Study on Microstructure and Mechanical Properties of Tantalum Carbide Ceramics Fabricatied by SPS

Author:Zhong Lian Bing

Supervisor:ye feng liu li meng


Degree Year:2018





Aimed at the difficulty of densification of TaC ceramics,the technology of low-temperature sintering TaC ceramics was optimized by using the SPS sintering processes.Effects of adding Al,IB metals of Cu,Ag and Au,Si and Si3N4 particels on densification,phases,microstructure and mechanical properties of the as-sintered TaC ceramics were investigated systematically.TaC ceramics with different stoichiometric ratio were fabricated using TaC and Ta powders as raw material.Microstructure evolution and the relationship between microstructure and properties were illuminated.Densification temperature of TaC ceramics can be significantly reduced by adjusting the SPS processes.Microstructure evolution and densification of non-stoichiometric TaCx ceramics(fabricated using TaC and Ta as raw materials)were composed of 2 or 3 distinct stages,and the stage happening at the temperature range of1450-1800℃ was crucial.Because evaporation-recondensation at high temperature could lead to the coarsening of microstructure.In addition,the pores are trend to converge at the poly-crystalline joint which will prevent them to be discharge.Dense TaCx ceramics could be obtained by spark plasma sintering at a lower temperature of1400℃,by avoiding the non-densifying mechanism of evaporation-recondensation,though bulk diffusion and boundary diffusion were limited at low temperatures,by reasonable using the limited TaC+Ta→TaC0.9 reaction diffusion,optimizing the sintering parameters could still result in>96%relative density.Densification of TaC ceramics was enhanced by additions of passive metal,Si and Si3N4.These sintering aids can also optimize the type and content of the scondary phases,regulate the interface structure,increasing the mechanical properties of TaC ceramics.(1)In case of metallic liquid sintering of TaC ceramics,when temperature reached the respective melting point of the Al,Cu,Ag and Au metals,the metal liquids wetted TaC particles to facilitate rearrangement of the particles.Coating of TaC particles by a thin layer metal liquid helped inhibit evaporation of oxides and also replaced the deleterious surface diffusion by grain boundary diffusion.When 5mol%Au was added and the SPS condition was 1700oC/30Mpa/5min,relativety densities of the TaC ceramics could reached values>97%.(2)In case of Si reactive sintering of TaC ceramics,when temperature reached 1413oC and above,Si melted to wet TaC particles and promoted particle rearrangement.Refractory phases of TaSi2 and SiC were fromed by reaction of Si with TaC.TaC grain growth was slowed down by distribution such secondary particles of TaSi2 and SiC on the TaC grain boundaries.At temperatures>1650oC,plastic deformation of the TaSi2 phase took place after brittle-to-ductile transformation.(3)In case of adding Si3N4 particles,reaction of Si3N4 with TaC and Ta2O5 formed SiC phase and various tantalum silicides,which will decrease O impurity,nail the TaC grain boundary,and promote the densification of the ceramics.TaC sintered by adding Si3N4 possessed excellent maechanical properties at high temperature.Densification of non-stoichiometric TaCx ceramics is closely related to the reaction of TaC+Ta→TaCx and the diffusion of C element.C diffusing from TaC into Ta particles formed intermediate Ta2C.Further C diffusion formed equilibrium TaCy orζ-Ta4C3-x.However,phase formation and densification processes took place incoinstantaneously.For the C/Ta=0.5,0.7 compositions,densification preceded phase formtions,whereas phase transformations were completed before starting to densify in the C/Ta=0.8-1.0 compositions.In case of research on TaC chemistry effects,results showed that C/Ta=0.5,0.7and 0.8-1.0 compositions produced monolithic hexagonal Ta2C,ζ-Ta4C3-x+TaCy composite,and cubic TaCy ceramics,respectively,which further effect the mechanical properties.The cubic TaC0.8-1.0 ceramics were consisted of brittle fine equiaxed grains in microstructures and showed high Vickers hardness;Largeζ-Ta4C3-x lamina dominated microstructures of the TaC0.7 composites and the composition had high flexural strength and fracture toughness;Microstructures of the monolithic hexagonal Ta2C were determined by sintering temperatures,to form fine equiaxed and elongated grains in the1500℃ and the 1700℃ material,respectively.At 1900℃ porosity was trapped intragranularly by the coalescing of the neighbouring grains.