PSI - Issue 22

Andrey Burov et al. / Procedia Structural Integrity 22 (2019) 243–250

248

6

Author name / Structural Integrity Procedia 00 (2019) 000 – 000

3.2. Residual stress state for the regular TGO shape considering the interfacial debonding

For the case of the regular TGO, the debonding upon cooling occurs along both the TC/TGO and BC/TGO interfaces (Fig. 3). The locations of crack nucleation correspond well to the distribution of normal and tangential stresses (Fig. 2a). First on time, the crack is initiated at the valley of the TC/TGO interface (Fig. 4), and then the debonding progresses towards the peak location with the mixed-mode mechanism. At the end of cooling near all contact elements between the TC and TGO layers are damaged. Close to the valley location the debonding parameter reaches the threshold value. The results are consistent with work of Bäker (2012), who has shown that, in most cases, the valley location in the TC layer is more favourable for crack propagation than the peak one. Later on (about 900 s after the cooling start that corresponds to 550 0 C), the new debonding appears at the peak of the TGO/BC interface. The interfacial cracking develops reaching the length of 13.8  m, but is then arrested at the asperity middle due to the presence of high compressive stresses. The mode I debonding is dominant in this damage process.

Fig 3. Stress  y distribution at the end of cooling (the regular TGO).

Fig 4. Evolution of contact gap distance at different locations (the regular TGO).

The interfacial debonding induces the stress redistribution in all the TBC layers. As shown in Fig. 5 and Fig. 6, the level of tensile out-of-plane stresses decreases in both the TC and BC layers compared with the case of intact interfaces. Most stress relaxation appears in areas of failed interface. 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 endures a higher level of compressive stress (Fig. 7). The change in the stress state following the interface delamination has been also reported by Jiang et al. (2017) and later by Cen et al. (2019). The stress relaxation in the BC and the accompanied stress increase in the TC layer have been shown after the BC/TGO delamination occurs. The authors have attributed this phenomenon 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 have observed, except that in our study the interfacial debonding occurs on both sides of the TGO layer.

Fig. 5. Stress  y distribution in the BC layer (the regular TGO): (a) without and (b) considering the interfacial debonding.