Oxidization of Thermal Barrier Coatings and Spalling Stress Analyzed with X-Rays
Kenji SUZUKI, Takahiro KUBO, Keisuke TANAKA, Yoshiaki AKINIWA and Hideki OKADO
Abstract:As the bond coating, NiCoCrAlY powder was atmospheric plasma-sprayed on the Ni based super-alloy (In738LC), and the thickness of the bond coating was 0.15mm. Zirconia powder with 8 mass% yttria was atmospheric plasma-sprayed as the top coating, and the thickness was 0.3mm. To oxidize the specimen, the specimens were kept in air at 1373 K for 0, 500, 1000 and 2000 h. 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 500 h 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 1000 h exposure was similar to that for the case of 500 h. The TGO promotes the spallation of the top coating, and the distribution of the spalling stress corresponds to the observed position of spalling cracks. Key Words:Thermal barrier coating, Spalling stress, Synchrotron radiation, Thermally grown oxide, Residual stress