Oxidation Behavior Of Plasma Sprayed Thermal Barrier Coatings

With the development of aerospace industry, the efficiency of the gas turbine has been improved by increasing inlet temperature. However, the superalloys and cooling technique available cannot satisfy this need. Therefore, plasma sprayed thermal barrier coatings (TBCs) are applied to increase heat resistance of the superalloys. Although the TBCs have been used for a long time, their oxidation resistance is still the main problem to be solved. In this paper, the oxidation resistance of three kinds of plasma sprayed TBCs has been studied. Isothermal oxidation weight gains and the thicknesses of TGO were measured to investigate the oxidation thermodynamics and kinetics. A scanning electron microscope was used to examine microstructures. X-ray diffraction analysis was carried out to determine phases of coatings and electron probe microanalysis was conducted to investigate elements distribution.First, isothermal oxidation test was conducted using two kinds of ZrO₂ thermal barrier coatings stabilized by MgO and Y2O3, respectively. It was found that parabolic law could be used to describe oxidation kinetics for the two kinds of coatings. With increasing the oxidation temperature, porosity of ceramic layer decreases and monoclinic phases increase. Although the bond coat for both the coatings is NiCrAlY, Y2O3 stabilized ZrO₂ thermal barrier coatings display better oxidation resistance due to more uniform distribution of Al elements.A low-pressure plasma spraying coated bond coat TBC system was also tested at 800-1000 ℃. The results showed that oxidation resistance of this kind of TBCs was much improved. The Al element diffused from bond coat to the interfaces of Top coat/bond coat after oxidation for a long time at higher temperature and generated the two layers of Al₂O₃.Based on the experimental results, it has been proposed that the oxidation process of TBCs includes six stages: absorption of oxygen, diffusion of oxygen in the top coat, the scale of Al₂O₃ formed by selective oxidation, the growth of thin scale, the growth of thick scale and the failure of thick scale. The distribution of Al element has effect on the growth of scale and then affects oxidation resistance of thermal barrier coatings.