Sintering Densification And Properties Of (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C High Entropy Ceramics

Supersonic aircraft has always been the commanding heights of strategic competition between major powers.Its service environment is extremely harsh.The nose and wings,with a higher radius of curvature,reach more than 2000°C,because of the high-speed airflow.It requires materials with excellent high-temperature mechanical and thermal properties.High-entropy ceramics composed of more than five transition metal carbides have extremely promising applications.High-entropy ceramics can inherit the advantages of each mono-carbide,also,lattice distortion can further improve the mechanical properties.The high-entropy effect of thermodynamics and the delayed diffusion effect of kinetics can give them more excellent thermal properties,especially at high temperatures.However,the delayed diffusion effect brought by multiple components makes the sintering densification of high-entropy ceramics require high-temperature and pressure field assisted sintering.There is no case of pressureless sintering.At the same time,the existing mechanical and high-temperature performance tests of high-entropy ceramics are not perfect.This article focuses on the sintering densification behavior,microstructure and performance analysis of(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C high-entropy ceramics,and mainly explores its structure-activity relationship.After confirming that the use of the most widely used the spark plasma sintering（SPS）can obtain nearly fully densified single-phase(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C ceramics,the mechanical and high-temperature ablation resistance properties are tested;Afterwards,the pressureless sintering（PS）densification it was focus on,by optimizing the sintering system and additive content,regulating the microstructure of the material,and exploring the mechanical and thermal properties.The main research contents include:（1）Single-phase(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C high-entropy ceramics was sintering by SPS.The lattice constants were basically the same,with the relative density>99%.the Vickers hardness～19 GPa and fracture toughness～3 MPa·m^1/2.In the ablation test,the linear ablation rate of(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C decreased,with the increase of the ablation test time and the microscopic morphology becomes smoother after the surface gradually melts.Through the analysis of phase and valence,it can be found that Ti oxides are formed and lost preferentially,followed by Ta,Nb,and finally Hf,Zr.The part of the oxide film close to the substrate was denser,which could protect the substrate better.（2）SPS was used to sinter Si C-(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C multiphase high-entropy ceramics.The relative density was above 99.0%and the Si C-second phase was evenly distributed.In the ablation test,it is found that Si C would preferentially oxidize to form glassy oxide,and provide protection for high-entropy ceramic substrate.The addition of 10 vol%Si C could reduce the linear ablation rate to 1/10.With the increase of Si C content,the surface shows more obvious network structure and holes,the linear ablation rate increased,and the ablation resistance decreased.（3）The carbon obtained from the pyrolysis of 1-6 wt%phenolic resin was used as a sintering aid to obtain a single-phase(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C high-entropy ceramic by PS.With different contents of pyrolysis,the lattice constants was similar.With 4 wt%phenolic resin,(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C has the highest density,96.6%,and exhibits the best thermal properties.While a large amount of carbon causes more weak bonding surfaces and lower mechanical properties.With 2 wt%phenolic resin,(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C exhibits the best mechanical properties.The fracture toughness of(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C is significantly higher than the corresponding mono,binary and ternary carbide ceramics.（4）A two-step sintering experiment was carried out on the sintering of(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C.With the increase of preservation time and two step temperature,the density increases,eventually reaching 97.9%.But the size of graphite at the grain boundary also increases significantly,which damages the mechanical properties.The sample sintering at 2000℃for 4 h have the best comprehensive mechanical properties.