Study Of Rare Earth Hexaaluminate Environmental Barrier Coatings

Silicon carbide fiber-reinforced silicon carbide composites（SiC_f/SiC）are considered as the structural materials for hot sections of next generation of aero-engines due to their outstanding mechanical properties at high-temperature and their density is only one-third that of nickel-based superalloys.However,SiC_f/SiC is susceptible to water vapor corrosion during the high-temperature gas environment of aero-engines,and its mechanical properties deteriorate rapidly.Therefore,it is necessary to prepare dense environmental barrier coatings（EBCs）on the surface of SiC_f/SiC to insulate the high-temperature gas,in order to achieve long-term durability and stability.Ytterbium silicate coatings are promising EBCs materials due to their excellent high-temperature stability,low thermal conductivity and good corrosion resistance.However,the volatilization of Si（OH）₄ and degradation of coating cause the failure of coating due to the reaction between water vapor and ytterbium silicate coatings.In this work,the LaMgAl₁₁O₁₉ coating with high melting point and low thermal conductivity serves as top ceramic coating so as to improve the water vapor corrosion resistance of coating.In addition,the Sibond coat was modified by HfO₂,and the influence of HfO₂ relative content on the thermal cycling lifetimes of Si-HfO₂ bond coat was also investigated.Furthermore,multi-layer Hf20Si（HfO₂+20wt.%Si）/YbSi1.5（Yb₂O₃:SiO₂=1:1.5）/Yb₂SiO₅/LaMgAl₁₁O₁₉ coating were designed and fabricated by APS,and its thermal cycling and water vapor corrosion behavior were also investigated.Tri-layer Si/Yb₂SiO₅/LaMgAl₁₁O₁₉ EBCs were fabricated by atmospheric plasma spray（APS）,and the water vapor corrosion behaviors of SiC_f/SiC coated with and without EBCs at 1300℃were investigated.This result indicated that coating can dramatically improve the stability of SiC_f/SiC.The water corrosion behaviors of Yb₂SiO₅and LaMgAl₁₁O₁₉ coatings were also investigated at 1300℃.It was found that the single-layer coating has excellent water vapor corrosion resistance.The thermal mismatch stress as well as the formation and phase change of TGO was the main causes of tri-layer coating failure.The Sibond coat was modified by HfO₂.The Si-HfO₂ powders with four different compositions were spray-dried,and then the Si-HfO₂ bond coat was obtained by the method of APS.Results indicated that the Si-HfO₂ coating showed a non-linear expansion in air environment due to the continuous HfO₂ networks accelerated the oxidation of Siand formation of HfSiO₄.The thermal cycling results of the Si-HfO₂bond coat at 1300℃suggested that the Si-HfO₂ bond coat with lower HfO₂ content has relatively better thermal shock resistance,owing to its low coefficient of thermal expansion（CTE）and better oxidation resistance.The HfSiO₄ phase formation reaction between SiO₂ and HfO₂ alleviates the coating spallation caused by cristobalite phase transformation.The Ostwald ripening of dispersed HfSiO₄ particles also restrains the surge of thermal stress and formation of cracks.However,the coefficient of thermal expansion mismatch between the Si-HfO₂ bond coat and the SiC substrate as well as the oxidation of SiC caused the spallation of the Si-HfO₂ bond coat along the edges of the SiC surface.Multi-layer Hf20Si/YbSi1.5/Yb₂SiO₅/LaMgAl₁₁O₁₉ coating was fabricated on the surface of the SiC_f/SiC and SiC substrate,which exhibited relatively dense structure and the microstructure of coating is indifference.The thermal cycling lifetimes of coating coated SiC_f/SiC substrate at 1300℃and 1350℃is about 171 cycles and 107cycles,respectively.The thermal cycling lifetimes of coating decrease with the thermal cycling temperature,due to the increase of tensile stresses in the coating.There are two different forms of crack propagation in the coating after water vapor corrosion at1300℃,due to the reaction of ytterbium silicate coating with water vapor.The thermal cycling lifetimes of coating coated SiC substrate at 1400℃and 1450℃is about 20cycles and 12 cycles,respectively.During the thermal cycling tests,the tensile stresses caused by thermal mismatch in the coatings result in the coating spalled along the surface of the SiC substrate.