Investigation On Yb₂SiO₅-Yb₂Si₂O₇ Environmental Barrier Coatings And High Temperature Water Vapor Corrosion Resistance Mechanism

SiC_f/SiC matrix composites are regarded as one of the promising candidates in aero engine hot-section parts owing to their excellent high-temperature mechanical properties and low density.However,the service life of SiC parts is limited by the poor water vapor corrosion resistance in the combustion chamber.Fortunately,environmental barrier coatings（EBCs）prepared on the surface of SiC matrix can effectively improve the high temperature water vapor corrosion resistance.Among all the EBCs materials,ytterbium silicates（Yb₂SiO₅and Yb₂Si₂O₇）are one of the research hotspots of EBCs owing to their low coefficient of thermal expansion,excellent corrosion resistance,good phase stability and chemical compatibility.In this thesis,the effects of Yb₂Si₂O₇doping on the microstructure,mechanical properties,thermal conductivity and high temperature sintering behavior of Yb₂SiO₅-Yb₂Si₂O₇composite ceramics were studied.The stoichiometric Yb₂Si₂O₇middle layer and Yb₂SiO₅-Yb₂Si₂O₇working layer were prepared by plasma spraying technology.The deposition mechanism,thermal cycling performance and high temperature water vapor corrosion behavior of the coatings were studied.The research results lay a theoretical foundation for promoting the application of SiC_f/SiC matrix composites in aero engine hot section components.The main results are as follows:（1）The effects of Yb₂Si₂O₇doping on the thermal conductivity,mechanical properties and high-temperature sintering resistance of Yb₂SiO₅-Yb₂Si₂O₇ composite ceramics were studied.The 0.5Yb₂SiO₅-0.5Yb₂Si₂O₇composite ceramics were only composed of monoclinic X2-Yb₂SiO₅andβ-Yb₂Si₂O₇phases. Compared with pure Yb₂SiO₅ceramics,the fracture toughness of 0.5Yb₂SiO₅-0.5Yb₂Si₂O₇composite ceramic was increased by 60%,and high-temperature sintering performance was greatly improved.The thermal conductivity remained at a low level.The grain size and density remained unchanged after sintering at 1400℃ for 100 h.（2）The preparation and phase structure control technology of plasma-sprayed Yb₂Si₂O₇coating were mastered.The plasma-sprayed Yb₂Si₂O₇coating has a typical layered structure composed of dark gray structure（Yb₂Si₂O₇）,light gray structure（Yb₂Si₂O₇+Yb₂SiO₅）and light strip structure（Yb₂SiO₅）.It was mainly due to the volatilization of SiO₂with the different degrees in the sprayed powder during the spraying process.The plasma-sprayed Yb₂Si₂O₇coating after heat-treatment was composed of 90.68 mol.%Yb₂Si₂O₇and 9.32 mol.% Yb₂SiO₅two phases.The content of Yb₂Si₂O₇phase after heat-treatment reached 99.41 mol.%when added 10 mol.%SiO₂to the Yb₂Si₂O₇sprayed powder.It was proved that adding excessive SiO₂to Yb₂Si₂O₇can effectively offset the problem of SiO₂volatilization in the process of coating preparation,so as to prepare Yb₂Si₂O₇coating with high purity.（3）Thermal cycling failure performance and high-temperature water-oxygen corrosion resistance of the plasma-sprayed Si/Yb₂Si₂O₇/0.5Yb₂SiO₅-0.5Yb₂Si₂O₇ coatings at 1350℃ were clarified.The microstructure of the plasma-sprayed 0.5Yb₂SiO₅-0.5Yb₂Si₂O₇coating was relatively dense.The relative content of Yb₂SiO₅in the coating was 52.96 mol.%,and the relative content of Yb₂Si₂O₇ was 47.04 mol.%due to a small amount of volatilization of SiO2.It still maintained good high-temperature phase stability after sintered at 1350℃ for 100 h.The Si/Yb₂Si₂O₇/0.5Yb₂SiO₅-0.5Yb₂Si₂O₇coating peeled off due to the thickening and cracking of the SiO₂TGO layer after 672 thermal cycles at 1350℃.Compared with the traditional Si/Mullite/Yb₂SiO₅coating（257 cycles）, its thermal cycling performance was improved by 160%.The water vapor corrosion resistance of Si/Yb₂Si₂O₇/0.5Yb₂SiO₅-0.5Yb₂Si₂O₇coating was comparable to that of traditional Si/Mullite/Yb₂SiO₅coating at 1350℃.Although the Yb₂Si₂O₇phase in the working coat decreased due to the reaction with high-temperature water vapor,the structure of the coating still remained stable, compact,and no spalling occurred after the water-oxygen corrosion for 200 h.