Study On Mechanical And High Temperature Oxidation Resistance Of Environmental Barrier Coating Si-Yb-Gd/Si-Yb-Gd-Hf Composite Bond Coat

At present,the most advanced environmental barrier coatings(EBCs)used in silicon carbide ceramic matrix composites(SiC_f/SiC)still rely on the use of Si bond coat.Although the bond coat can improve the durability and adhesion of the coating,the maximum operating temperature of the EBCs is limited by the melting point of Si(1414°C).The turbine inlet temperature has exceeded this temperature,and the Si bond coat itself has many problems.In order to reduce or eliminate the cooling of advanced components and improve the efficiency of the engine,EBCs need new bond coat materials.In this study,a composite bond coat composed of Si-Yb-Gd and Si-Yb-Gd-Hf was designed based on literature,and their bulk samples were prepared by spark plasma sintering(SPS)and characterized,and then their resistance to high temperature oxidation was studied.Finally,atmospheric plasma spraying(APS)was used to preparation the multi-component composite bond coat-Yb₂Si₂O₇ coating system.Through the characterization and analysis of the experimental results,a preliminary study of the multi-component composite bond coat was systematically carried out.The main research contents are as follows:(1)Two bulk materials,Si-Yb-Gd and Si-Yb-Gd-Hf,were prepared by SPS method,and their microstructure and mechanical properties were characterized and analyzed.Results showed that due to the large difference in the melting and boiling points of the components,the material failed to react uniformly,especially for Hf,which existed independently in the form of flakes.By comparing the mechanical properties of the two materials,it was found that the hardness and Young's modulus of the material were reduced after doping with Hf.(2)High temperature oxidation experiments were carried out on the two bulk materials,and it was found that the oxide layer of the materials exhibited Yb enrichment and Si depletion with increasing oxidation time and temperature.Moreover,Gd appeared near the surface of the oxide layer,but showed a depletion at the oxide layer and the matrix interface.Studies have found that this phenomenon is the results of the synergistic effects of the selective oxidation of the bond coat material and the diffusion coefficient of oxygen in oxides,as Gibbus free energy changes in Gd oxidation and Yb oxidation change is substantially the same and lower than those of Si and Hf.Moreover,as the oxygen diffusion coefficient in Gd₂O₃ is higher than in Yb₂O₃,and the formation of Yb₂O₃ prevents further oxidation of Gd.In addition,due to the small volume change caused by Yb oxidation,large internal stress can be avoided,suggesting Yb is very suitable for doping in the composite bond coat.(3)The traditional APS method was used for the preparation of the coating.Previous indicated that the bond coat prepared by traditional APS may experience oxidation to a certain degree,and get damaged as a consequence.In addition,it is also difficult to ensure the uniform distribution of elements in the multi-component composite bond coat using APS,which results in having multiple phase components,and mechanically weak areas prone to failure.This served as a preliminary feasability study of the coating preparation procedure and potential means of process optimization.