13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- h l a) b) c) TC BC Figure 6. Comparison of interface microstructure / morphology in actual thermal barrier coatings. TC BC BM TGO a) b) c) Figure 7. Crack patterns observed in thermal barrier coatings. (a) black interface crack, (b)ceramic white crack and (c) mixed black/white crack with the crack partly at the interface, partly in the top coat. b l h + + d TC BC Figure 8. Definition of measurement on deflected kink crack. Different crack types can theoretically be observed in the ceramic / metal material system. The three typical crack patterns are shown in Error! Reference source not found.. The different crack patterns (a), (b) and (c) are identified as black TC/BC interface cracks, white ceramic TC cracks and mixed kink cracks with portions of the crack both at the TC / BC interface as well as portions in the ceramic TC respectively. Cracks of type (c) are contained inside the TGO layer especially if the oxide is formed on an alloy with relatively low Al concentration. In such cases, the oxide is often observed to contain many defects and can be described as layered. During thermal fatigue exposure of the coatings, damages of type (a) are common. Also damage of type (c) is frequently observed after furnace testing and engine exposure. Damage of type (b) is not frequently occurring. This damage pattern is more related to high heat flux situations in burner test rigs and corresponding engine situations with high thermal loads A schematic picture of the measurements taken is shown in Error! Reference source not found.. The measure d is defined as the distance from a ridge to the location where a crack deflects away from the interface into the top coat projected on a plane parallel to the interface (and the TBC outer surface). Also the distance vertically (perpendicular) from the ridge to the location of crack deflection is measured, in Error! Reference source not found. defined as b. At the location of crack deflection from interface to TC crack the curve shape tangent dy/dx, or curve shape first derivative, is measured (a). On the corresponding model, the curvature has been described by a tangent as shown in (b) and (c). The tangent (b) is the slope at the inflexion point
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