Preparation,Properties,and CMAS Corrosion Behavior（NdGdYErYb）₂Zr₂O₇ High-entropy Ceramics

With the development of high-performance gas turbines,the high-pressure turbine inlet temperature is increasing,which brings more severe service environment to the combustion chamber and blades and other hot-end components,and it is already difficult to meet the requirements of using single-crystal nickel-based high-temperature alloy alone.The application of thermal barrier coating on the alloy surface can effectively solve the above problems.Compared with the most widely used yttrium oxide stabilized zirconia（YSZ）thermal barrier coated ceramic materials,high entropy rare earth zirconate ceramic（HE-RE₂Zr₂O₇）materials are rapidly developing due to lower thermal conductivity,excellent high temperature stability and sintering resistance.Therefore,in this paper,sintered blocks of（Nd Gd YEr Yb）₂Zr₂O₇ high-entropy ceramics were prepared by high-temperature solid-phase reaction method using Nd₂O₃,Gd₂O₃,Y₂O₃,Er₂O₃,Yb₂O₃,and Zr O₂ as raw materials.The thermal conductivity,thermal expansion coefficient,thermal stability,hardness and corrosion behavior of HE-RE₂Zr₂O₇ with CMAS（Ca O-Mg O-Al₂O₃-Si O₂）were investigated around the performance of HE-RE₂Zr₂O₇ and the actual service environment.The main studies and conclusions of the paper are as follows:（1）By calculating the difference value of cationic radii（δ）,equimolar high-entropy rare earth zirconate ceramics(Nd_0.2Gd_0.2Y_0.2Er_0.2Yb_0.2)₂Zr₂O₇ and non-equimolarhigh-entropyrareearthzirconateceramics(Nd_0.58Gd_0.05Y_0.05Er_0.05Yb_0.27)₂Zr₂O₇ withδgreater than 5.2%were prepared,and single-component Rare Earth Zirconate ceramic Nd₂Zr₂O₇was used as a control.The phase structures of the three ceramic materials were analyzed by XRD and Raman spectroscopy.It was found that Nd₂Zr₂O₇had a pyrochlore structure,non-equimolar(Nd_0.58Gd_0.05Y_0.05Er_0.05Yb_0.27)₂Zr₂O₇ had a dual-phase structure of pyrochlore and defect-fluorite,and equimolar(Nd_0.2Gd_0.2Y_0.2Er_0.2Yb_0.2)₂Zr₂O₇had a defect-fluorite structure.This indicates that High-Entropy Rare Earth Zirconate materials with a difference value of cationic radii（δ）greater than 5.2%can form ceramics with dual-phase structure of pyrochlore and fluorite.（2）Within the temperature range of room temperature to 1200℃,the average thermal expansion coefficients of(Nd_0.58Gd_0.05Y_0.05Er_0.05Yb_0.27)₂Zr₂O₇and(Nd_0.2Gd_0.2Y_0.2Er_0.2Yb_0.2)₂Zr₂O₇ are measured to be 12.6×10^-6 and 12.9×10^-6 K^-1,respectively.These values are slightly higher than the thermal expansion coefficient of the single-component Nd₂Zr₂O₇ ceramic,which is 11.2×10^-6 K^-1.This suggests that high-entropy design can increase the thermal expansion coefficient of the material to some extent.This observation may be attributed to the relatively lower electronegativity and lattice energy among the constituent elements.Furthermore,the thermal conductivity of high-entropy ceramic materials is lower than that of Nd₂Zr₂O₇,with the dual-phase structured high-entropy ceramics exhibiting the lowest thermal conductivity.The mechanical analysis results indicate that the Vickers hardness of(Nd_0.58Gd_0.05Y_0.05Er_0.05Yb_0.27)₂Zr₂O₇ and(Nd_0.2Gd_0.2Y_0.2Er_0.2Yb_0.2)₂Zr₂O₇was better than that of YSZ,however,due to the lack of low-temperature phase transformation toughening and high-temperature iron-ballistic toughening mechanisms,their fracture toughness is relatively lower.The study of the high-temperature phase stability of HE-RE₂Zr₂O₇ materials showed that they exhibited good phase stability without phase transformation occurring after annealing at 1300℃for 100 h and rapid cooling at different temperatures,which is in line with the selection requirements for ceramics used as thermal barrier coatings.（3）The corrosion behavior of(Nd_0.2Gd_0.2Y_0.2Er_0.2Yb_0.2)₂Zr₂O₇ high-entropy ceramic was studied in a molten CMAS（calcium-magnesium-aluminum-silicate）environment at 1250 and 1300℃.The corrosion mechanism is described as follows:Firstly,Ca O and Si O₂ infiltrate along the defects or grain boundaries within the high-entropy ceramic and react with rare earth oxides to form apatite.The formation rate of apatite increases with the increasing ionic radius of RE³⁺ions.Simultaneously,the high-entropy ceramic undergoes decomposition,resulting in the formation of calcium and rare earth-stabilized Zr O₂.With prolonged corrosion time,Mg O and a portion of Al₂O₃ on the sample surface form spinel compounds,specifically Mg Al₂O₄.Under corrosion at 1250℃,a continuous and dense layer of apatite is formed,which may serve as a barrier to the continuous penetration of CMAS.However,under the same conditions,the high-entropy ceramic exhibits persistent corrosion at 1300℃.