| In this paper, CoCrAlY bond coat(BC) is subjected to be surface aluminized with the electric beam vacuum deposition(EBVD) and high-current pulsed electron beams(HCPEB) to improve its high temperature oxidation resistance.CoCrAlY bond coat were manufactured by air plasma spraying(APS) technique on the substrate of nickel-based superalloy GH4169. HCPEB source was used to irradiate the CoCrAlY bond coat before and after 600 nm thick aluminum film deposition with electron beam vacuum deposition(EBVD). The high temperature oxidation resistance test at 1050? were carried out to evaluate the performance of the aluminized BC. All the samples were characterized by X-ray diffraction(XRD), scanning electron microscope(SEM), energy dispersive spectrometer(EDS) and 3D laser scanning microscope(LSM). Surface microstructural characterization and high temperature oxidation resistance were investigated, and the influence of surface aluminized BC on the microstructure and the growth behavior of thermally grown oxide(TGO) was analyzed.The APS-CoCrAl Y bond coat was composed mainly of ?-CoAl, ?-Co, Al2O3 and Co-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, and the microstructure was featured by typical lamellar structures with black oxides. 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. After isothermal oxidation for 30 min, the surface was rough and loose with splash-type spherical oxides and arborization structure and contained ?-Co, ?-CoAl, ?-Al2O3, ?-Al2O3 and mixed oxides with Co and Cr.(2) a slow instantaneous oxidation stage. In this stage the mass weight increased slowly. After oxidation for 20 h, the phase of ?-CoAl and ?-Al2O3 disappeared while the content ?-Al2O3 increased. The spherical oxides and arborization structure also decreased.(3) a complicated oxidation stage. In this stage the weight of oxides increased accelerated, and TGO primarily consisted of the mixed oxides with spinels after isothermal oxidation for 200 h.After HCPEB irradiation, the coarse surface of CoCrAlY bond coat was melted apparently, and the entire surface was filled with many discrete bulged nodules with a compact appearance and a small amount of porosities and micro cracks. As the pulses increased, the bulged nodule size grew larger. Simultaneously, the BC after HCPEB treatment was composed mainly of ?-CoAl, ?-Co and Al2O3. Significantly, after HCPEB irradiation and aluminum deposition, the coat produced surface alloying with aluminum to form the aluminized BC with about 12 ?m thickness of refined layer, and Al phase was recorded. Moreover, in comparison with the coat with the same irradiation pulses, the nodules of aluminized coat were bigger and mutually connected, and the black district among the nodules primarily consisted of YAlO3 and Al2O3. Besides, hundreds of nanometers diameter bubbles were observed closely adherent to the surface of the nodules, which were ?-Co bubbles in which the element Cr was solid dissolved. According to the high temperature oxidation test of aluminized BC, the oxidation process also had a three-stage growth phenomenon. A uniform and compact ?-Al2O3 film was formed to avoid the appearance of Al-depletion after oxidation for 200 h, which indicates that the aluminized coating has a much higher oxidation resistance. |
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