Spark Plasma Sintering And Characterization Of Ta₄HfC₅-Based Ultrahigh Temperature Ceramics

In this paper,Ta₄HfC₅ powder and Ta₄HfC₅-MoSi2 powder were used as raw materials to prepare Ta₄HfC₅ ternary ceramics by spark plasma sintering?SPS?.The effects of SPS sintering process parameters on the phase composition of sintered Ta₄HfC₅ ceramic were studied.Structure,mechanical properties,sintering mechanism and densification process were analyzed,and its antioxidant activity was studied.The XRD patterns of Ta₄HfC₅ powder and Ta₄HfC₅-Mo Si2 powder were studied.The results showed that the powder was single phase with high crystallinity.Ta₄HfC₅ and Ta₄HfC₅-MoSi2 ceramic materials were prepared by SPS preparation process under different sintering pressure and different sintering temperature.With the increase of sintering pressure and sintering temperature,the grain size became larger and the number of intergranular pores decreased,The density of Ta₄HfC₅ ceramics increased from 81.61% to 88.60%,and the maximum bending strength and fracture toughness were 244 MPa and 2.56MPa·m1/2.The density of Ta₄HfC₅-Mo Si2 ceramics increased from 91.15% to 99.19%,and the bending strength and fracture toughness were 722 MPa and 5.61MPa·m1/2,respectively.The sintering mechanism of Ta₄HfC₅ ceramic materials prepared by SPS was studied.During the preparation of SPS,there were pores between the particles in the initial stage,and the gas in the pores collapsed under the action of the external electric field,resulting in spark discharge,release a large number of plasma.High speed plasma Impinging on the surface of the powder particles can remove the surface oxide film and adsorbed gas.Compared with the traditional sintering process,The powder particle surface was more easily purified and activated.The sintering process was accelerated with the increase of sintering pressure and temperature Densification was greatly improved with doped Mo Si2 second phase as sintering aid.During the oxidation of the Ta₄HfC₅ powder TG-DSC,the carbon retention caused the maximum increase over the theoretical calculation.Under different air traffic,the higher of the air mass flow,the initial oxidation temperature was decreased slightly.And the DSC exothermic peak shift to the right.TG-DSC of Ta₄HfC₅ ceramic under the same oxidation condition,Ta₄HfC₅ initial oxidation temperature?850??was higher than the ceramic powder?540??.But there was no carbon retention phenomenon as a result of Ta₄HfC₅ ceramic reaction interface less than Ta₄HfC₅ powder.The Ta₄HfC₅ ceramics prepared by adding Mo Si2 were subjected to high temperature thermostatic oxidation experiment.After the oxidation of 1000¹200?,the porous structure ?-Ta2O5 and HfO2 were formed on the surface,and the residual phase was not observed on the surface.The microstructures of the surface of the oxidized material were analyzed by SEM.It was found from the SEM that the oxidized layer was filled with a large number of pores,and the pore size and the number increased with the increase of the oxidation temperature.The isothermal oxidation dynamics of Ta₄HfC₅-MoSi2 ceramics were studied.The oxidation process was controlled by diffusion reaction in the range of 1000¹100? for 15 min,and the oxidation process was controlled by diffusion and the oxidation of 1200? was the diffusion control process.The apparent activation energy of the oxidation reaction in the range of 1000¹200? was calculated,and the apparent activation energy was 33.54KJ/mol in the oxidation reaction at 1000¹200?.