| At present,SiCf/SiC composites have been successfully applied to some hotsection components of aero-engines.Under the extremely harsh combustion environment in engine,SiCf/SiC composite is easily corroded by high-temperature water vapor and CMAS melts,resulting in material failure and performance degradation.Applying environmental barrier coatings on SiCf/SiC hot-section components is of great significance to improve engine efficiency and thrust-to-weight ratio.Rare earth silicates are widely studied as the next-generation environmental barrier coating material.Rare earth mono-silicates possess excellent high temperature stability and good corrosion resistance;and rare earth disilicates show matched thermal expansion coefficient with SiCf/SiC and have good chemical compatibility with Si bond coat.However,these materials also demonstrate certain various disadvantages that could not satisfy the critical requirements for advanced environmental barrier coating.Through the doping of different rare earth elements and thereafter initiate synergistic effects,the multi-component rare earth silicates may have optimized thermal expansion coefficient,thermal conductivity,corrosion resistance and other key aspects.When plasma spraying technique is used to prepare environmental barrier coatings,it is usually accompanied by high temperature and plasma-induced volatilization of multi-components and the amorphous phase caused by excessive cooling rates.The composition changes caused by volatilization and the subsequent high temperature crystallization pose significant negative impacts on the performance and long-term service of the coating.Specially for the multi-component rare earth silicates,the preferential volatilization of various rare earth elements as well as the residual amorphous phases may be more significant due to the complex vapor pressures of each component and the consequent phase competitions and evolutions.Therefore,it is necessary to understand the effects of APS technology and post heat treatment technology on the evolution of coating composition and microstructure.The results are expected to be important for the controlled fabrication and development of new EBCs.In this thesis,(Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 was focused for the systematic study of preparation and evaluation as anew EBC.The pure phase powders were prepared by the solid-phase reaction method.The powders were thereafter agglomerated by spray drying to obtain feedstocks;and the coatings were prepared by atmospheric plasma spraying technology.The composition,microstructure,mechanical properties and CMAS corrosion resistance of the coating were investigated.The main research contents and results are as follows:(1)Preparation and characterization of feedstock for atmospheric plasma spraying.Single-phase(Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O powders were prepared by solid-state reaction method.The powders were agglomerated by spray drying method to obtain spherical feedstocks for plasma spraying.The characterization results of spherical feedstocks showed that the rare earth elements were uniformly distributed within the spherical feedstocks without obvious segregation.The spherical feedstocks possess uniform particle size distribution(D50=42 μm),high fluidity(50 s/50 g)and high loose density(1.29 g/cm3),which benefit powder feeding in the atmospheric plasma spraying process and effectively preventing the blockage of the spray gun.(2)The influence of spraying parameters on coating composition and microstructure were studied.The results showed that with the increase of plasma spraying current and spraying distance,the feed particles were more sufficiently melted.Meanwhile,the coatings possessed more amorphous phase and lower porosity.The coating prepared by plasma spraying with current of 650 A and distance of 90 mm was better.(3)The effects of different heat treatment temperature,time duration and atmosphere on the evolution of coating composition and microstructure were studied.The results showed that with the increase of the heat treatment temperature and the prolongation of the holding time,the crystallization of the coating become more sufficient;and the relative proportions of RE2Si2O7,RE2SiO5 and RE4.67(SiO4)3O phases in the coating were significantly different when annealed in different atmospheres.The coatings annealed in air showed more severe oxidation at the interface of disilicate topcoat and the Si bond layer,which also resulted in slightly lower adhesion than coatings annealed in argon.(4)The CMAS corrosion resistance of the coating after heat treatment was investigated.The results showed that apatite Ca2RE8(SiO4)6O2 was formed after CMAS corrosion at 1300℃.With the prolongation of corrosion time,the amount of corrosion products increased,the ratio of Ca/Si in the residual CMAS decreased gradually,and the coating was gradually penetrated.Within the corrosion time of 1 h and 4 h to prevent CMAS corrosion than the coating annealed in air.However,after 25 h and 50 h corrosion,all the coatings were penetrated,and the surface layer and bonding layer were peeled off. |
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