Evaluation Method for Inelastic Constitutive Equation of Thermal Sprayed Ceramic Coating by Bending Strain of Coating System

Thermal sprayed ceramic coating system consists of top coat (TC), bond coat (BC), and substrate. Mechanical properties of the TC differ from those of bulk ceramic due to micro cracks and voids in the coating. A stress-strain constitutive equation of TC is one of the most important mechanical properties for stress analysis of ceramic coating system. However, in the case of the evaluation methods using a freestanding coating, the specimen breaks at a quite low strain because of rapid crack propagation. On the other hand, using a coating system specimen, the substrate prevents the specimen from rapid fracture, hence constitutive equation up to high strain is obtainable. All the previous evaluation methods using a coating system specimen are based on bending deflection, however, high accuracy is required for the evaluation of porous TBC. In this study, highly accurate evaluation method for inelastic constitutive equation using bending strain of the coating in thermal sprayed coating system was proposed. This method can evaluate the stress-strain constitutive equation of coating by calculating the coating secant modulus of elasticity from bending strain at each load. First, it was confirmed by FEM analysis that the proposed method was able to provide the accurate inelastic constitutive equation. Thereafter, we experimentally evaluated the inelastic stress-strain constitutive equations of ceramic coatings. As a result, mechanism of non-linearity of ceramic coating was able to be explained by the static friction and micro crack opening / closure, which are specific to thermal sprayed ceramic coatings.