| In this paper, a typical two-layer structural thermal barrier coating involving CoCrAlY bond coat (BC) and yttria stabilized zirconia (YSZ) ceramic top coat was prepared on the superalloy K444 matrix via electron beam physical vapor deposition(EB-PVD). Based on the results of the experiments of high temperature oxidation, hot corrosion and thermal shock, the surface phase, organizational structure and the elements diffusion rules as well as the performance of the coating during the tests were characterized by X-ray diffraction(XRD),scanning electron microscope(SEM) equipped with energy dispersive spectroscopy(EDS).The results show that the coating is totally anti-oxidation during the process of 200hrs high temperature oxidation. A dense layer of thermally grown oxide (TGO) whose main component was Al2O3 generated between bond coat and ceramic coat,and a depleted zone of aluminum appeared under the TGO after high temperature oxidation, accompanied by internal oxidation. In the first 0?120h stage,the thickness of TGO and the width of depleted zone were thickened with oxidation time, which could be up to 2.28?m (120h). Then TGO turned to be thinner, which, together with the weightlessness at the points of 50h and 160h,was related to the production of volatile oxides. The embedded volatile oxides were observed among the columnar grains. At the point of 200h, a small amount of monoclinic ZrO2 appeared. The emergence of local fragmentation and bubbling during the oxidation resulted in obvious differences in grain orientation between bulges and smooth, where the cracks began to generate and expand. During the process of 200hrs high temperature oxidation, the elements interdiffusion zone between the bond coat and matrix broadened with the extension of oxidation time. Meanwhile, the Al-rich phases distributing in the matrix connected with each other and grew up, which changed the interface morphology.The rate of hot corrosion was low in the first 60hrs and then started to accelerate. During the whole process of hot corrosion, the ceramic layer only happened to peel off and blackened near the hanging holes and edge of the sample and the ceramic layer was still in good condition. The Y2O3 of YSZ reacted with the mixed salt to form YVO4. The porosity of ceramic layer firstly decreased and then increased. Al2O3 was the main component of the TGO with its thickness being relative thin, which might be caused by the fact that the filling corrosion products hindered the penetration of oxygen. Molten salt has penetrated into the bond coat, whereas it didn't destroy the barrier layer of Al2O3 fully for the low content. TGO has played a role of preventing the further penetration of corrosive medium. The interdiffusion rule between bond coat and matrix resembled but not as evident as high temperature oxidation. Moreover, no failure phenomenon was observed in each layer and between them.In the process of thermal shock, the ceramic kept basically integrated, except for the slight flaking in extreme positions of the edge and hanging holes. With thermal shock cycles,the size and number of the bulges increased. Part of the bulges even peeled to the bond coat and oxidized to be black. Al2O3 dominating TGO formed by high temperature oxidation in thermal shock test was thickening as high as 3.1 ?m after the 100th thermal shock cycle. The Al depleted zone has expanded into the internal bond coat. The interdiffusion rule of the interface between bond coat and matrix resembled but a deeper level than high temperature oxidation, and no failure phenomenon was observed in the coating.Combined with the performance of the thermal barrier coating in the tests of high temperature oxidation, hot corrosion and thermal shock, it shows that the coating has satisfactory comprehensive properties and can be effectively applied in gas turbine engines to protect the alloy from the destruction of high temperature oxidation, hot corrosion and thermal shock. |
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