Influence Of Phase Composition On Microstructure And Properties Of Rare Earth Silicate Environment Barrier Coatings

Silicon-based ceramics have the characteristics of low density,excellent high-temperature mechanical properties,making them suitable for application to hot sections of gas turbine engines.However,their corrosion resistance to high-temperature water vapor is poor.Therefore,environment barrier coatings?EBCs?have been developed to solve this problem.Rare-earth silicates have been regarded as one of the most promising EBC materials due to their excellent environment durability,relatively suitable coefficients of thermal expansion?CTE?and superior phase stability.Atmospheric plasma spray?APS?technique is an effective method for the preparation of EBCs.The plasma spray process has the characteristics of high temperature,high speed and fast cooling rate,which lead to the great difference of the phase composition,microstructure and basic properties of plasma-sprayed rare-earth silicate coatings compared with bulk ceramics.Therefore,it is necessary to study the influence of the phase composition on microstructure and properties of the rare-earth silicate coatings.In this work,the RE2SiO5?RE=Y,Gd,Er,Yb?coatings and the ytterbium silicate coatings with different compositions designed by controlling the Yb₂O₃/SiO2 molar ratios,were fabricated by APS.The phase composition and microstructure of the coatings were characterized.The basic properties and the corrosion resistance to high-temperature water vapor were investigated.The relationship between microstructure and properties of the coatings were analyzed.The thermal shock resistance of the multilayer EBC system fabricated on the surface of silicon-based ceramics was investigated,and the failure mechanism of the coatings was analyzed.The following major conclusions can be drawn: 1.RE2SiO5 and ytterbium silicate powders with different compositions synthesizd by solidstate reaction showed well fluidity and uniform distribution of particle size.The coatingsdeposited by the powders exhibited uniform microstructure and the particles were fullymelted.2.The RE2SiO5 coatings prepared by APS contained RE2O3 and amorphous phase,which had a typical layered structure,and there were more cracks in the Gd2SiO5 coating.The four kinds of RE2SiO5 coatings had similar mechanical properties,such as elastic modulus and hardness,excellent high-temperature phase stability and excellent corrosion resistance to high-temperature vapor.Compared with the Gd2SiO5 coating,the Y2SiO5,Er2SiO5 andYb2SiO5 coatings had similar CTE with silicon-based ceramics and better thermal shock resistance,which can be attributed to the low thermal stress formed in these coatings during thermal cycling.3.The Yb₂O₃-rich ytterbium silicate coatings were composed of Yb₂O₃ and Yb2SiO5 phases, which was characterized by higher crystallinity,Yb₂O₃ columnar grains,obvious interfaces between splats and more microcracks.However,the SiO2-rich ytterbium silicate coatings were composed of Yb2SiO5 and Yb2Si2O7 phases,which was characterized by well bonded splats with more spherical pores and lower crystallinity.4.The elastic modulus and hardness of the Yb₂O₃-rich and SiO2-rich ytterbium silicate coatings were lower than that of the Yb2SiO5 coating.Compared with the Yb₂O₃-rich ytterbium silicate coatings,the SiO2-rich ytterbium silicate coatings had smaller CTE, higher thermal conductivity,and better thermal shock resistance.The main reason for the failure of the Yb₂O₃-rich coatings was the formation of high thermal stress in the coatings during thermal cycling.The SiO2 oxide layer produced in YSx/Si bilayer EBC system would accelerate the failure of the coating.The dense Yb2SiO5 layers were formed on the surface of ytterbium silicate coatings with different phase compositions,which could effectively prevent the diffusion of the corrosive substance into the coatings,so that the coatings had excellent resistance to high-temperature water vapor.5.After high-temperature thermal aging,the phenomenon of crack healing of the Yb₂O₃-rich ytterbium silicate coatings was more remarkable,and their elastic modulus,hardness and CTE were significantly increased.However,larger grains were formed on the surface of the SiO2-rich ytterbium silicate coatings accompanied with the appearance of transgranular cracks,and the thermal conductivity of the SiO2-rich coatings increased significantly.