A mechanistic study of oxidation-induced degradation in a plasma-sprayed thermal barrier coating system.: Part I: model formulation

The effect of the oxidation induced degradation of a typical plasma-sprayed thermal barrier coating (PS-TBC) system on the local ceramic-metal interfacial stresses responsible for the nucleation of mesoscopic cracks is investigated. A coupled oxidation-constitutive approach is proposed to describe the effect of the phase transformations caused by local internal and external oxidation processes on the constitutive behaviour of the metallic coating. The coupled constitutive framework is implemented into the finite element method and used in parametric studies employing periodic unit cell techniques. The effects of service, microstructural and ceramic-metal interface parameters on the peak interfacial stresses during service and cooling to room temperature are quantified. The results of the parametric unit cell FE analyses revealed a strong dependency of the local stresses responsible for mesoscopic crack nucleation and growth on the local morphology of the oxidised interface, the sintering of the ceramic coating, stress relaxation effects due to creep, the thickness of the thermally grown oxide (TGO), and the applied mechanical loads. When no mechanical straining of the TBC system is considered, local tensile stresses normal to the coating surface within the ceramic top coating reach values of up to 330 MPa at room temperature for a critical TGO thickness of approx. 3 μm.