Microstructure Control And Mechanical Properties Improvement Of MeC(Me=Zr,Ti) Based Ultrahigh Temperature Ceramics

MeC（Me=Zr,Ti）ultra-high temperature ceramics have attracted much attention due to their excellent comprehensive properties such as high hardness,high modulus,and good corrosion resistance.MeC（Me=Zr,Ti）exhibits excellent potential in the application of ultra-high temperature environment and are urgently needed in the field of aerospace and nuclear energy.However,poor thermal shock resistance and fracture toughness limit its application under extreme conditions.In particular,using pure-phase transition metal carbides has great limitations.At the same time,because of its strong covalent bond and low self-diffusion coefficient,it is difficult to obtain fully dense ceramics by the conventional sintering process.Using high sintering temperatures to overcome this problem will lead to excessive grain growth,resulting in poor mechanical properties.Recent studies have shown that the addition of sintering aids can improve the sintering problem of materials and enhance the related properties of materials.For ZrC ceramics,dense（Zr,Ti）（C,N）solid solution ceramics with TiCN content of 5 mol%-50 mol%were prepared by spark plasma sintering at 1900-2000℃.In order to further reduce the sintering temperature,10 mol%ZrH₂ was added as the sintering additive.The effects of TiCN content and ZrH₂ addition on the microstructure and mechanical properties of ZrC-based ceramics were studied.The addition of TiCN and ZrH₂ to form solid solution ceramics and vacancies effectively promoted the densification process,reduced the critical resolved shear stress and promoted the diffusion of carbon and metal atoms.When the TiCN content reaches 5 mol%and 20 mol%,a single Zr（Zr,Ti）（C,N）-rich solid solution is formed in the ceramics.When the content of TiCN increases to 50 mol%,with the formation of Ti-rich phase solid solution,the（Zr,Ti）（C,N）solid solution ceramics show good microstructure,and the grain size is 0.3-0.4μm.The Vickers hardness and flexural strength of（Zr,Ti）（C,N）solid solution are higher than those of pure ZrC ceramics,which can be attributed to solid solution strengthening,smaller grain size and lower porosity.The improvement of fracture toughness is attributed to crack deflection and crack bridging formed by Zr rich and Ti rich solid solutions.ZrC-45 mol%TiCN-10 mol%ZrH₂ possesses good comprehensive mechanical properties(Vickers hardness of 24.5 GPa,flexural strength of 503 MPa,and fracture toughness of 4.3 MPa·m^1/2).For TiC ceramics,dense（Ti,Zr）C-Zr Si-Ti₃Si C₂-Si C ceramics were prepared by hot-pressing sintering at 1400-1600°C with Si and ZrC.The effects of ZrC content and sintering temperature on the microstructure and mechanical properties of TiC-based ceramics were studied.When ZrC content is 10 mol%,Ti₃Si C₂ phase exists.When the ZrC content increased to 30 mol%,Zr-rich solid solution appeared,Ti₃Si C₂ phase disappeared,and Zr Si phase content increased significantly.The increase in sintering temperature will cause the decomposition of Ti₃Si C₂ phase and the decrease of Zr Si content.The formation of the liquid phase,carbon vacancy and solid solutions during sintering contributes to the densification process.Intergranular and intragranular structures containing Si C nanoparticles were introduced into ceramics.The grain refinement is due to the pinned Si C phases and solid solution coupling effect.When the temperature reaches 1600°C,the Zr-rich phase can inhibit grain growth.The increase of solid solution and Si C content will improve the Vickers hardness of ceramics.The main toughening mechanisms were the crack deflections and bridging caused by the formation of Ti₃Si C₂.The synergistic effect of grain refinement,good fracture toughness and solid solution formation enhances the flexural strength.TiC-36 mol%Si-10mol%ZrC ceramics exhibit good mechanical properties(Vickers hardness of 21.8GPa,flexural strength of 580 MPa,and fracture toughness of 6.7 MPa·m^1/2).