Relationship between Interfacial and Coating Fracture Toughnesses of Thermal Barrier Coating

A thermal barrier coating (TBC) system consists of a substrate, a bond-coat (BC), and a top-coat (TC: TBC). The coatings are deposited by thermal spraying. The interfacial fracture toughness between TBC and BC is important mechanical property because it is necessary for damage assessment. The TBC system suffers from thermal exposure, and the interfacial fracture toughness is changed by the thermal exposure as well as the TBC toughness. However, there are few studies on the relationship between the interfacial and TBC toughnesses due to the difficulty of accurate comparison. The purpose of this study is to investigate the relationship between interfacial and TBC toughnesses. For this purpose, we evaluated the two toughnesses of thermally exposed TBC specimens using an indentation technique with an identical specimen and identical evaluation theory. Our results revealed that the interfacial fracture toughness was higher than the TBC fracture toughness for both the coatings as-sprayed and exposed at a moderate temperature. On the other hand, the TBC toughness became to be close to the interfacial toughness for the coatings exposed at high temperatures. Additionally, the convergence rate became to be high by higher treatment temperature. The interfacial cracks occurred not at the strict interface but at the TBC near the interface. These results suggested that interfacial toughness was essentially identical to coating toughness and the changes in the two toughnesses were caused by sintering of the TBC.