Strength Estimation and Residual Stress Analysis of Thermal Barrier Coatings by Synchrotron Radiations.

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As a bond coating, NiCoCrAlY was plasma sprayed oh the substrate of Ni super alloy and a top coating of the specimen was made by air-plasma spraying. High energy X-rays from a synchrotron source has a large penetration depth. A special furnace was made for the in-situ stress measurement in high temperature. This furnace was attached on a 4-circle goniometer at a beam line BL02B1 in SPring-8. Using high energy X-rays with 72keV, the internal stress of the bond coating was measured nondestructively through the top coating with the increase in temperature. At 1073 K or higher, the internal stress in the bond coat was released due to softening of the bond coat. The change of internal stresses in the bond coat with temperatures was explained by thermal mismatch of expansion between top and bond coats.A new hybrid method was proposed to evaluate a spalling stress. The in-plane stress in the thermal barrier coatin can be measured with laboratory X-rays, and the out-of-plane stress can be meas … More ured with high energy synchrotron X-rays. Combining these stresses obtained by the low and high energy X-rays, the distribution of the spalling stress was evaluated. To oxidize the specimen, the specimens were kept in air at 1373 K for 0,500,1000 and 2000h. The cross section of each oxidized specimen was observed with a scanning electron microscope. The thermally grown oxide(TGO) consists of the alumina layer and the composite oxide layer. The thickness of alumina layer stopped to increase after 500h exposure, while the thickness of the composite oxide layer incresed monotonically. The TGO grew at the convex part of the bond coating, and pushed up the top coating. As a result, the spalling crack was initiated near the convex part. The spalling stress for each oxidized specimen was estimated by the hybrid method using the stress data obtained by laboratory X-rays and high energy synchrotron X-rays. The top coating without the oxidization did not have the spalling stress. For the oxidized specimen, the spalling stress was small beneath the surface, and steeply increased near the interface between the top and the bond coating. The spalling stress near the interface was about 200 MPa. The distribution of the spalling stress for the case of the 1000h exposure was similar to that for the case of 500h. The TGO promotes the spallation of the top coating, and the distribution of the spalling stress corresponds to the observed position of spalling cracks. Less