High Temperature Corrosion Behaviors And Mechanism Of Rare Earth Silicate Environmental Barrier Coatings

Ceramic matrix composites?CMCs?are the highly potential candidate materials forthe hot sections of aero-engines.During service processes,CMCs are susceptible to corrosion by high temperature water vapor and silicate deposites,resulting in a sharp decline in mechanical properties.Depositing environmental barrier coatings?EBCs?on the surface of CMCs is an effective method to settle this problem.Studies indicated that rare earth silicates are promising candidates for EBC materials,due to their properties of high melting point,suitable coefficients of thermal expansion,low thermal conductivity and superior phase stability.Corrosion resistance of rare earth silicate coatings with different crystal structure or rare earth element under high temperature is different.Plasma spray?PS?has been adopted as a reliable technique for preparing EBCs.During spraying process,the phase composition of rare earth silicate coatings is changed due to the volatilization of silicon,which further has an influence on corrosion behaviors of the coatings.Additionally,rare earth silicate coatings are mainly applicated as top coating or intermediate coating in the EBCs multilayer system.While the system possesses defects such as interfaces and cracks,which will affect the penetration of the corrosion medium in rare earth silicate coatings.At present,the influence of the above factors on corrosion behaviors of rare silicate coatings in high temperature corrosion environments has not been fully understood.In this work,the plasma spray technology was adopted to prepare the following three kinds of rare earth silicate coating system:?1?Four kinds of rare earth monosilicates?RE₂Si O₅,RE=Gd,Y,Er or Yb?coatings.?2?Ytterbium silicate coating with different phase compositions?YSx coatings,Yb₂O₃:Si O₂=1:x mol,x=0.5,1.0,1.5 or 2.0?.?3?RE₂Si O₅/Yb₂Si₂O₇/Si?RE=Er or Yb?coating system sprayed on the surface of the substrate.The corrosion behaviors and mechanism of the above coatings under 1400?water vapor and molten deposition?Ca O-Mg O-Al₂O₃-Si O₂,CMAS?environments were investigated respectively.The mainly conclusions were as following:1.The study of the high temperature water vapor corrosion behaviors and mechanismof rare earth silicate coatings showed that the X2-RE₂Si O₅?RE=Y,Er or Yb,spacegroup P2₁/c?coatings possessed excellent water vapor corrosion resistance than theX1-Gd₂Si O₅ coating?space group C2/c?.As the RE³⁺radius decreased,thecorrosion resistance of the X2-RE₂Si O₅ coatings increased,and the Yb₂Si O₅coatingwas the best.The corrosion mechanism of the RE₂Si O₅ coatings was that RE₂O₃and RE₂Si O₅ grains on the surface of the coatings reacted with water vapor togenerate volatile RE?OH?₃ and Si?OH?₄.For the ytterbium silicate coatings withdifferent phase compositions,the corrosion resistance of Yb₂O₃-rich coatings wasbetter than that of Yb₂Si₂O₇-rich coatings.The volatilization of Yb₂O₃ or thedecomposition of Yb₂Si₂O₇ would increase the content of pores and then weaken the corrosion resistance of the ytterbium coatings.2.The study of the high temperature CMAS corrosion behaviors and mechanism ofrare earth silicate coatings showed that X2-RE₂Si O₅?RE=Y,Er or Yb?coatingspossessed better CMAS corrosion resistance than the Gd₂Si O₅ coating,and theYb₂Si O₅ coating had the best corrosion resistance.A corrosion layer composed ofCa₂RE₈?Si O₄?₆O₂ and RE₃Al₅O₁₂phases was formed on the surface of the RE₂Si O₅coatings,which further hindered the penetration of CMAS.The Gd₂Si O₅ coatingonly precipitated Ca₂RE₈?Si O₄?₆O₂phase.As the RE³⁺radius decreased,the X2-RE₂Si O₅ coatings were prone to precipitate RE₃Al₅O₁₂,which reduced coating losand thus exhibit better corrosion resistance.For the ytterbium silicate coatings withdifferent compositions,the corrosion resistance of Yb₂O₃-rich coatings was betterthan that of Yb₂Si₂O₇-rich coatings,and the corrosion resistance of the coatingsdecreased with the increase of Yb₂Si₂O₇ phase content.The exist of Yb₂O₃accelerated the consumption of Si O₂ in CMAS and promoted the precipitation ofYb₃Al₅O₁₂.While the reaction of Yb₂Si₂O₇ with CMAS consumed Ca O andsimultaneously generated Si O₂ dissolving in CMAS,which led to the coatings suffering serious CMAS penetration.3.The corrosion behaviors and failure mechanisms of the RE₂Si O₅/Yb₂Si₂O₇/Si?RE=Er or Yb?coating system on the surface of the substrate were studied.Resultsshowed that the RE₂Si O₅ coating produced penetration cracks due to thermalmismatch among the coatings,which accelerated the penetration of corrosionmediums including water vapor,CMAS and oxygen.During the water vaporcorrosion,RE₂Si O₅ and Yb₂Si₂O₇ produced more pores,which accelerated theoxidation of the Si coating to form Si O₂?Thermally grown oxide,TGO?layer.Owning to Si O₂ phase transition and corrosion,transverse cracks was produced inthe coatings.During CMAS corrosion,a corrosion layer composed ofCa₂RE₈?Si O₄?₆O₂ was formed on the surface of RE₂Si O₅ layer.Simultaneously,CMAS melt penetrated along the cracks to the intermediate layer and bonding layer.The Yb₂Si₂O₇ and Si coatings were dissolved,resulting in the failure of the coating system structure.