Development and implementation of a viscoplastic model for air plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are widely used in gas turbines to reduce the temperature in hot components, which allow a combination of increased part life and improved system performance. Experiments have shown that plasma sprayed ceramic coatings exhibit large tension-compression flow stress asymmetry and rate-dependent deformation behaviors. While in recent years there has been extensive activity in the analytical and numerical simulation of TBC behaviors, none of the research has focused on the realistic behavior of TBCs mentioned above. Instead, simple elastic and a few elastic-plastic models have been used. The purpose of this study is to investigate and implement a realistic viscoplastic model for a plasma sprayed ceramic TBC. Temperature-dependent material constants in the model are found for a specific material using a novel approach based on genetic algorithm. A finite element simulation of a plasma sprayed TBC system represented as a three-layer structure with an interface asperity is presented. The validity of results obtained using simpler and more convenient nonlinear constitutive models for the TBC is assessed by comparison. It is shown that elastic residual stresses at room temperature never become large enough to propagate asperity size scale cracks based on linear elastic fracture mechanics. Failure may need to be described in terms of volumetric damage associated with exceeding the limit stress of the TBC.