High Sensitive NDE of Subsurface Local Damage in Thermal Barrier Coating System by the Laser Speckle Method

Thermal barrier coatings (TBCs) have been used for insulating a substrate from high temperature in gas turbine plants. High temperature fatigue strength has been an important problem for these components. However, nondestructive evaluation (NDE) of subsurface delamination for a thermal-barrier-coated material has been so difficult that the effect of delamination on the fatigue fracture life of a substrate hasn't been well studied. The relationship between surface strain behavior and the state of any subsurface delamination for a TBC material is described in this paper. Tension-compression fatigue tests were carried out at 873K and room temperature for ZrO₂-8Y₂O₃-sprayed and Al₂O₃-sprayed specimens. Surface strain behavior during the fatigue test was measured using the laser speckle method. It was found that surface strain was fully reflected by substrate strain unless subsurface delamination was initiated, while surface strain was found to decrease when subsurface delamination was initiated. All the surface strain behaviors were classified in five patterns, and they were related to the state of subsurface delamination. It was concluded that the surface strain beside a crack was sensitive to the subsurface damage and applicable for evaluating subsurface damage