Mechanism of transient thermal cycle failure in ZrO₂ /NiCrAl functionally graded thermal barrier coatings

In this paper, the failure mechanism under transient thermal cycle loading is studied for a functionally graded thermal barrier coating (FGTBC) system consisting of ZrO₂ and NiCrAl plasma sprayed on a steel substrate. Controlled experiments and numerical analysis (thermoelastic finite element method) were performed and the results indicate that the surface tensile stress is responsible for the initiation of surface cracks and the tensile stress normal to the interface is large enough to break the interfacial bond (the magnitude of the interfacial bond strength of the ZrO₂ layer in the FGTBC is below the strength of the ZrO₂, i.e. the interface is weaker than the ZrO₂ material). By comparing the mode I stress intensity factor and the fracture toughness, the surface cracks are considered to be arrested, this characteristic is confirmed by SEM studies. The interfacial crack propagation causes a segment of the ZrO₂ layer in the FGTBC to spall.