For the case of the regular TGO, the debonding upon cooling occurs along both the TC/TGO and BC/TGO interfaces (Figs. 1, 2). The locations of crack nucleation correspond well to the distribution of normal and tangential stresses . First on time, the crack is initiated at the valley of the TC/TGO interface (Fig. 1c), and then the debonding progresses towards the peak location with the mixed-mode mechanism. At the end of cooling, 2/3 of the contact elements between the TC and TGO layers are failed as the debonding parameter reaches the threshold value (Fig. 2). These results are consistent with a previous work that, in most cases, the valley location in the TC/TGO interface is more favorable for crack propagation than the peak one.
About 900 s after the cooling start (Fig. 1c), the debonding also appears at the peak of TGO/BC interface. The interfacial crack develops reaching the length of 13.8 μm, but it then arrested at the middle of asperity due to the presence of high compressive stresses. The mode I debonding is dominant in this damage process.
The interfacial debonding induces the stress redistribution in all the TBC layers. As shown in Fig. 3, the level of tensile out-of-plane stresses decreases in both the TC and BC layers compared with the case of intact interfaces (Fig. 4). Most stress relaxation appears in areas of failed interfaces. Note also that the zone of maximal tensile stresses in the TC layer moves upward from near the interface locations. In contrast to the reduction in stress level in the TC and BC layers, the TGO layer experiences higher compressive stress than those obtained when the debonding is not permitted.
Fig.1. The stress oy distribution at the end of cooling (a-b) and evolution of the contact gap distance(c).
Fig. 2. The damage parameter distribution for TBCs with the regular (a) and irregular (b) TGO layer.
Fig. 3. The stress σy distribution in TBC with the regular TGO considering interfacial debonding.
Fig. 4. The stress σy distribution in TBC with the regular TGO without considering interfacial debonding.
The change in the stress state following the interface delamination has been also reported by other authors. The stress relaxation in the BC and accompanied stress increase in the TC layer have been shown after the BC/TGO delamination occurs. This phenomenon is attributed to the decrease in thermal mismatch between the TGO and BC layers due to reduction of contact area. Apparently, the same reason is the source of stress redistribution we observed, except that in our study the interfacial debonding occurs on both sides of the TGO layer.
For the case of the irregular TGO, the debonding is first occurs at the middle of the TC/TGO interface caused by a pure II mode, and the damage is developed towards the peak. There are no mode I damages near the peak locations until the temperature drops to 800 ◦C (about 500 s of the cooling time, Fig. 1c.) From this time, the mode I crack is initiated and then grows up to about 8 μm to the middle of the TC/TGO interface. There is almost no damage along the TGO/BC interface, except near the peak locations where the maximum debonding parameter amount to 0.6 (Fig.2).
As well as for the case of the regular TGO, the debonding leads to the stress redistribution in the TC and TGO layers. As shown in Fig. 5, the debonding of the TC/TGO interface results in change of the stress distribution pattern and decrease of out-of-plane stresses in the TC layer, especially near the peak location. At the same time, the minimum principal stresses increase (in absolute value) by more than 15% compared with simulation without considering the interfacial debonding. As expected, the stress state in the BC layer with and without simulation of interface failure remains practically the same due to a negligible damage at the TGO/BC interface
Fig. 5. The stress σy distribution in the TC layer of TBC with the irregular TGO with (a) and without (b) considering interfacial debonding.
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