The Effect of Conical Indenter Shape on TBC Interfacial Toughness Measurements

Abstract

Thermal barrier coatings (TBCs) are thin ceramic coatings used to insulate gas turbine hot section components. The degradation of interfacial fracture toughness between the coating and the metallic substrate is a key concern for TBC systems. Previous research by the authors has explored the use of conical (Brale) indentation to measure the interfacial fracture toughness of electron beam physical vapor deposition (EB-PVD) TBCs. However, indentation using a standard 120° Brale indenter can fail to yield an ideal debond size for accurate measurement for poorly adhered coatings subjected to long-term thermal exposures, or for as-processed coatings that are well-adhered to the metallic substrate. These limitations of existing conical indentation tests have lead to the study of different shapes of indenters to obtain optimal debond sizes. In this paper, the differences in measuring the interfacial fracture toughness in thermal barrier coating systems due to indentation by rigid cones with various tip angles, namely, 60°, 90°, 120°, and 150° are addressed. Interfacial stress intensity factor distributions, i.e., curves of the interfacial stress intensity factor (K) vs. normalized radial distance (R/a), are obtained through numerical simulations coupled with thin film fracture mechanics relations. Results from experimental studies on an exposed EB-PVD TBC specimen are presented. The goal of this work is to obtain larger debond sizes for high toughness specimens and smaller debond sizes for low toughness specimens, while maintaining an adequate indentation depth.