Reaction Solid Solution Behavior And Microstructure And Properties Of ZrB₂-TiC Composite Ceramics

Borides and carbides of transition metals have excellent physical and chemical properties such as high hardness,modulus,thermal conductivity,and high thermal shock and ablation resistance,which has broad application prospects in extreme environments.In general,the refractory carbides are used as the sintering aids for removing the oxide impurities.In fact,in addition to the oxide removal reactions,some carbides could react with diborides to produce a series of multi-component solid solutions.In this work,with TiB₂-ZrC as the control system,the diffusion kinetics and phase equilibrium in ZrB₂-TiC ceramics were explored.The densification mechanism and grain growth behavior were systematically studied.Besides,the internal relationships between the phase formation,microstructure evolution and mechanical properties of the ceramics were also clarified.The effects of boride-carbide composition on the room and high temperature mechanical properties,thermophysical properties and oxidation performance have been investigated.The thermodynamic calculation results show that the ZrB₂-TiC and TiB₂-ZrC systems have exactly the same final state which are consist of（Ti,Zr）B₂ and（Zr,Ti）C solid solutions.However,the mixed powder sintering and the diffusion couple test show that ZrB₂-TiC exhibits much higher sintering activity than that of TiB₂-ZrC.Exchange reaction in TiB₂-ZrC is based on the pure diffusion process of metal atoms;while the reaction process in ZrB₂-TiC system is dominated by the diffusion of B andC atoms,it will promote the solid solution process of metal atoms.The inter-diffusion mass transfer driven by the reaction&solid-solution coupling effect in ZrB₂-TiC system promotes the densification of the material.Fully reaction and rapid densification could be achieved at 1800°C for 10 min in ZrB₂-50mol.%TiC.The sintering activation energy is 531±16 k J/mol,and the densification controlling factor is the lattice diffusion of Tiand Zr atoms.When the volume fraction of the two phases of the material is close,the Zener pinning grain boundary model fails,forming a coupled growth behavior of the two phases.The grain growth activation energy is 933 k J/mol,which greatly limits the grain growth.The average grain size of ZrB₂-50 mol.%TiC ceramics sintered at 1750°C is only～0.3μm.The optimized sintering process of ZrB₂-TiC ceramics is 1800°C/1 h/30 MPa.ZrC can dissolve into TiB₂ matrix to form a super-hard structure（Ti,Zr）（B,C）solid solution with close-packed hexagonal structure.At 2000°C,the solid solubility of ZrC in TiB₂ is 9 mol.%.At this time,the material has the best mechanical properties,and its hardness,elastic modulus,flexural strength and fracture toughness are 31.7GPa and 500 GPa,693 MPa and 5.58 MPa·m^1/2,respectively.When the ZrB₂ in the original powder is excessive,the remaining ZrB₂ will dissolve with the reaction product TiB₂ to form a solid solution which will reach equilibrium with（Zr,Ti）C;when TiC is excessive,the ZrC and TiC could dissolve each other to form a single（Zr,Ti）C phase when the temperature rises or approaches the TiC end,and the spinodal decomposition may occur if the component is in the corresponding phase diagram region.Based on the reaction and solid solution behavior between ZrB₂ and TiC,there are strengthening and toughening mechanisms such as fine-grain strengthening,solid-solution strengthening,and plate-like crystal toughening in the composite ceramics.The hardness,flexural strength and fracture toughness of ZrB₂-TiC ceramics hot-pressed at 1800°C first increase and then decrease with the increase of TiC addition.The optimized comprehensive mechanical properties such as a hardness of 27.7 GPa,a strength of 659 MPa and an indentation fracture toughness of 6.5 MPa m^1/2 was achieved in ZrB₂-90 mol.%TiC ceramics.As the temperature rises to 1200°C,The high-temperature modulus of the material decreases linearly,the drop of pure ZrB₂ is up to～11.6%.The high-temperature strength of the material decreases significantly due to the softening of the high-temperature grain boundary caused by the introduction of low-meltingCo impurity during the ball milling process.When the material composition transfers from the ZrB₂ end to the TiC end,the thermal expansion coefficient between room temperature and 1500°C increases from 6.8×10^-6K^-1 to 8.6×10^-6K^-1,and the room temperature thermal conductivity decreases from 57 W·m^-1·K^-1 to 24 W·m^-1·K^-1.As the temperature rises,the electronic heat conduction gradually increases,while the phonon heat conduction gradually decreases.At high temperatures,ceramics mainly rely on electronic heat conduction.The oxidation process of all the ceramics satisfy the diffusion control kinetic equation.ZrB₂-10 mol.%TiC exhibit the best oxidation resistance,and the weight gain is～2.4 mg·cm^-2 at 1300°C for 4 h.The oxidation resistance performance gradually deteriorates with the increase of carbide content.