Coupled effects of temperature gradient and oxidation on thermal stress in thermal barrier coating system

The thermal stress fields in thermal barrier coating (TBC) system are studied in the present paper. The thermal stress fields are induced by the non-linear coupled effect of temperature gradient, oxidation, thermal fatigue, creep, morphology of TBC system as well as cooling rate. TBC system is assumed to be partially stabilized ZrO₂ by 8 wt.% Y₂O₃ (PSZ) or mullite over a NiCrAlY bond coat sprayed on nickel superalloy or steel substrate. The TBC system is a composite medium with four layers in cylindrical coordinate system. The temperature fields for the non-homogeneous problem with energy generation in medium are analytical solved by using Taylor transformation and Green's function approach. The analytical solutions for thermal stress fields in composite medium are obtained when eigenstrain rate is taken into consideration. The constitutive equations, such as the creep of ceramic coating (PSZ and mullite) and substrate (Ni-superalloy), plasticity of bond coat are given by a general formula. Thermal growth oxidation (TGO) and the temperature dependence of thermal-mechanical parameters are taken into consideration. The calculated results of temperature fields and thermal stresses fields are given and the related results are discussed. TGO does not affect the temperature fields in PSZ coating systems. But it has influence on temperature fields in mullite coating systems. The residual stress with TGO considered is larger than that with TGO non-considered. It is very interesting to have the conclusion that TGO may make the residual tangent stress from tensile to compressive. The characterization of thermal stresses in PSZ coating system is very different from that in mullite coating system. It may be due to the difference of mechanical behavior such as creep, thermal mismatch as well as mechanical mismatch which is reflected by Dundurs' parameters a and b. The geometrical radius not only affects the quantum of thermal stress but also affects the characterization of thermal stress. The effect of cooling rate on residual stress is due to the high creep rate of ceramic coating operating at high temperature. The effect of cooling rate on residual stress is not large for system operating at relative low temperature.