Abstract
The major problem in thermal barrier coatings applied to gas turbine components is the spallation of ceramic coating under thermal cycling environments. In order to study the spalling behavior and durability of functionally graded thermal barrier coatings, the well controlled burner-heating-test is conducted. Three types of FGM coatings as well as duplex coatings, each designed to have the same thermal resistance, are fabricated by air plasma spraying process on stainless steel substrates. The spallation mechanism has been discussed on the basis of the crack observation, frequency analysis of AE events and the variation of effective thermal conductivity. The sequence of spalling behavior is found to be ; orthogonal crack formation on the top surface during cooling, then transverse crack formation in the graded layer during heating, and subsequent growth of transverse cracks and their coalescence which leads eventually the ceramic coat to spall. Comparing with duplex coatings, it has been revealed that functionally graded TBCs possess the desirable effect for improvement of spallation life under cyclic thermal loads. The dependence of spallation life on composition profile in functionally graded coatings has been discussed.
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