Influence of High-Temperature Exposure on Interfacial Fracture Toughness of Thermal Barrier Coating

Abstract

In thermal barrier coating (TBC) sprayed on the surface of high-temperature components in a land-based gas turbine (GT), stress is induced thermally or mechanically in the coating with the start-steady-stop operation of the GT. The coating stress leads to a crack initiation at the coating interface, and consequentially to TBC spallation. Thus, it is necessary to ascertain TBC interfacial strength, in order to assess precisely TBC spallation life. The aim of this study is to present the interfacial fracture toughness of TBC by using a TBC specimen exposed under a high-temperature environment. For accomplishing this, the fracture test is conducted using a mixed-mode interfacial fracture test device that has been proposed by our group. Thermal grown oxide (TGO) formation at the interface and residual stress of TBC, which are known as the origins of a reduction of the interfacial strength of TBC, also are examined. The intrinsic interfacial fracture toughness of TBC, which was estimated with the residual stress energy stored in TBC by thermal spray process, is related with cohesion of splat boundary and pore characterizing microstructure of TBC, in order to make clear why the TBC interfacial fracture toughness varied with exposure condition.

A physics-based model of temperature-exposure-dependent interfacial fracture toughness of thermal barrier coatings

“…In this paper, the interfacial fracture toughness measured by Arai et al [30] at 970 K and 1373 K were used to fit parameters in our proposed physics-based interfacial toughness model at elevated temperatures. In Arai et al paper [30], the ceramics topcoat is conventional 8 wt % YSZ, the CoNiCrAlY as the bond coat and Co-based superalloys as the substrate. The thickness of the bond coat is 100 μm, and 1.0 mm for the topcoat and 1.0 mm for the substrate.…”

“…Based on this Figure 1. Schematic illustration of the mixed-mode interfacial fracture testing method using a combination of compressive load applied by edge-flat and slingling load by thin tape hooked in a coating beam [30].…”

“…Based on this specifically designed pre-crack distributed in the TBC system of figure 1, the energy release rate for this specific mixed-mode crack tip was derived in [30],…”

“…During high temperature exposure of TBC systems (∼1000°C), the ceramic YSZ topcoat experiences significant sintering, leading to an increase of topcoat Young's modulus E. This increase in Young's modulus affects considerably fracture toughness, failure behavior and the life of TBCs. Arai et al [30] evaluate experimentally the change of Young's moduli E due to the top coat sintering at both 973 K and 1073 K, and obtained curves of Young's moduli E versus exposure time depicted in figure 3. It is observed that E initially drops within the first 100 h exposure, and then increases gradually.…”

“…Arai et al [30] established a mixed-mode interfacial fracture toughness measurement method by using a combination of the compressive and slinging loads enforced to the coating beam, figure 1. They showed that TBC fracture toughness can be measured in a wide range of loading phase angles ahead of the interfacial crack tip.…”