Preparation And High Temperature CMAS Molten Salt And Water Vapor/Oxygen Corrosion Behavior Of Yb₂Si₂O₇ Ceramics

Aero-engines which are the typical representative of advanced equipment,committed to the development of high efficiency and high thrust-to-weight ratio.Silicon-based ceramics have become a candidate material for the hot parts of aero-engines due to their excellent high temperature performance.High temperature corrosion failure in service limits the practical application of silicon-based ceramics.Environmental barrier coatings can alleviate the corrosion of silicon-based ceramics,providing the possibility for the application of silicon-based ceramics in practice.In this paper,Yb₂Si₂O₇ ceramics were used as the research object,preparing Yb₂Si₂O₇ powders by high-temperature solid-phase synthesis process.The effect of synthesis process and raw powder mixing ratio on the phase composition of powder materials is explored,the densification sintering process of Yb₂Si₂O₇ ceramics was investigated by adjusting the sintering method and process parameters.High temperature CMAS molten salt corrosion experiments and high temperature water vapor/oxygen corrosion experiments under different conditions were designed and carried out to explore the high temperature corrosion behavior and corrosion mechanism of Yb₂Si₂O₇ ceramics.Yb₂O₃ and Si O₂ were mixed with the molar ratio of 1:2.2,and calcined at1500°C/4h high-temperature solid-phase process can synthesize pure phase Yb₂Si₂O₇ powder.Sintering by hot pressing and under the condition of 1550°C,30MPa argon holding 2h,obtaining pure phase Yb₂Si₂O₇ ceramic block with a density of 98%and an average thermal expansion coefficient of 4.1×10^-6K^-1 from room temperature to 1400^oC.High temperature CMAS molten salt corrosion of Yb₂Si₂O₇ ceramic material includes two processes:molten salt infiltration and corrosion reaction.CMAS molten salt penetrates into the substrate by the grain boundary,and reacts to form a needle-like Ca₂Yb₈（Si O₄）₆O₂ apatite phase and Si O₂ of amorphous.As the corrosion temperature increases,the corrosion reaction process changes from interface reaction control type to diffusion control type.As the corrosion temperature increases,the degree of corrosion reaction between CMAS molten salt and Yb₂Si₂O₇ceramics increased.The content of Ca element in CMAS molten salt is the key to determining the corrosion reaction process.When the content of Ca element is higher,the reaction produces apatite phase.After reaching the critical value,reaction reached the limit and smaller Yb₂Si₂O₇ phase precipitated on the side close to the substrate.The corrosion reaction between Yb₂Si₂O₇ ceramics and high temperature water vapor/oxygen is a cyclic corrosion process.Reaction process can be divided into two stages.At the initial stage of the reaction,Si element reacts with high temperature water vapor/oxygen to form Si（OH）₄,and the reaction is mainly dominated by grain boundary corrosion.When the Si element content reduced,corrosion reaction is dominated by the reaction between Yb element and high temperature water vapor/oxygen,generating Yb₂Si O₅ phase,gaseous Si（OH）₄and gaseous Yb（OH）₃ in the process.At this time,the degree of intragranular corrosion is higher than the grain boundary.Yb₂Si O₅ formed in corrosion reaction improves the corrosion resistance of the material,the interfacial reaction constant and temperature satisfy the relationship K=exp（25.26–51.36/T）.Reducing the content of Si element in Yb₂Si₂O₇ helps to improve the corrosion performance of the material in high temperature water vapor/oxygen.