58 Issue 9 Pages 759-766
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.
