Size effects on damage evolution of ceramic coatings under thermal loading

Ceramic coatings are widely used in the thermal protection of hot section components of aero-engines, such as turbine blades, turbine casings and combustion chambers. The damage and failure of ceramic coatings have a direct impact on the stability and reliability of hot section components while in operation, and thus have attracted extensive attention. Especially, the size effect of damage of ceramic coatings under thermal loading has to be investigated. This study employs numerical simulation methods to explore the size effect of damage evolution in ceramic coatings with a series of thicknesses under thermal loading. The simulation results show that the transverse cracks, perpendicular to the interface between the coating and the substrate, first initiate inside the ceramic coating, and then propagate toward the outer surface of the ceramic coating and the interface. As the thickness of ceramic coatings increases, the number of transverse cracks decreases, and the interfacial delamination becomes more pronounced. By combining the simulation results of the crack evolution with the mathematical damage model, the relationship between the thickness of the ceramic coating and the damage rate under thermal load is studied. The results indicate that the damage follows a power-law characteristic with an exponent of 0.5, and the damage rate increases with the enhancement of coating thickness. Moreover, the influences of variations in coatings' elastic modulus and the difference in thermal expansion coefficients between the coating and substrate on the initial damage temperature of the coatings are also studied. The results show that doubling the elastic modulus of coatings leads to an average reduction of 43.7 % in the initial damage temperature of the coatings. When the difference in thermal expansion coefficient between the coating and the substrate increases 1.5 times, the average initial damage temperature of the coating reduces 22.3 %.