Issue 35

K. Slámečka et alii, Frattura ed Integrità Strutturale, 35 (2016) 322-329; DOI: 10.3221/IGF-ESIS.35.37 326 Fig. 4. The initial stresses follow the waviness of the surface with tensile stresses (i.e. the negative contact pressure) located at the bond coat peaks and compressive stresses located in the valleys. When the TGO of the thickness of 3  m is introduced, not only do the magnitudes of these stresses increase but they are also redistributed and the correlation with the surface topography and the initial stress state is significantly diminished, Fig. 4. Such evolution corresponds to the stress conversion, the onset of which depends on the local geometry of the interface [5, 11, 16]. The TGO layer is nearly under biaxial compression with the first principal stress  1 being negligible, and with the second and third principal stresses being predicted to be approximately -2.6 GPa, Fig. 5. Given the compressive strength of alumina of 2-4 GPa, the waviness by itself might at this stage induce local cracking of the TGO during the cooling part of the cycle, if it is not counteracted by sufficient creep stress relaxation at high temperatures. Figure 4 : Contact pressure at the TGO/BC interface for t TGO = 0 µm (left), and t TGO = 3 µm (right). Figure 5 : The principal stresses  2 ,  3 in the TGO layer ( t TGO = 3 µm). The stresses in the top coat, which are essential when the loading involves fast thermal cycling and the failure occurs due to delamination cracks initiated at microstructural defects such as pores or de-bonded splat boundaries, initially decrease with BC oxidation, Figs. 6, 7. The highest tensile stresses are located near the bond coat peaks. The correlation with local interfacial features remains high when the TGO layer is present, the broadening of the tensile regions due to the stress conversion (from tensile to compressive above the peaks and from compressive to tensile above the valleys) is, however, already observed, Fig. 6. Previous calculations on regular surfaces revealed that for the TGO of thickness of 3 µm, the surface with the wavelength of  = 125  m and the amplitude of A = 10-30  m exhibits substantial shift of the tensile region towards the valley, while for the surfaces of  = 500  m, the tensile stresses are still located above the BC peaks [16]. Moreover, with further oxidation, the tensile stresses generally increase and the microcracks formerly arrested in the compression zone above the valleys can continue to propagate and interconnect to form larger delamination cracks leading to the final failure [5]. Figure 6 : The first principal stress  1 in the top coat for t TGO = 0 µm (left) and t TGO = 3 µm (right).