Finite element simulation of stresses in a plasma-sprayed thermal barrier coating with a crack at the TGO/bond-coat interface

Plasma-sprayed TBCs typically consist of a ceramic top-coat and a metallic bond-coat with an irregular metal/ceramic interface, which is required for better adhesion and, at the same time, generates tensile stresses responsible for in-service failure. Exposure to high-temperatures results in formation of a third, thin layer of thermally grown oxide (TGO) at the top-coat/bond-coat interface. One of the cracking mechanisms experimentally observed in these coatings is delamination at the TGO/bond-coat interfacial peaks. Such delamination cracks increase tensile stresses in the ceramic top-coat layer and may induce its cracking. This article provides the numerical study of the residual stress state of the YSZ (yttria partially-stabilized zirconia) top-coat in a conventional plasma-sprayed TBC system with a delamination crack centred at the TGO/bond-coat 3D cosine interfacial peak. The results show that the delamination crack induces considerable stresses in the YSZ layer; the small interfacial peaks are more likely to cause cracking above the peak, whereas the large peaks are more likely to cause off-peak cracking. Furthermore, it is shown that the aspect ratio (the ratio of amplitude to wavelength), which is often used in numerical studies to distinguish between various geometries, is not a unique parameter, since the changes in amplitude and wavelength are not equivalent.