| Thermal barrier coatings(TBCs)can improve the service temperature and high temperature corrosion resistance of aero-engines,thus prolong the service life due to their good thermal insulation property.TBCs together with the advanced high-performance air film cooling technology,high temperature structural materials are regarded as three key technologies for advanced aero-engines high pressure turbine blade.Currently,the traditional fabrication technology of TBCs mainly includes atmospheric plasma spraying(APS)and electron beam physical vapor deposition(EB-PVD).The lamellar 7YSZ coatings fabricated by APS possess have high deposition rate,good thermal insulation performance and the bad thermal shock resistance whereas the columnar 7YSZ coatings deposited by EB-PVD have good thermal shock resistance and strain tolerance,poor thermal insulation performance,low deposition rate and high cost.With the increasing thrust weight ratio of aero-engines,the integrated service performance demands of TBCs would be more hash.To meet these demands of the next-generation aero-engines with higher thrust weight ratio,developing a new preparation process and new material system for TBCs is necessary.Recently,the advanced and structure controllable preparation technology,plasma spraying physical vapor deposition(PS-PVD),is expected to fabricate the TBCs with a new structure combined the advantages of APS with EB-PVD technology based on the development of very low pressure plasma sprayingIn this paper,the feather-like columnar structure 7YSZ coatings whose raw materials are agglomerated and sintered ZrO2-7%Y2O3(7YSZ)nanopowders,are prepared onto the K417 nickel-based superalloy using PS-PVD technology.In addition,the conventional APS 7YSZ laminar structure coatings are also fabricated.The high temperature oxidation resistance,particle erosion and thermal shock resistance of these two kinds of coatings have been compared and investigated systematically.Furthermore,the relationship among the process-structure-property of coatings is established by adjusting the coating preparation process parameters(spray distance).Finally,the microstructure evolution of coatings during high temperature service is analyzed by the alternating current impedance spectroscopy method.The results show that:1.The feather-like columnar structure 7YSZ coatings are composed of secondary columnar crystals with nanostructure and their gaps,columnar dendritic pores and micro/nano solid particles distributed on columnar dendrites.Along the growth direction of the coating,the "cauliflower" head diameter and the columnar structure spacing gradually increased,and the coating evolves from dense structure to loose structure.All the 7YSZ coatings are feather-like columnar structure at different processes(spray distance).The"cauliflower" surface of the coatings changes from "small and dense" to "large and sparse"and the surface roughness and porosity are increased with the increase of the spray distance.Meanwhile,there is no significant change about the bonding strength between 19 and 21MPa.When the spray distance is 1250 mm,the micro hardness of the single column structure is the highest,about 441.24HV0.1.2.The micro cracks of the feather-like columnar structure 7YSZ coatings initiate at the interface between thermal grown oxide layer(TGO)and ceramic coating(7YSZ),propagate along the TGO/7YSZ interface under TGO growth stress,stop at the columnar structure gap releasing part of the inner stress during the high temperature oxidation.The new micro cracks would initiate due to the introduction of the residual stress from columnar structure gaps to inside of the columnar structure.Finally,the propagation of new micro cracks leads to the internal fracture failure of the columnar crystal under the thermal stress,thus the oxide spallation pits on the coatings surface are formed.The oxidation rate of the feather-like columnar 7YSZ with high porosity TBCs whose bond coatings are not been pretreated is significantly faster than the lamellar structure 7YSZ coatings.However,the high temperature oxidation resistance of the feather-like columnar 7YSZ TBCs whose bond coatings undergo polishing and pre-oxidation is 5 times higher than that of 7YSZ coatings with lamellar structure.In addition,the high temperature oxidation resistance of the feather-like columnar structure 7YSZ coatings decreases gradually with the increase of spraying distance.When the spray distance increases to 1250 mm,the oxidation rate constant of coatings have no significant change because the external oxygen pressure of TGO is close to the critical saturation value.3.The erosion failure process of the feather-like columnar structure 7YSZ coatings mainly includes three stages:The rapid erosion stage,namely initial erosion stage,the erosion of the "cauliflower" shape surface.The medium erosion stage,i.e.,the erosion of the columnar structure below the "cauliflower" shape surface and above the dense ceramic layer.The slow erosion stage,i.e.,the abrasive wear process at the bottom of the dense ceramic layer.The erosion resistance of the feather-like columnar structure 7YSZ coatings is worse than that of lamellar structure 7YSZ coatings according to the weight and thickness loss testing.Considering that the microhardnesss of the single feather-like columnar structure coatings is the main factors affecting their erosion resistance,the coatings with spray distance 1250 mm have the best erosion resistance.Furthermore,The PS-PVD and APS composite technology can effectively improve the erosion resistance of feather-like columnar 7YSZ coatings.Although the dense 7YSZ coatings with 20 ?m thick are broken,the composite coatings still remain slow erosion rate and their erosion resistance is about 4 times higher than that of the feather-like columnar structure 7YSZ coatings.4.The failure form of the feather-like columnar structure 7YSZ coatings is mainly pitting corrosion and peeling-off during the thermal shock and the main reason is quenching stress and thermal mismatch stress due to the differences of thermal expansion coefficient between the substrate and ceramic layer.The micro cracks will generate the single feather-like columnar structure coatings due to the residual thermal stress.With the increase of the number of thermal shock,the micro cracks penetrate through the single feather-like columnar structure,resulting in pitting corrosion and peeling-off.When the adjacent feather-like columnar structure is constantly pitted and peeled,a large area of pitting corrosion will be formed,resulting in failure of the coatings.The average thermal shock life(208 times)of feather-like columnar 7YSZ coatings is 1.5 times of that of lamellar 7YSZ coating(139 times).The porosity of the feather-like columnar 7YSZ coatings increases with the increase of the spray distance,which would enhance the strain tolerance and improve the thermal shock resistance of the coating.5.The equivalent circuit feather-like columnar structure 7YSZ coatings consists of four R-C series circuits.The impedance spectroscopy analysis of as-sprayed coatings indicates that the number of capacitive arc impedance decreases with the increase of temperature.There is an exponential relationship between grain boundary resistance value(y)and porosity(x)of YSZ,i.e.,y=(1.3)x+1.43×103).The impedance spectroscopy analysis of coatings after high temperature oxidation shows that the resistivity of TGO layer increases due to densification after of 50 h high temperature oxidation.The sintering shrinkage cracks in the coatings surface forming during the high temperature oxidation result in decrease of capacitance and increase of resistance of YSZ grain boundary.However,the molten sintering of the secondary columnar crystal is the main factors restraining the decrease of capacitance and the increase of resistance of the YSZ layer.Thermal shock impedance analysis shows that the resistance value of TGO layer increases and the capacitance decreases with the increase of the number of thermal shock,which indicates that the TGO layer is growing continuously during thermal shock.Thermal shock has no obvious influence on the internal structure of the secondary columnar nano-grains,but the shrinkage cracks on the surface of the coating cause the increase of the resistance and the decrease of the capacitance of YSZ grain boundary. |
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