Optimization Of Processing And Mechanical Properties Of TaC_x(x=0.3-0.6)-Ni Composites

Tantalum carbide ceramics have been investigated for potential applications in high temperature structural components,thermal protection and electronic industry due to extremely high melting temperatures,good thermal/electrical conductivity and excellent mechanical properties.Among the tantalum carbides,i.e.hexagonal Ta₂C,rhombohedralζ-Ta₄C_3-x and cubic TaC_y which are defined by both C:Tastoichiometry and lattice structure,the hexagonal Ta₂C phase has relatively better oxidation resistance,thus is worth more detailed understanding.In this work,TaC_x（x=0.3-0.6）-Ni composites with Ni as a sintering additive were prepared by hot pressing under conditions of 1100-1900℃/10-30MPa/30-60min,using TaC,Taand Ni powder as starting materials.Effects of C:Tastoichiometries,Ni additive and hot pressing parameters on densification,phases microstructures and mechanical properties were investigated by X-ray diffraction,scanning electron microscopy,transmission electron microscopy,X-ray energy dispersion spectroscopy,selected area electron diffraction,indentation hardness test,three-point bending,and single edge-notched beam bending.The research results include:（1）In composition TaC_0.5-20 vol%Ni,effects of hot pressing temperature change in range of 1100-1900°C was revealed in view of reaction densification,phase formation and microstructure development.Intermetallic compound Ni₃Tawas formed by reaction 3Ni+Ta=Ni₃Tatook place at 1100-1300°C.At high temperatures over the melting point of Ni₃Ta（T_m=1520℃）,the Ni₃Tafacilitate densification of the composites;Therefore,samples that were hot pressed at 1600°C and above showed relative densities>98%.In the same temperature range,Ta₂C started to form due to reaction TaC+Ta=Ta₂C+TaC_y and at higher temperatures continuous happening of TaC_y+Taresulted inζ-Ta₄C_3-x phase formation due to consumption of Taby Ni₃Ta,deviating residual C:Tastoichiometry into theζ-Ta₄C_3-x phase filed in the Ta-C phase diagram.The samples prepared at 1100-1300°C had low mechanical properties for poor densification while the 1600°C material showed the best bending strength（686 MPa）and fracture toughness(7.02 MPam^1/2)for formation a uniform microstructure consisted of fine laminar grains.When a higher hot pressing temperature of 1900°C was used,coarsening of the microstructure lead to lower strength.Additionally,squeezing and Ni₃Taloss was observed when hot pressing of TaC_0.5-20 vol%Ni at 1600and 1900°C.（2）In composition TaC_x（x=0.5,0.55,0.6）-20 vol%Ni₃Ta,Ni as an additive was replaced by Ni₃Ta.Samples were prepared by hot pressing under 1800℃/20MPa/30min and effects of C:Tastoichiometries at x=0.5,0.55,0.6 on densification,phases,microstructures and mechanical properties were investigated.The results showed phase components in line with Ta-C phase diagram predication,i.e.the predominant phase in the x=0.5,0.55,0.6 material was Ta₂C,Ta₂C+ζ-Ta₄C_3-x andζ-Ta₄C_3-x respectively.Microstructures of the composites were consisted of laminar grains and accordingly showed good bending strength and fracture toughness.（3）In composition TaC_0.3-20 vol%Ni,C:Tastoichiometry was lowered to x=0.3to counteract effect of Taconsumption due to Ni₃Taformation.The hot pressing temperature was varied in range of 1100-1900℃ to investigate its effects on densification,phases,microstructures and mechanical properties of the composites.The research results showed that densification increased with temperature and fully dense composites could be obtained by hot pressing at T>1600°C.The samples prepared at1900℃was consisted of single phase Ta₂C according to XRD.However,over-grown laminar grains decreased mechanical properties.The samples prepared at 1600℃showed good bending strength（689±71 MPa）and fracture toughness(5.72±0.57MPa·m^1/2).Mass loss due to squeezing of Ni₃Tawas also observed.