Fabrication,Structure And Properties Of SiC_f/SiC Composites Modified By Yttrium Oxide

SiCf/SiC composites are considered as the potential materials for hot-section components in advanced aircraft engine due to the outstanding properties,such as low density,superior toughness,high temperature resistance and non-brittle fracture failure.However,because SiC matrix can react with water vapor and oxygen to form volatile silicon hydroxide?Si?OH?4?,the wet oxygen environment in aircraft engine can lead to serious erosion or even failure for SiCf/SiC composites.Environmental barrier coatings?EBCs?can be applied on SiCf/SiC composites to overcome the drawbacks as they can isolate the composites from the corrosion and avoid the reaction between SiC and wet oxygen environment.Due to the coefficient of thermal expansion?CTE?mismatch,the peeling-off of EBCs may occur during the long-term service in aircraft engine and lead to the failure of the composites.The problem has a serious impact on the service safety of aero-engines.To solve the above problems,Y₂O₃ is proposed to modify the SiC matrix of the composites,and regulate the presence and distribution structure of rare earth compounds in the matrix by combining the preparation process of SiC matrix,such as chemical vapor infiltration?CVI?method and polymer impregnation and pyrolysis?PIP?method.In the high temperature wet oxygen environment,the in-situ reaction in the matrix generates the rare earth silicate with low volatilization rate,which hinders the permeation of oxidizing atmosphere and reduces the degradation degree of the matrix,thus improving the intrinsic wet oxygen resistance of the composite.For SiCf/SiC-Y2Si2O7 composite,Y₂O₃ is generated by the Y?NO3?3·6H2O/alcohol solution impregnation and pyrolysis method in SiC fiber preform firstly,and layered z-Y2Si2O7 is obtained from the transformation of Y₂O₃ during CVI process.SiCf/SiC-Y2Si2O7 were finally prepared.It was shown that the introduction of Y₂O₃ will block some pores and hinder the permeation of reactive gases during the CVI process,which may lead to the increase of porosity and decrease of mechanical properties.In 1400? wet oxygen environment,the crystal type of Y2Si2O7 transformed from z to ?,and SiC was oxidized to Si?OH?4.The volatilization of Si?OH?4 will promote the redistribution of ?-Y2Si2O7 in the surface layer of silicate melt.Finally,the ?-Y2Si2O7 could form a protective layer over the sample surface and block the oxidizing atmosphere.After the oxidation for 80 hours,the retention rate of bending strength of the composite with three layers and one layer of z-Y2Si2O7 reaches 71.93% and 60.38%,respectively.By contrast,it is only 50.11% for SiCf/SiC composites without z-Y2Si2O7.For SiCf/SiC-Y₂O₃-YSi2 composite,carbon source and Y₂O₃ are generated by phenolic resin and Y?NO3?3·6H2O/alcohol solution respectively,by PIP method.Then,the reactive melt infiltration?RMI?method was used to prepare SiC matrix.In the high-temperature vacuum infiltration environment,part of Y₂O₃ can react with Si melt to generate YSi2,and form a dispersion structure around the fiber bundle.SiCf/SiC-Y₂O₃-YSi2 were finally prepared.Since Y₂O₃ in the preform can only be attached to the area near the fiber bundle,YSi2 and Y₂O₃ in the final matrix can only form a diffuse structure in the area close to the fiber bundle,and the matrix far from the fiber bundle is filled by Si.The existence of a large number of residual Si and the thermal damage of SiC fibers in the melting and infiltration process lead to the low bending strength of the composites.For SiCf/SiC-Y₂O₃ composite,SiC and Y₂O₃ are generated by PCS/gasoline and Y?NO3?3·6H2O/alcohol solution respectively,by PIP method.SiCf/SiC-Y₂O₃ were finally prepared and Y₂O₃ forms a diffuse structure in the matrix.The result in this paper shows that the introduction of Y₂O₃ does not hinder the impregnation process of PCS/gasoline solution,and has little impact on the mechanical properties of the composites.In 1400? wet oxygen environment,SiC matrix oxidizes to form SiO2,and Y₂O₃ reacts with the SiO2 to form ?-Y2Si2O7,which inhibits the SiC matrix from volatilization in the steam environment.At the same time,the content of cristobalite from the oxide layer is reduced,and the number of cracks in the oxide layer is reduced after cooling,the structure is dense,so as to avoid further oxidation corrosion inside the composite.After the oxidation for 75 hours,the strength retention rates of composites with Y₂O₃ mass fraction of 17% and 30% reaches 53.05% and 61.26%,respectively.By contrast,it is only 43.73% for SiCf/SiC composites without Y₂O₃.This study shows that the introduction of rare earth compounds into SiC ceramic matrix can improve the intrinsic water-oxygen resistance of the composites,thus improving the safety service level of SiC ceramic matrix composites in extreme environments.