Study on Delamination Fatigue Strength of Thermal Barrier Coating at Elevated Temperature

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In this study, it is investigated experimentally and analytically that fracture process and fatigue behavior of Thermal Barrier Coatings (TBCs) used in a high temperature component in a land based gas turbine. The effects of the thermal load on TBCs and delamination strength are investigated by measuring residual stress of top coat, using the finite element method and various cyclic shearing, tensile and torsion tests.It is found that (1)the TGO layer thickness increases with increasing heating time for both cases of 900 and 1000℃, and it saturates gradually after 20 hours for the both cases. (2)A delamination of top coat under thermal load occurs at a peak of undulation in top coat and bond coat interface. This is because that tensile tearing stress which is normal to the interface occurs at the peak area after cooling. So, about failure patterns of TBCs, we can find two patterns, the first one is that a crack initiates at TC surface and propagates in the thickness direction. After th … More e crack arrives near the interface, a delamination occurs at the interface and grows. Another pattern is that a local delamination can be observed without any crack grown from the TC surface. In the both cases, it is observed that delaminations run from a peak to peak of undulation of BC near the interface. (3)It is found that the mechanical property of bond coat changes after thermal aging and the shear fatigue behavior of TBCs is affected by bond coat strength. (4) Delamination energy increases quickly with increasing heating time up to 40 hours and then decreases slightly. So, the delamination strength of TC is thought to decrease apparently after long time heating. This is because that residual stress in TC increases and so elastic strain energy increases with increasing heating time. (5) Softening of BC caused by long heating makes fatigue life of TBCs small.(6)We can find that the shearing using small plating specimens gives reliable data with small scatter bands on delamination strength. (7)Also, the torsion pin-test was found to be a good method which obtains a delamination strength of thermal sprayed coating under combined shearing with tensile stresses. Less