Influence of Load Dependency and Coating Anisotropy on Calculated Young’s Modulus of TBCs by Indentation Test

Abstract

Thermal Barrier Coatings (TBCs) are developed as a method for increasing the operation gas temperature of gas turbines for electric power stations, and recently they are applied to 1500 � C class gas turbines. On the other hand, they have been reported that the repetition of thermal stress caused by a difference in the thermal expansion coefficient between thermal barrier ceramic layer and metallic layer in TBCs promotes coating damage. Therefore, understanding the accurate value of top-coat (TC) Young’s modulus that can estimate the extent of generated thermal stress is necessary to ensure material reliability. However, because TBCs are formed as a composite material of combined thin layer coatings, the essential problem that TBCs can’t apply to general materials estimation experiment is obvious. We have studied the estimation of TC Young’s modulus by using indentation hardness tests. In the former paper, we reported that the calculated TC Young’s modulus for TBCs specimens depends on the indentation load. Therefore, we first discuss such dependency in the present paper. As a result, we clearly show that the reason for the load dependency of the calculated TC Young’s modulus below 147 N of indentation load is the effect of under-coat (UC) and calculating TC Young’s modulus by using this procedure can eliminate the UC effect until an indentation load of 9.8 N. In addition, we examine the fracture behavior of TC following the indentation process and discuss the reason why does calculated TC Young’s modulus decrease above 147 N. We also discuss the influence of TC anisotropy on calculated TC Young’s modulus, and then we confirm that the influence of TC anisotropy rarely appears using this procedure.