ABSTRACT
Microcracks have been observed to initiate near the bond coat/TGO interface after only a few thermal cycles in electron beam – physical vapor deposition (EB-PVD) TBC systems. The origin of these microcracks can be attributed to the buildup of thermal stresses that may magnify due to asperity of the thermally grown oxide (TGO) layer and the bond coat. The TGO/bond coat interface was modeled as a rough periodic surface. Finite element calculations were conducted to determine the magnitude of stresses at the bond coat/oxide interface (modeled as a sine wave). Two different cases were analyzed for this purpose; axisymmetric and plane strain. It is found that large normal interface stresses arise at the peaks, while large shear stresses arise at the mean line of the rough interface. These residual stresses can exceed the interfacial tensile or shear strengths. As a result, microcracks can nucleate at the bond coat/oxide interface that may promote debonding. The plane strain case was found to have about 25 percent higher stresses than the axisymmetric case. In particular, the condition for microcrack nucleation was investigated. The results show that asperity can result in stresses high enough to initiate microcracks at the TGO/bond coat interface.
