Abstract
A large penetration depth of high energy X-rays makes it possible to measure nondestructively the residual stress in the interior of the top coating in the thermal barrier coating (TBC). A new hybrid method is proposed to estimate the distribution of the spalling stress in the top coating by combining the synchrotron data with the stress data measured by the conventional X-ray method utilizing a Cr-K_α radiation. The new hybrid method was applied to estimate the distribution of the spalling stress in the top coating of TBC which had a zirconia top coating with a thickness of 0.24 mm and a NiCoCrAlY bond coating with a thickness of 0.2 mm. The residual stress, σ_<11>-σ_<33>, within the top coating was determined by synchrotron X-rays of 73keV energy level, where σ_<33> was the stress perpendicular to the surface and σ_<11> was an inplane stress. The distribution of residual in-plane stresses, σ_<11> and σ_<22>, in the top and the bond coating was determined with the conventional X-ray method by repeating the measurement after successive removal of the surface layer. From the data obtained by synchrotron and conventional X-rays, the distribution of stress component, σ_<33>, responsible for spalling was determined. The estimated value of the spalling stress was very small beneath the surface and increased to about 75 MPa near the interface between the top and the bond coating.
