Evaluation Method of High Temperature Nonlinear Constitutive Equation of Thermal Barrier Coating Based on in Situ Curvature Measurement during Thermal Cycling

Thermal barrier coatings (TBCs) are important in improving the efficiency of gas turbine engines. TBCs are generally produced by atmospheric plasma spraying, resulting in a lamellar microstructure with pores and cracks. The stress-strain relations at high temperatures are one of the most important mechanical properties for the stress analysis of TBC. The previous studies by curvature measurement during thermal cycling revealed that TBC shows in-plane nonlinear behavior attributed to the unique microstructure. However, there has been little discussion on temperature-dependence of non-linear stress-strain relationship. In this study, we propose a new method to determine the nonlinear stress-strain relation at each temperature. This method evaluates the stress-strain responses based on the temperature- and stress-dependence of coating secant modulus of elasticity, where the nonlinearity was quantified by the stress-dependent decrease rate of secant modulus. The experiments were made on two-layered specimens, consisting of ceramic top coats and Ni-based superalloy substrate, with the different ratios of coating thickness to substrate thickness. The temperature range of thermal cycling was between room temperature and 915℃. Specimens in as-sprayed and heat-treated at 1000℃ conditions were tested to investigate the effects of sintering. It was found from the results of as-sprayed specimens that the nonlinearity of stress-strain relation decreased above 600℃ due to sintering densification. For heat-treated specimens, the nonlinearity of stress-strain relation became high with rising temperature.