水氧腐蚀对Yb_(2)Si_(2)O_(7)涂层裂纹扩展的影响

Effect of Water Oxygen Corrosion on Crack Propagation of Yb_(2)Si_(2)O_(7)Coating

双硅酸镱(Yb_(2)Si_(2)O_(7),YbDS)与碳化硅复合材料(CMC)基体具有良好的热匹配能力,是目前环境障涂层(EBCs)中常用材料。然而,在高温水氧腐蚀环境中YbDS向单硅酸镱(Yb_(2)SiO_(5),YbMS)的转变导致涂层发生开裂和分层。为了分析水氧腐蚀对相转变和由此引发的EBC内应力的影响,本工作从实验观察和仿真计算,采用大气等离子喷涂制备Si+YbDS涂层,通过循环热载荷水氧腐蚀实验分析YbDS涂层的腐蚀产物和腐蚀速率,计算YbDS腐蚀过程中导致EBC内应力的再分布特征,分析由此引发的裂纹扩展。结果表明,完成沉积后的YbDS面层受残余压应力作用,而在蒸汽循环腐蚀后的YbDS涂层中出现了拉应力,较大的应力导致EBC过早开裂。YbDS成分挥发导致的孔隙增加对腐蚀区的应力影响较大,孔隙也提高了Ⅰ型裂纹的扩展速率。

Introduction The aimed environmental barrier coatings(EBCs)need to meet the characteristics of low oxygen permeability,match thermal expansion coefficient,and keep phase stability under service conditions,which usually need to be designed as a multi-layer structures to avoid premature failure of gas turbine engines during operation.Yb_(2)Si_(2)O_(7)(YbDS)is considered the most promising candidate due to its excellent resistance to water vapor corrosion and damage tolerance,a similar coefficient of thermal expansion(4.1×10^(-6)/K)to that of the matrix material(SiC,4.7×10^(-6)/K),and does not generate high thermal cycling stress in EBCs systems.However,when YbDS was exposed to an environment containing with high-temperature water vapor,corrosion transformation may occur,which would reduce the durability of EBCs.To analyze the corrosion process,stress characteristics,and crack propagation laws of YbDS under the coupling effect of water-oxygen corrosion and thermal cycling,the compositional and structural changes of YbDS under high-temperature water-oxygen action were determined through experiments,also the influence of water-oxygen corrosion on the stress evolution of the coating was explored using finite element software Abaqus.The extended Finite Element Method(XFEM)was used to simulate the propagation path of cracks in YbDS,and the accelerating effect of water-oxygen corrosion on crack propagation was diScussed finally.Methods The Si powder was used for bond coat deposition,and Yb_(2)Si_(2)O_(7)powder was used for top coat deposition.Powders used in this experiment were designed with a particle size distribution suitable for thermal spray applications.Both the bond coat and top coat were deposited onto theφ25.4 mm×3 mm SiC substrate using the Praxair 3710 air plasma spraying system.Thermal cycling tests of the EBC system was conducted in an aluminum oxide tube furnace at 1325℃,The ratio of water vapor to oxygen was controlled to 9:1 by a gas-liquid mixing device,samples were took out and cooled to room temperature when insulation for 5 h.Repeated this steps until the coating failed.The cross-sectional and surface morphology changes of the coatings were observed via a Scanning electron microScope for both secondary electron mode(SE2)and backScattered electron mode(BSE).Energy dispersive spectroScopy was used for elemental trace analysis,and X-ray diffraction was used to analyze the phase composition of the coating surface at different stages of water oxygen corrosion.The fracture toughness of the coating was calculated via the indentation methods.The changes in phase,structure,and mechanical properties collected from the experiment were compiled into the USDFLD and UEXPAN subroutines in Abaqus,and then substituted them into the established two-dimensional finite element model for calculation.By prefabricating XFEM cracks in the model,the influence of the cracks on the coating was analyzed.Results and diScussion The XRD results showed that the Yb_(2)SiO_(5)increased from 21.6%in the initial spray state to 58.1%and 95.1%,and the phase content of Yb_(2)Si_(2)O_(7)decreased from 73.8%in the spray state to 33.6%and 1.5%when the sample surface suffered 20 h and 40 h of water oxygen corrosion.After 60 h corrosion,phase composition of Yb_(2)Si_(2)O_(7)was not detected,and the content of Yb_(2)SiO_(5)increased slightly to 91.5%,and some Yb_(2)O_(3)was observed results show that the Yb_(2)Si_(2)O_(7)would preferentially decompose into Yb_(2)SiO_(5)under high temperature water vapor environment,and the Yb_(2)SiO_(5)has better high temperature stability than that of Yb_(2)Si_(2)O_(7).The finite element results showed that the corrosion zone of YbDS changed from compressive stress to tensile stress under the joint action of corrosion and thermal cycle.After cooling,the stress increased to 631.4 MPa,resulting in an early corrosion EBC cracking and rapid water vapor penetration.The volume shrinkage caused by the volatilization of the components in corrosion area made a great influence on the stress of the corrosion layer,and the residual tensile stress caused by it was about 490 MPa,while the effect of thermal mismatch in cycle stage was relatively small,only about 130 MPa.It should be noted that the new pores in high temperature corrosion stage would change the stress field in the coating,and the more the corrosion depth,the more the new pores effect on the stress field.Through the simulation of crack growth process,it was found that the volume shrinkage in high temperature corrosion stage exhibited a more important impact on the surface crack growth when the corrosion aera was shallow.The thermal mismatch caused by water oxygen corrosion gradually became the main driving force for the crack propagation when corrosion depth reached 147μm.The above two alternate actions led to the surface cracking.Conclusions The composition of environmental barrier coating would decompose to Yb_(2)SiO_(5)corroded under high temperature water-oxygen environment.The volatilization of Si—O caused the increase of shrinkage stress in surface coating,and thermal mismatch between Yb_(2)SiO_(5)and the matrix material also increased,the residual stress accumulated as the cyclic thermal load continued.The alternating action of these two factors caused the surface layer to crack.At the same time,the pores generated during the high-temperature corrosion stage could also resulted in the internal stress field of the coating,and its impact increases with the increase of corrosion degree.