Preparation And Densification Mechanisms Of Tantalum Carbide Ceramics

Tantalum carbide（TaC）is one of the important ultrahigh temperature ceramics due to its excellent properties,such as,high melting point,high elastic modulus,high hardness,high flexural strength,good thermal conductivity and good ablation resistance.That makes it become a promising candidate in the fields of cemented carbide,aviation aerospace,biology medicine and electronic materials and so on.However,TaC powder is extremely hard to sinter due to the strong covalent bonding of Ta–C and low self-diffusion coefficient.The particle size,shape,distribution,composition,agglomeration degree and chemical purity of the powder have significant impacts on the subsequent molding and densifying.Commercially available TaC powder exhibits extremely poor sinterability owing to the large grain size and low purity.However,the synthesis of high-purity and ultra-fine TaC powder is so difficult,where few studies have been done.Moreover,it is vital to clarify the densification mechanism of TaC ceramics for the formulation of sintering process and the quality control of sintering products.In this work,high-purity TaC nanopowder was synthesized using a combination of sol–gel method and spark plasma sintering（SPS）technique,and dense TaC ceramics were prepared by SPS sintering.The microstructure evolution and densification kinetics during the sintering process were investigated,and the mass transfer and grain growth mechanisms at different sintering stages were revealed.With that,high-performance TaC ceramics were fabricated using SiC as sintering additive.This work provides a theoretical support for reasonably establishing and optimizing sintering process of TaC ceramics.It is expected to realize the low temperature preparation of ultra-high temperature ceramics with excellent mechanical properties.The main contents and conclusions of this work are as follows:（1）Synthesis of TaC nanopowder using a combined sol–gel method and SPS technique:Gels of Ta-containing chelate with good uniformity and high stability were prepared by solution-based processing using TaCl₅ and phenolic resin as the sources of Ta and C,respectively.The composite powder composed of the carbon shell and Ta-containing oxide core were obtained after pyrolysis.Further heat treatment by SPS resulted in the fast formation of TaC.The average particle size,oxygen and free carbon contents of TaC powder synthesized at a pyrolysis temperature of 800℃,synthesis temperature of 1500℃,soaking time of 5 min and heating rate of 200℃/min are 50 nm,0.43 wt.%and 0.52 wt.%respectively.Furthermore,this process had its unique advantages in the synthesis of TaC powder.On the one hand,the core-shell structure of pyrolysis products greatly increased the contact area between the Ta-containing oxide and C,and shorten the diffusion distance of Ta,C and O atoms.So that,the Ta-containing oxide was more prone to be carbonized in the subsequent carbothermal reduction process.On the other hand,compared with the traditional heating method,fast heating rate of SPS（≥100℃/min）could not only shorten the time required for the synthesis of TaC powder,but also reduce the synthesis temperature.（2）Effects of carbon content on synthesis and densification of TaC powder:The effects of carbon content on the reaction yield,microstructure,chemical compositions and sinterability of TaC powder were investigated by controlling the C/Ta ratio in the Ta-containing precursor.It was found that a high C/Ta ratio was more favorable to formation of the TaC powder.With the increasing C/Ta ratio,the average particle size and oxygen content of TaC powder decreased,whereas the free carbon content increased,which was conducive to eliminate the oxide on the powder surface and enhance the densification of TaC powder.However,that had a negative impact on the mechanical properties of TaC ceramics.The hardness and fracture toughness of TaC ceramics increased firstly and then decreased with the increase of C/Ta ratio.The optimal mechanical properties for TaC ceramics were obtained at a C/Ta ratio of 4.00.It was mainly because that at a high C/Ta ratio,the residual carbon was enriched at the grain boundary,resulting in weak interface bonding between TaC grains.It was recommended to sinter the TaC powder prepared at a C/Ta ratio of 4.00 for obtaining dense TaC ceramics at 1900℃for 5 min under 80 MPa,whose relative density,average grain size,Vickers hardness and fracture toughness were 97.86%、4.28μm,16.54 GPa and 3.72 GPa·m^1/2,respectively.（3）Sintering and densification mechanisms of TaC powder:The sintering densification and grain growth behavior of TaC powder under different sintering conditions were studied,when TaC ceramics were prepared by SPS.The results showed that the rise of sintering temperature had little contribution to the densification of TaC ceramics,whereas the TaC grains were obviously coarsening when the sintering temperature exceeded 1800℃.TaC ceramic with the fine and dense microstructure was obtained at 1800℃for 10 min under 80 MPa,which had a relative density of 97.19%,average grain size of 2.67μm,Vickers hardness of 15.6GPa and fracture toughness of 3.59 MPa·m^1/2.With increasing sintering pressure from30 MPa to 100 MPa,the relative density of TaC ceramics increased significantly（93.06%→98.06%）,while its increase also caused an increase in the grain size of TaC ceramics（1.53μm→3.16μm）.Furthermoreover,based on Bernard-Granger creep model,the densification mechanisms of TaC powder at 1600～1800℃/80 MPa was analyzed.Diffusion and grain boundary sliding were confirmed to be the dominant densification mechanisms based on the stress（n）and grain size exponents（m）combined with the microstructure evolution.The grain growth mechanisms at final sintering stage were grain boundary diffusion and lattice diffusion.（4）Preparation and properties of TaC–SiC ceramics:The sinterability of TaC was improved to some extent by using high-purity TaC nanopowder as raw material.However,the fully dense TaC ceramics were still unable to be obtained,resulting in the poor mechanical properties of TaC ceramics.To improve the mechanical properties of TaC ceramics,TaC–SiC composites were fabricated at 1800℃/80 MPa using SiC as sintering aid.The effects of SiC on the microstructure,densification behavior and mechanical properties of TaC–SiC ceramics were investigated.It was concluded that the addition of SiC greatly promote the densification of TaC–SiC composites.The SiC particles pinned at the TaC grain boundary,which effectively suppressed the growth of TaC grains and increased mechanical properties of TaC/SiC composites.The relative density,average grain size,Vickers hardness and fracture toughness for the TaC–SiC ceramic with a SiC content of 40 vol.%were 99.37%,1.36μm,20.73 GPa,5.62 MPa·m^1/2,respectively.Compared with pure TaC ceramics,the hardness and fracture toughness of TaC–SiC composites were increased by 32.9%（15.60 GPa→20.73 GPa）and 57.5%(3.59 MPa·m^1/2→5.62 MPa·m^1/2),respectively.The main toughening mechanisms of TaC–SiC composites were crack deflection and crack branching.In addition,the oxidation resistance of TaC–SiC composites was improved with the increase of SiC content.