Influence of cracks in APS-TBCs on stress around TGO during thermal cycling: A numerical simulation study

The failure of thermal barrier coatings (TBCs) fabricated by atmospheric plasma spraying (APS) during thermal cycling is often attributed to the accumulation of the thermal stress. However, in the end it comes down to the growth of thermally grown oxide (TGO) and propagation of the cracks around the TGO. Based on the 8. wt.% yttria stabilized zirconia (YSZ) TBCs fabricated by APS (APS-TBCs), the geometrical shape of the TGO layer which is located between the bond-coat (BC) and top-coat (TC) was simplified as sinusoidal curve with a certain amplitude and wavelength. The influence of the horizontal crack and vertical crack in the ceramic layer on the stress around the TGO layer during thermal cycling has been calculated using finite element method. The investigation results indicate that the stress concentration in the TBCs is dependent on the location of the vertical and horizontal crack. The vertical crack can partially release the stress concentration around the TGO layer compared with the horizontal crack if the crack is located above the peak of the TGO layer. When there is horizontal crack, the maximum tensile stress is located at the peak of the TGO/BC interface, while the maximum compressive stress is located at the spinodal position at the inner of the TGO. When there is vertical crack, the stress concentration tends to appear near to the crack tip, and the influences of horizontal crack and vertical crack on the stress around the TGO are distinctly different. In addition, the location of the vertical crack also has a certain influence on the distribution of the maximum stress. The vertical crack and the TGO have mutual effect on the stress concentration of each other. As for the horizontal crack, if it is located below the peak of the TGO layer (at the inner of the top-coat and near to the TGO layer), the crack tip has also exhibited stress concentration effect compared with the case that the horizontal crack is located above the peak of the TGO layer. The fracture mechanic parameters of the crack which may propagate, the propagation patterns of single crack and failure mechanisms of the APS-TBCs have also been calculated and discussed in this paper.