Preparation And Performance Study Of (5RE_0.2)₂Zr₂O₇ High Entropy Ceramics

High entropy ceramics（HECs）are inorganic compound solid solution with one or more Wyckoff positions occupied by multiple atoms of equal or near equal proportions.The emergence of this new material has brought new opportunities for material design and performance tailoring.At present,the most widely used thermal barrier coating material is8 wt.%Y₂O₃partially stable Zr O₂（8YSZ）,which has high melting point,low thermal conductivity,and coefficient of thermal expansion close to the base material.The suitable working temperature for 8YSZ can only be below 1200℃.When the temperature exceeds1200℃,phase transformation and sintering phenomena will occur,leading to a decrease in its thermal physical and mechanical properties,causing coating cracking,and ultimately leading to the failure of the thermal barrier coating.New characteristics such as low thermal conductivity,high thermal expansion coefficient and good mechanical properties have been found in HECs,which brings opportunities to develop new thermal barrier materials.RE₂Zr₂O₇generally has two types of structures:pyrochlore structure and defective fluorite structure.Compared with 8YSZ,it has lower thermal conductivity,higher hardness and fracture toughness,and is an excellent candidate material for thermal barrier coatings.However,its low coefficient of thermal expansion limits its development.In order to improve the coefficient of thermal expansion of RE₂Zr₂O₇and further reduce the thermal conductivity,five rare earth elements were used to dope A site i n equal proportion,and five components(5RE_0.2)₂Zr₂O₇high entropy ceramics and La₂Zr₂O₇ceramics were prepared by Solution precursor plasma spraying（SPPS）:(L a_0.2Nd_0.2Sm_0.2Y_0.2Yb_0.2)₂Zr₂O₇（LNSYYb Z）,(La_0.2Nd_0.2Sm_0.2Y_0.2Lu_0.2)₂Zr₂O₇（LNSYLu Z）,(La_0.2Nd_0.2Sm_0.2Yb_0.2Lu_0.2)₂Zr₂O₇（LNSYb Lu Z）,(La_0.2Nd_0.2Y_0.2Yb_0.2Lu_0.2)₂Zr₂O₇（LN YYb Lu Z）,(La_0.2Sm_0.2Gd_0.2Y_0.2Yb_0.2)₂Zr₂O₇（LNSYYb Z）and La₂Zr₂O₇（LZ）。The thermal decomposition process of precursor solution is divided into three stages.The first stage:between room temperature and 250℃,the decomposition products include anhydrous nitrate and water of crystallization;Phase 2:Between 250℃and 400℃,anhydrous nitrate decomposes to form basic nitrate,NO and NO₂,and Zr（C₂H₃O₂）₄decomposes to form Zr O₂and H₂O.The third stage:between 400℃and 600℃,the basic nitrate decomposes to form rare earth oxides and the residual Zr（C₂H₃O₂）₄decomposes to form Zr O₂and H₂O.After the crystallization process of the precursor solution at 600℃,rare earth oxides（La₂O₃,Nd₂O₃,Sm₂O₃,Gd₂O₃,Y₂O₃,Yb₂O₃,Lu₂O₃）in the precursor react with Zr O₂to form(5RE_0.2)₂Zr₂O₇high entropy rare earth zirconate.Five components(5RE_0.2)₂Zr₂O₇high entropy ceramics and LZ ceramic powders were prepared by solution precursor plasma spraying.The prepared ceramic powders are all of defect fluorite single-phase structure,with a spherical microstructure and uniform distribution of rare earth elements,indicating the successful preparation of high entropy ceramic powders.Five components(5RE_0.2)₂Zr₂O₇high entropy ceramics and LZ ceramic blocks were prepared using pressureless sintering.The XRD results indicate that all high entropy ceramic blocks have a dual phase structure of pyrochlore defect fluorite,while LZ ceramics have a single-phase structure of pyrochlore.The smaller the r _A/r _Bof(5RE_0.2)₂Zr₂O₇,the fewer the pyrochlore phase and the more defective fluorite structure.Five components(5RE_0.2)₂Zr₂O₇high entropy ceramics have excellent thermal physical and mechanical properties.Among them,the coefficient of thermal expansion is improved,and the average coefficient of thermal expansion of(5RE_0.2)₂Zr₂O₇high entropy ceramics is about 10.31 to 10.78×10^-6K^-1,the average coefficient of thermal expansion of LZ ceramics is(9.02×10^-6K^-1,200～1400℃),14～20%higher.The thermal conductivity further decreased,with the thermal conductivity of high entropy ceramics ranging from 1.23 to1.45 W·m^-1·K^-1at room temperature,which is 15-25%lower than that of LZ ceramics(1.70 W·m^-1·K^-1at room temperature).The thermal conductivity of high entropy ceramics at 1400℃is about 20%lower than that of LZ ceramics.The hardness of high entropy ceramics is about 10 GPa,which is about 60%higher than the hardness of LZ ceramics（6.34±0.41 GPa）.The fracture toughness is about 1.1 MPa·m^-1/2,which is about 80%higher than the fracture toughness(0.59 MPa·m^-1/2)of LZ ceramics.In conclusion,the prepared five component(5RE_0.2)₂Zr₂O₇high entropy ceramics show excellent performance.The coefficient of thermal expansion and mechanical properties are improved through the high entropy effect,and the thermal conductivity is further reduced.It is a new ceramic thermal barrier coating material with good comprehensive performance.