Development of a micromechanical life prediction model for plasma sprayed thermal barrier coatings

A widely used method to produce thermal barrier coating (TBC) systems is the vacuum plasma spraying of a highly dense bondcoat layer with a defined surface roughness and the atmospheric plasma spraying (APS) of a porous (10–15%) Y2O3-stabilized zirconia top coat. In thermal cycling operation these systems often fail by crack initiation and propagation close to the bondcoat–top coat interface. This failure is attributed to stresses arising from the formation of a thermally grown oxide (TGO) layer on the rough bondcoat surface. The actual stress situation is rather complex due to TGO formation, creep effects in both bondcoat and top coat and due to the roughness of the bondcoat. All these factors have been take into account in the present work by using a finite element method (FEM) to calculate stress development during thermal loading. These results can then be introduced into a crack propagation model to estimate crack development during the thermal cycling operation. The predictions of this approach are compared to experimental results on the influence of bondcoat roughness on coating life. In these experiments TBC systems with bondcoat layers having three different levels of roughness were cycled in a gas burner rig until failure.