PSI - Issue 68

Elena Fedorova et al. / Procedia Structural Integrity 68 (2025) 908–914 Elena Fedorova et al./ Structural Integrity Procedia 00 (2025) 000–000

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For all the variants of the oxide thickness considered, initial damage occurs at a temperature of about 950 °C in the middle part of the TGO/TC interface as a result of interlayer shear. For a TGO of 5 and 10 μm, further interface delamination drived by the mixed-mode failure occurs mainly in the direction of the valley, whereas at t = 1 μm, damage evolution is directed toward the peak. The distribution of the damage parameter along the TGO/TC interface at the end of cooling is shown in Fig. 6a. As follows from the presented data, the debonding crack has its maximum length ( ~ 19 μm) if t = 10 μm with its greatest opening in the valley area. With the exception of the thinnest TGO, the development of debonding along the interface in the peak area is hindered by residual compressive stresses acting there. On the contrary, at t = 1 μm tensile stresses at the peak contribute to the failure of the interface according to the normal mode failure, which is consistent with the results of other studies on the stress state of similar TBCs. It should also be noted that for t = 2.5 μm, the simulation results predict the shortest crack length (about 3 μm) located at the central part of the TGO/TC interface.

Fig. 6. The damage parameter d m distribution along (a) TC/TGO and (b) TGO/BC interfaces.

A slightly different picture of the failure process is observed at the interface between the BC and oxide layers. Here, there is a pronounced tendency for the interfacial crack length to increase with increasing TGO (Fig. 6b) from the maximum value of 21.2 μm to 3.5 μm at t equals to 10 and 1 μm, respectively. Such a ratio of the length of the damage zones at the BC/TGO interface is characteristic of all stages of the cooling process. In all cases, the mode I crack has the greatest opening in the peak area. The stress state of the TBC also depends on the thickness of TGO. Despite the fact that the maximum compressive stresses are formed in the thickest TGO (- 4037.7, -2135.4, -2302.5 and -2515.6 MPa for t = 10, 5, 2.5 and 1 μm, respectively), their values averaged over the volume of the oxide layer increase with decreasing its thickness and are -753.0, -861.1, -1135.6 and -1435.3 MPa for t = 10, 5, 2.5 and 1 μm, respectively. A high level of compressive stresses in combination with lower buckling resistance, compared to thick TGOs, can lead to buckling failure of the oxide layer, which is considered as a possible mechanism of TBCs fracture according to Ashofteh A. and Rajabzadeh M., (2024). 5. Conclusions The data on the TBC components thickness, its uniformity and geometry of the interfaces were obtained by SEM microstructural analysis for two types of Ni-based single crystal superalloys: as-coated samples and samples of turbine blades with TBC after the high temperature cyclic oxidation tests. New designs of samples and loading fixtures were proposed to measure the interfacial adhesion parameters of TBC on Ni-based single crystal superalloy. Different configurations were tested in order to facilitate and ensure accuracy of the experimental measurement. The obtained values of energy release rate determined using mode II energy delamination criterion was in a range of 90-120 J/m 2 . The results of numerical study on the interface debonding process indicate a pronounced influence of the oxide