Microstructure And Thermal Cycling Behavior Of Thermal Barrier Coatings Irradiated By High-current Pulsed Electron Beam

Thermal barrier coatings?TBCs?have been extensively applied to important hot section components in gas turbine engines as thermal protective coatings.The service life of TBCs is directly related to the safety performance of gas turbines.The thermal cycling behavior is an important performance index that determines the service life of TBCs.Because the current inlet temperature of the gas turbine is getting higher,of course higher requirements are also put forward for improving the thermal cycling resistance of TBCs.In this paper,ceramic top coat?ZrO₂-8wt%Y₂O₃,YSZ?prepared by air plasma sprayed?APS?technology was irradiated by high-current pulsed electron beam?HCPEB?.The research can realize the improvement of thermal cycling performance of TBCs by purposely regulating the YSZ topcoat structures at micro level.The thermal cycling behavior of TBCs at different heating temperatures before and after HCPEB treatment was tested by alternate heating in the furnace and then coolingin the water.X-ray diffraction?XRD?,scanning electron microscopy?SEM?and other analysis methods were used to analyze the microstructures and phase structures of the YSZ topcoat before and after HCPEB irradiation and thermal cycling test.The effect of HCPEB irradiation on the microstructures of YSZ coating as well as the final effect on the thermal cycling behavior of TBCs was analyzed.The YSZ topcoat prepared by APS technology?APS-YSZ?has a typical lamellar structure.The top surface was was apparently rough and consisted of many unmelted or semi-melted particles,accompanied by a large number of pores and cracks.The phase composition of the APS-YSZ ceramic layer at room temperature consisted of the tetragonal phase of t?-ZrO₂ and a small amount of monoclinic phase of m-ZrO₂.Microstructure observations of HCPEB-YSZ topcoat showed that after HCPEB irradiation,the coating surface was remelted accompanied by the disappearance of thermal sprayed defects,and the surface roughness was reduced.Besides,as the number of irradiation was increased,the surface roughness was gradually decreased,and the thickness of the remelted layer was gradually increased.The remelted layer was mainly composed of refined equiaxed grains on the surface and dense columnar grains in the fracture cross-section.Besides,a continuous micro-crack network oriented along the electron beam direction was generated throughout the the entire remelted layer.The residual m-phase in the irradiated YSZ ceramic layer almost disappearred,and the surface generated high amplitude of compressive stress.The results of thermal insulation test showed that the thermal conductivity of TBCs was reduced from the original value of 20.528 W/?m·K?to that of 16.861W/?m·K?after HCPEB irradiation,indicating that the thermal insulation performance of TBCs after HCPEB irradiation was significantly improved.The results of thermal cycling test at 950?showed that after 250 cycles,overall spalling occurred within the APS-TBCs,and the spalling site showed a gradient separation interface,meaning that the coating has failed.In comparison,after 450 cycles of HCPEB-TBCs,the spalling area accounted for less than 1%,indicating that the thermal cycle performance of TBCs after HCPEB treatment was greatly improved.The failure of coating at 950?mainly occurred inside the YSZ topcoat,and the thermal stress generated during the thermal cycling was the main cause of the failure of TBCs.After HCPEB irradiation,the thermal stress level of TBCs during the thermal cycle was greatly reduced under the synergistic action of various irradiation effects,thus the HCPEB-TBCs exhibited superior thermal cycling performance.The results of thermal cycling test at 1150?showed that after 35 cycles,overall spalling occurred within the APS-TBCs,which means that the coating has failed.By contrast,after 65 cycles of HCPEB-TBCs,35%of the spalling area appeared and the coating failed.The coating failure occurred mainly at the TC/TGO interface at 1150?,and the thermal cycling life of TBCs at 1150?was significantly lower than that at950?.Thermal stress gereated during thermal cycling process,residual stress of thermally YSZ coating,TGO growth stress and phase transformation stress were the causes that led to the coating failure,among which the level of thermal stress was the decisive factor.After HCPEB irradiation,the internal thermal stress level of the coating was greatly reduced and the TGO layer thickness was lower than that of the original coating,which were the main reasons that led to the enhancement of the thermal cycling performance of HCPEB-TBCs.