FEM simulation of crack propagation and oxidation induced stresses in a TBC model system

Plasma sprayed thermal barrier coating systems are used on top of highly stressed components e.g. on gas turbine blades to protect the underlying substrate from the high surrounding temperatures. A typical coating system consists of the bond-coat (BC the thermal barrier coating (TBC. The thermally grown oxide (TGObetween the BC and the TBC develops in service as a third layer which is caused by the diffusion of oxygen through the TBC. To study the behaviour of the complex failure mechanisms in thermal barrier coatings, a simplified model system is used to reduce the number of the system parameters. The artificial system consists of a bond-coat material (fast creeping Fecralloy or slow creeping MA956as the substrate with a Y2O3 partially stabilised plasma sprayed zirconium dioxide TBC on top and a TGO between the two layers. Alongside the experimental studies a FEM model was developed to calculate the stress distribution inside the system [1]. The simulation permits the identification of compression and tension areas which are established by the growth of the oxide layer and the stresses which occur during the heating and cooling processes. Furthermore, a 2-dimensional finite element model of crack propagation in the model system was developed in which the crack direction is calculated by using short test cracks in different directions. The direction of the crack in the coating system is defined as the crack direction with the maximum energy release rate [2, 3]. The simulated stress distributions and the obtained crack path provide an insight into the possible failure mechanisms in the coating and allow to draw conclusions for optimising real thermal barrier coating systems.