Thermal barrier coatings (TBCs) have been utilized for more than four decades to increase the efficiency and durability of aircraft engines as well as land based gas turbines. Though the function of the thin TBC layer is heat insulation, it is critically important for it to maintain adherence in service. In this paper, we address some fundamental aspects of TBC debonding failure in electron beam physical vapor deposition (EB-PVD) TBC systems that had been considered in a previous study. We demonstrate that the energy release rate formulation for EB-PVD TBC systems based on as-processed conditions and a thin thermally grown oxide (TGO) layer overestimates the energy release rate for exposed TBC systems having thicker TGO layers. A modified formulation is given, applicable to exposed TBC systems. A finite element contact and fracture model is used to validate the formulation as well as study crack mode mixity. Mode II interfacial cracking is shown to be dominant for practical TGO thicknesses seen in exposed EB-PVD TBC systems.

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