Research On Preparation And Performance Of Gd₂Zr₂O₇ Agglomerated Powedrs And Coatings

With the development of aero-engine towards high efficiency and high thrust to weight ratio,higher requirements are put forward for the application of TBCs on the surface of engine hot end components.For yttria-stabilized zirconia(YSZ)materials can not meet the requirements of higher temperature and better thermal insulation due to phase transformation,sintering and other problems.The development and application of new thermal barrier coating materials are attracting attention worldwide.Gadolinium zirconate(Gd₂Zr₂O₇)is one of the most promising ceramic materials for TBCs due to its low thermal conductivity,high phase transition temperature and high temperature stability.Plasma spraying-physical vapor deposition(PS-PVD)has the advantages of both atmospheric plasma spraying(APS)and electron beam-physical vapor deposition(EB-PVD),the unique feather columnar structure coatings with excellent comprehensive performance can be obtained by adjusting the process parameters.PS-PVD is regarded as the most potential new thermal barrier coating preparation technology.At present,on the one hand,the research on PS-PVD mainly focuses on YSZ materials,while the research on new thermal barrier coating materials such as gadolinium zirconate is less,and there is no commercial gadolinium zirconate powder suitable for PS-PVD spraying.On the other hand,previous PS-PVD studies focused on the effects of process parameters on the growth mechanism and properties of coatings,and few studies on the effects of spraying powder on the structure and properties of coatings.Therefore,in this paper,the spray drying method was used to prepare gadolinium powder suitable for PS-PVD spraying.The effects of additives and solid content on agglomeration powders were studied,and the best spray drying process for preparing high performance gadolinium zirconate agglomeration powder was obtained.On this basis,the influence of particle size of original gadolinium zirconate powder on agglomerated powder and coating was studied,and the controllable preparation from agglomerated powder to coating was realized.Finally,the single/double ceramic layer structure system and Yb doped gadolinium zirconate coating were investigated.The main research results are as follows:(1)The dispersant can stabilize the suspension,and the absolute value of Zeta potential decreases first and then increases with the increase of the content of dispersant-polyacrylic acid(PAA).When the PAA content is 6 wt.%,the Gd₂Zr₂O₇ suspension,with a maximum Zeta potential and the minimum settling height,was the most stable.The binder plays the role of bonding the original powder and maintaining the strength of the agglomerated powder.The sphericity,loose density and fluidity of agglomerated powder increase with the increase of binder content.Both polyvinyl pyrrolidone(PVP)and polyvinyl alcohol(PVA)can be used as the binder of Gd₂Zr₂O₇ agglomerated powders,but they have lower organic content and better fluidity when PVA is used.The particle size,sphericity and fluidity of agglomerated powders increased with the increase of slurry solid content.The optimal suspension ratio scheme for preparing Gd₂Zr₂O₇agglomerate powder was 6 wt.%PAA,1.5 wt.%PVA and 50 wt.%solid content.(2)With the increase of the original powder particle size,the particle size,sphericity and fluidity of agglomerated powder decrease.The agglomerated powders prepared by nanoscale original powders were hompgeneous and compact inside,and it is difficult to be completely gasified during PS-PVD spraying.The coating is mainly deposited in liquid phase and has a compact stacking structure inside,which is also the reason for the high hardness and bonding strength but low thermal shock resistance of the coating.There are some pores in the agglomerated powder prepared by submicron original powder,and co-deposition of gas phase and liquid phase occurs in the spraying process.The coating has many internal defects and poor microstructure uniformity,so its hardness and bonding strength are low.The agglomerated powder prepared by micron original powder are porous and loose inside and can be completely gasified in plasma flame flow.The coating has a typical feather-columnar crystal structure with high bonding strength,hardness and thermal shock resistance.(3)The thermal expansion mismatch between Gd₂Zr₂O₇ coating and matrix material will lead to the spalling failure of the coating in the preparation process,YSZ as the intermediate transition layer can mitigate the impact of this thermal expansion mismatch.The bottom of the single-ceramic-layer(SCL)GZO coating prepared by PS-PVD is a dense layer formed by equiaxed grains,and above the dense layer is a feather-columnar crystal structure composed of radial dendrites,while the top of the GZO coating presents an undulating"cauliflower"morphology.Both the YSZ layer and the GZO layer in the double-ceramic-layer(DCL)YSZ/GZO coating exhibit typical feather-columnar crystal structure,and the GZO layer extends along the direction of YSZ columnar crystal growth.Yb doping can significantly improve the comprehensive properties of GZO coating.The(Gd_0.9Yb_0.1)₂Zr₂O₇(GYb Z)coating showed indentation hardness of 5.4 GPa,the indentation elastic modulus of 111.6 GPa,and the tensile strength of 41.3 MPa,which are significantly higher than that of GZO coating.