| The preparation of thermal barrier coatings (TBCs), which have better resistance to high temperature oxidation and heat barrier properties, has been extensively applied to improve the efficiency and durability of hot section components in aviation turbine engine. Thermally grown oxide (TGO) is the main factor in the failure of TBCs due to its uncontrolled growth during thermal exposure in air, which is the critical technical problem in the design of advanced aero-engine. In order to improve the service performance of TBCs at high temperature, this project is devoted to present a prospective route to control the growth and evolution behaviors in the TBCs-TGO treated by high-current pulsed electron beams (HCPEB).In this paper, two-layered TBCs were manufactured by air plasma spraying (APS) technique on the substrate of nickel-based superalloy GH4169, using CoCrAlY bond coat (BC) powder and ZrO2-8wt%Y2O3 (YSZ) top coat (TC) powder. HCPEB source was used to irradiate the MCrAlY bond coat and YSZ top coat, respectively. The high temperature oxidation resistance test, thermal cycling test and tensile strength test were carried out to evaluate the performance of TBCs. All the samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM),3D laser scanning microscope (LSM), atomic force microscopy (AFM), transmission electron microscopy (TEM) and micro-Raman spectroscopy system (MRS). The influence between the microstructures and HCPEB irradiation was investigated, and the relationship between the microstructures and the performance of TBCs was also established. This research can present some necessary theoretical and experimental storage for revealing the failure mechanism and improving the service life of TBCs under the surface treatment condition.The APS-MCrAlY bond coat was featured by typical lamellar structures with many layered "composites" made out of alloy splats surrounded by oxides. The top surface was apparently rough and consisted of a number of structural defects like large cavities, non-molten or re-solidified particles and oxide inclusions. The results of high temperature oxidation test at 1050℃ reveal that the high temperature oxidation process had a three-stage growth phenomenon:(1) an instantaneous oxidation stage. The oxidation kinetic curve in this stage obeyed linear regularity, which means the TGO had an obvious thickening. (2) a slow instantaneous oxidation stage. In this stage TGO was comprised of two distinct layers:mixed oxides layer and α-Al2O3 layer, which showed a straight-line growth with a small slope. (3) an accelerated oxidation stage. In this stage TGO primarily consisted of the mixed oxides, which showed a straight-line growth with a big slope. After isothermal oxidation for 200h, the TGO thickness was 11.12μm. Besides, extensive cracks, which were propagated in the direction parallel to the interface, were formed in mixed oxides. The results of thermal cycling test show that the spalling area of APS-TBCs after 200 cycles accounted for around 34% of its total area, and the average residual stress inside the TGO reached up to 1.28GPa after 150 cycles.After HCPEB irradiation, the coarse surface of MCrAlY bond coat was melted apparently, and the entire surface was filled with many mutually connected bulged nodules with a compact appearance. Simultaneously, pores and large cavities were ultimately sealed on the irradiated surface. Columnar grains with size approximately 1～3μm in the perpendicular direction from the surface inside the bulged nodules were formed. Besides, nano-grains in various forms and deformed structures like dislocations, stacking faults, twins and etc. were obtained. The numerical simulation results of the temperature field depict that both melting and evaporation modes led to forming these special morphologies. According to the high temperature oxidation test, after HCPEB irradiation the treated coating was consisted mainly of a mono-layered TGO, whose growth obeyed Wagner’s theory on selective oxidation. The growth of TGO also appeared to have a three-stage growth phenomenon. Once the uniform and compact α-Al2O3 scale was formed at the onset of oxidation associated with the irradiated effect, the growth rate of oxide scale at the interface would be reduced. The average thickness of the TGO in HCPEB-TBCs reached a value close to 4.41μm after oxidation for 200h, which indicates that the irradiated coatings have a much higher oxidation resistance. The results of the tensile test reveal that the adhesion strength of thermally sprayed TBCs with the bond coats conducted by HCPEB treatment was increased obviously. The results of thermal cycling test show that the spalling area of APS-TBCs after 200 cycles accounted for only 1% of its total area, and the average residual stress inside the TGO reached up to 860MPa after 150 cycles, which means the residual stress inside the TGO was released effectively after HCPEB irradiation.Thermally sprayed YSZ coatings subjected to HCPEB with different pulses were investigated. Microstructure observations reveal that the coarse surface was melted and a flat and compact melted layer was formed accompanied by removal of all the inherent shortcomings existing in the APS coatings. The irradiated surface presented a significant reduction of the surface roughness. At the same time, rapid thermal cycles during HCPEB irradiation led to the very fine grains formed on the surface and the fully dense columnar grains in the fracture cross-section. Besides, a continuous micro-crack network oriented along the electron beam direction was generated throughout the whole melted layer due to the shrinkage and relation of thermal stress during the HCPEB irradiated process. The cracks developed in a much broader way with the increase of HCPEB pulses. The formation of columnar grains and vertical cracks is conducive to improving the strain tolerance of ceramic coating. XRD results demonstrate that APS-YSZ consisted of the tetragonal phase of t’-ZrO2 and a small amount of residual monoclinic phase of m-ZrO2. By contrast, the irradiated coating led to the almost disappearance of the residual m-phase due to the homogenization of composition during the HCPEB irradiated process. After oxidation for 200h, TGO layers on both coatings were comprised of two distinct layers. However, the thickness of the TGO layer of the HCPEB-irradiated coating was much thinner than that of the initial coating. |
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