The time dependent degradation of first-stage blades for gas turbine was investigated. First, Two used blades were
prepared from the operating facilities, and their operating hours were 25,000 and 52,000 hrs, respectively.
Microstructural comparison of coating surfaces showed that exposure to the environment increased thickness of TGO
(Thermally Grown Oxide) and β depletion region, and then debonding and spallation of TBC (Thermal Barrier Coating)
occurred finally. In the case of substrate, Ni-based superalloy, the shape and size of γ' were changed with increasing
operating time. Especially, in the leading and trailing edges of blades, the size of γ' was bigger and the shape was more
elliptic than other region. It can be thought that the leading and trailing edges of blades can be the starting point of
degradation in first-stage blades. Second, TBC failed blade used for 22,000 hrs was prepared and compared with other
two blades. Large interfacial cracks were formed between top and bond coat, and TGO was not uniform and continuous.
The chemical composition analysis on bond coat showed that low chromium content is responsible for the formation not
only of Al oxide but also of Co, Cr, Ni oxides.