Thermal Shock Fracture Mechanism of Thermal Barrier Coatings for Gas Turbine Blades

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In high temperature components of gas turbine engines which include combustor liners, turbine blades and after-burners, thermal barrier coatings (TBCs) are being employed for improved performance, efficiency and extended component life by reducing turbine cooling air while keeping sufficiently low metal temperatures.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-method has been developed and evaluation process of number of cycles to spall has been proposed.In this research the cyclic thermal fracture behavior and the evaluation of spallation life of PSZ/NiCrAlY functionally graded thermal barrier coatings has been investigated. Three types of FGM coating and conventional duplex coating were fabricated to demonstrate the marked advantages of … More functionally graded structure on the improvement of spallation life of the TBCs under cyclic thermal loads. The effect of compositional profile in the FGM coatings on spallation life has been discussed.The sequence of spalling behavior has been 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. It is shown that the spallation life of the coating can be defined as the critical number of thermal cycle, evaluated from either the rapid increase of AE hits during heating or the sudden drop of the effective thermal conductivity. 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. In addition, the dependence of spallation life on the composition profile in functionally graded coatings has been shown. Less