Thermal barrier coatings serve as thermal insulation and antioxidants on the surfaces of hot components. Different from the frequent thermal cycles of aero-engines, a heavy-duty gas turbine experiences few thermal cycles and continuously operates with high-temperature gas over 8000 h. Correspondingly, their failure mechanisms are different. The long-term failure mechanisms of the thermal barrier coatings in heavy-duty gas turbines are much more important. In this work, two long-term failure mechanisms are reviewed, i.e., oxidation and diffusion. It is illustrated that the growth of a uniform mixed oxide layer and element diffusion in thermal barrier coatings are responsible for the changes in mechanical performance and failures. Moreover, the oxidation of bond coat and the interdiffusion of alloy elements can affect the distribution of elements in thermal barrier coatings and then change the phase component. In addition, according to the results, it is suggested that suppressing the growth rate of uniform mixed oxide and oxygen diffusion can further prolong the service life of thermal barrier coatings.

A film/substrate system is a common structural form. In its fabrication and/or operation process, diffusion is a basic and key procedure. However, there still exist unclear points in the diffusion process, i.e., the effects of stress, creep, and interface properties. To clarify these unclear points, in this paper, a coupled diffusion model including stress, creep, and interface property is established. The obtained results indicate that compressive stress retards the diffusion of guest atoms. Meanwhile, creep reduces the retardation of diffusion through releasing the induced compressive stress, and then the concentration of the guest atoms can reach the prescribed value, which overcomes the much lower concentration predicted by the previous models without creep. In addition, interfacial diffusivity affects diffusion and the maximum stress in the film.

In the presence of temperature gradient, the non-uniform redistribution of solute atom usually occurs in an initially uniform solid, e.g. the segregation of alloy element in the gas turbine blade, and this phenomenon is called as Ludwig-Soret effect. The redistribution process of solute atom accompanies a significant stress in solids, which was not taken into account in previous theories. In this paper, in the framework of the irreversible thermodynamics, we establish a constitutive model of temperature gradient driving diffusion. The redistribution process of solute atom and the effect of stress are investigated, respectively. The results reveal that our established model describes the redistribution law of solute atom and clarifies the implied mechanism. And the redistribution of solute atom firstly starts at the hot end and gradually extends towards the cold end. Stress retards the diffusion driven by the temperature gradient and then reduces the non-uniform redistribution of solute atom. Moreover, the obtained quantitative relationship predicts the direction and magnitude of the formed concentration gradient. According to our results, researchers can optimize the distribution of element and evaluate the service performance of engineering materials.