Font Size: 

Thermal barrier coatings are used to protect turbine blades from the hightemperature of the process gas inside a turbine. They consist of a bond coat which protectsthe substrate from corrosion and of a ceramic top coat with low thermal conductivity.During service, an additional oxide layer forms between bond coat and top coat. Experimentally,it is known that these layers fail by spallation when the oxide layer exceeds acritical thickness of about 10 micron. Finite element simulations show that the roughnessof the interface between top and bond coat is crucial for determining the stress state.Lifetime models have been inferred that assume that cracks form in the peak positions atsmall oxide thickness and propagate when the oxide layer grows.In this contribution, a two-dimensional finite element model of crack propagation inthis system is presented. Since the cracks propagate near a material interface, standardtools of fracture mechanics for predicting the crack propagation direction are difficult toapply. This problem is circumvented by propagating short “test cracks” in different directionsand optimising to find the crack direction with the maximum energy release rate.It is shown that the energy release rate of cracks initiated at the peak position stronglydepends on the creep properties of the TBC and the TGO. For creep-soft materials, the energyrelease rate is small, but increases during crack growth. Implications for the lifetimeof TBCs are discussed.