Thickness effects on the delamination energy release rate of thermal barrier coating subjected to thermo-mechanical loads

Abstract

The delamination energy release rate of the thermal barrier coating (TBC) under steady thermal gradient coupled with mechanical loads is theoretically investigated. We first derive a set of integral equations to calculate the energy release rate and mode mixity phase angle of the bilayer TBC with thermo-mechanical loads. Then from the cut and paste procedure, a closed form energy release rate solution is obtained for the thermal gradient case, which is demonstrated to be identical to the numerical results for the coat/substrate thickness ratio less than 0.4. As the coat/substrate thickness ratio increases, when the substrate thickness is fixed, the energy release rate increases from zero except for a drop corresponding to the transition from crack model I to model II. When the film thickness is fixed, the energy release rate decreases from the isothermal result to a minimum point and then increases to a maximum point and then decreases. The mechanical loads are shown to reduce the energy release rate. A new model transition point emerges for larger thickness ratio due to the fact that the compression of the coating is relieved by the mechanical loads. The effects of the stress free reference temperature, Young's modulus, Poisson’s ratio, thermal conductivity and thermal expansion coefficient on the energy release rate are also disclosed. The obtained results can shed considerable insights for the assessment and design of TBCs and other layered structures subjected to thermo-mechanical loads.