Preparation And Thermophysical Properties Of The （La₀.2Nd₀.2Sm₀.2Eu₀.2Me₀.2）₂Zr₂O₇（Me=Gd,Y,Yb,Lu） High-entropy Ceramics

Thermal barrier coatings（TBCs）are ceramic coating materials deposited on the surface of turbine engine blades,which have high temperature resistance and good corrosion resistance.State of the art,the most widely used TBC material is 6-8wt%yttrium oxide partially stabilized zirconia（YSZ）.When the temperature exceeds 1200°C,YSZ will undergo phase transformations and sintering densification with a consequnence of premature failure.Therefore,the development of new TBC materials becomes crucial.A₂B₂O₇-type oxide ceramic Gd₂Zr₂O₇ is expected to become a new candidate TBC material due to its low thermal conductivity and high phase stability.However,it has the disadvantages of low fracture toughness and low thermal expansion coefficient,which affect its application.Inspired by high-entropy ceramics（HECs）,the“high-entropy”design of Gd₂Zr₂O₇ at A-site is expected to improve the performance of this material.In this paper,(La_0.2Nd_0.2Sm_0.2Eu_0.2Me_0.2)₂Zr₂O₇（Me=Gd,Y,Yb,Lu）HEC powders and Gd₂Zr₂O₇powder were prepared by Solution Precursor Plasma Spray（SPPS）method,and the corresponding ceramic bulks were prepared by pressureless sintering.The results can be drawn based on the experiments investigation as follows:（1）The thermal decomposition process of the(La_0.2Nd_0.2Sm_0.2Eu_0.2Me_0.2)₂Zr₂O₇HEC precursor undergoes four stages:the first and second stages include the volatilization of adsorbed water and partial dehydration of nitrate;the third stage is the removal of nitrate crystal water and the decomposition of zirconium acetate;the fourth stage is that the rare earth nitrates decompose into the corresponding oxide,which enter into the Zr O₂ lattice forming a solid solution with defective fluorite structure.（2）The SPPS prepared spherical(La_0.2Nd_0.2Sm_0.2Eu_0.2Me_0.2)₂Zr₂O₇HEC powders are single crystal with defective fluorite structure and uniformly distributed elements.The LNSEGZ and GZ bluks are pyrochlore single-phase and defect fluorite single-phase,respectively,while the other three HECs bulks are pyrochlore-defect fluorite dual-phase.All the five ceramic bulks have uniform distribution of elements,good phase stability and relative densities greater than 87%.The average grain size of the four HEC bulks is smaller than the GZ bulk.（3）The LNSEYb Z HEC bulk has the highest microhardness of 11.5±0.75 GPa,which is46.11%higher than the GZ bulk.The fracture toughness of the LNSELu Z HEC bulk is the highest with a value of 1.40±0.06 MPa·m^1/2,which is 25.00%higher than the GZ bulk.The thermal conductivity of the LNSEGZ HEC bulk is the lowest with a value of 0.90 W·m^-1·K^-1（1200°C）,which is 10.9%lower than the GZ bulk.The average thermal expansion coefficient of the LNSEGZ HEC bulk is 10.95×10^-6K^-1（200-1400℃）,which is 5.09%higher than the GZ bulk.（4）The high temperature phase stability of the five ceramic bluks is good.The microhardness of the four HEC bulks dosen’t change significantly before and after heat treatment at 1500 ^oC,whereas the microhardness of the GZ bulk increases to some extent after heat treatment due to sintering densification.The fracture toughness of the LNSEGZ HEC bulk decreases,and the fracture toughness of the LNSEYZ,LNSEYb Z and LNSELu Z HEC bulks don’t change significantly,whereas the fracture toughness of the GZ bulk increases after heat treatment.The thermal conductivities of the five ceramic bulks increase after heat treatment,and the LNSEGZ HEC bulk has the lowest thermal conductivity of 1.16W·m^-1·K^-1（1200℃）after 300 h heat treatment,which is 3.33%lower than that of the GZ bulk.The thermal expansion coefficients of the five ceramic bulks don’t change significantly before and after heat treatment,and the thermal expansion coefficient of the LNSEGZ HEC bulk is 11.42×10^-6K^-1（200-1400℃）after 300 h heat treatment,which is 8.56%higher than the GZ bulk.In summary,the comprehensive properties of the(La_0.2Nd_0.2Sm_0.2Eu_0.2Me_0.2)₂Zr₂O₇（Me=Gd,Lu）HECs are better than that of the Gd₂Zr₂O₇ ceramic,which can be considered as new TBC candidate materials.