Contribution to Life Prediction of Thermal Barrier Coatings using the Concept of Critical Strain
R. Herzog, D. Sebold, F. Schubert, L. Singheiser
CNKI
Abstract
An empirical model has been developed, which describes the correlation between the cyclic strain range of a Ni-base superalloy and the number of cycles to failure of an overlaying plasma-sprayed thermal barrier coating (TBC) under thermomechanical fatigue loading conditions. Failure was de- fined as macroscopic delaminations greater than 1 mm in diameter. The presented life prediction model is based on the Miner rule and includes in a phenomenological manner the time controlled damage and degradation effects due to bond coat oxidation and sintering of plasma-sprayed TBCs by means of a time dependent critical strain. Thermomechanical fatigue data were determined for IN 792 cc specimens with a VPS NiCo- CrAlY bond coat and an APS top coat of zirconia stabilized with 7-8 wt% yttria. The temperature range was RT - 950 C, the total mechanical strain range has been varied between 0.2 and 0.8% (tensile). The defect structure within the TBC after failure consisted of segmentation and delamina- tion cracks. The crack morphology and the results of calculations using the presented model allow the conclusion, that the failure was primarily caused by fatigue crack growth within the TBC, accel- erated by sintering of the TBC. The degradation due to bond coat oxidation under the applied load- ing conditions was of secondary importance. The TGO thickness was in any case below 0.8 m.
Keywords:cracks; cyclic; loading conditions; strain range; number of cycles; segmentation; TBC; thermal barrier coating; empirical; bond coat; Life Prediction; top coat; thermomechanical fatigue; oxidation and sintering
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