Microstructure And Properties Of The Spark Plasma Sintered TaC-Based Ceramic

The (Zr,Ta)C ceramic was spark plasma sintered (SPS) at 1600-1000 oC using TaC,ZrC ,Ta,Zr and C as the starting powders. Densification process, microstructure development, phase evolution, mechanical properties and thermal shock performance with the influence of temperature, content of ZrC and ratio of carbon were investigated by means of XRD,SEM and TEM.The result indicated that the theoretical density of the (Zr,Ta)C ceramics rise from 84.7%to 95.4% with the temperature increase from 1600℃to 1900℃,the densification process was studied by analysis of the densifying shrinkage, and three distinct stage of densification process were determined. The curve of the vacuum indicated an evaporation-condensation process possibly exist.The XRD pattern indicate that there were two distinct phase in the sample at 1600℃.At around¹700℃,only one phase was found in the XRD result. The peak of the X-Ray diffraction shift to the small angle along with the increase of the temperature, which indicated the solid solution between Arc and Taste results of TEM also demonstrated this phenomenon, the concentration gradient of the Or atom in the grain was observed,also.The theoretical density of (Zr,Ta)C material significant improved to ⁹8% with rapid grain growth from 2μm to around 5μm.Little entrapped residual pore existed in the grain.Vickers hardness and flexure strength increase along with the sintering temperature. The irregular distribution graphite in the microstructure harm the properties of the material。The fractgraphs show typical intergranular frature mode with no large scale material flaws,and no toughening mechanis.The theoretical density of the (Zr,Ta)C ceramics increase with the content of ZrC, though not obviously. The XRD pattern indicated that only one phase exist in the material. The grain size increase with the ZrC content, while the mechanical properties of the material are opposite. Best and lowest flexure strength is around 653MPa and 421MPa,respectively. The fractgraphs show typical intergranular frature mode with no large scale material flaws, and no toughening mechanis, same with the earlier results. SEM image of the materials show that the microstructure was compose of lighter ZrO₂ grains uniformly distributed in the (Zr,Ta)C matrix, the ZrO₂ probably come from the ZrO₂ grinding ball.Different phase of TaC ceramics were prepared through changing the ratio of the C atom. The results indicated that two indistinct phase Ta2C and Tay exist in the material when the ratio is 0.55,respectively.A new rhombohedral phaseζ-Ta₄C_3-x appeared in the XRD pattern along with the C ratio to 0.68.Only one phase was observed when the ratio increase to 0.8,which was demonstrated to be the face-centered cubic TaC, The diffusion of C explained the Ta₂C andζ-Ta₄C_3-x phase. SEM images indicated theζ-Ta₄C_3-x was lamellar, around 20³0μm, the presence of the lamellarζ-Ta₄C_3-x regions enhanced the flexure strength. The deflection and bifurcation of the crack enhanced the fracture toughness of the material.Thermal shock performance after 600,800,1000 oC were investigated. Results show that Ta₂O₅ formed at around⁸00℃,the residual strength of material decreased along with temperature,while with a little pick up around ¹000℃.SEM results indicated that grains crack at 600 and 800℃,and the interface adhesion strength decrease at the same time, which lead the decrease of the strength. Dense oxidation layer was found on the face of the material shocked at 1000℃.The strength pick up with less crack in the grains.