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.
| 