Influence of the TGO creep behavior on delamination stress development in thermal barrier coating systems

The lifetime of ceramic thermal barrier coatings (TBCs) is significantly affected by the growth of a dense alumina scale at the interface between TBC and the bond coat (BC), the so-called thermally grown oxide (TGO). Occurring growth stresses and thermal expansion mismatch can lead to system failure by spalling and delamination. The evolving stress state depends crucially on BC oxidation kinetics, TGO creep behavior and thermal–mechanical properties of the coating materials. Using a finite element model of a TBC system, the influence of TGO creep on the stress development was studied. The performed calculations consider the effect of TGO grain size, the roughness of the TBC/bond coat interface as well as time and temperature-dependent oxide growth. A high temperature induced sintering of the TBC is also taken into account. The obtained results show an oxidation induced development of compressive stresses above the peaks and of tensile stresses in the valleys of the rough TBC/BC interface. The results indicate that TGO creep can cause a significant stress relaxation above 1050 °C for grain sizes of 0.1 μm. At 1100 °C creep stress relaxation can already be expected for a grain size of 0.3 μm.