Research On Joining Process And Formation Mechanism For TiB₂-TiC-SiC Composite Ceramic And Metal Ta

Ultra-high temperature ceramic materials are suitable for extreme chemical and thermal environments,and have a wide range of applications in aerospace,military equipment and civil electronics.Ti B₂-Ti C-Si C composite ceramic is a new generation of ultra-high temperature ceramic material developed on the basis of Ti B₂ ceramic.The addition of Ti C improves the strength and fracture toughness of the Ti B₂ ceramic,and the addition of Si C improves the thermal shock and oxidation resistance of the Ti B₂ceramic.Ti B₂-Ti C-Si C composite ceramic exhibits the excellent chemical stability,good thermophysical properties and mechanical properties due to the synergistic effects,which make it widely used in modern air and space vehicle structures and thermal protection materials.However,the intrinsic brittleness of ceramic materials has always limited their engineering applications.It is of great theoretical significance and promising engineering applications to obtain composite ceramic-metal components that combine the excellent properties of both by achieving a reliable connection between composite ceramic and metal.Obtaining Ti B₂-Ti C-Si C composite ceramic with excellent properties is the basis for achieving high-quality joining of composite ceramic and metal.In this study,Ti B₂,Ti C and Si C powders were used as raw materials for the sintering of Ti B₂-20 vol.%Ti C-20 vol.%Si C（TTS）composite ceramic.The sintering process was optimized to obtain composite ceramic material with excellent performance in terms of densities,hardness,flexural strength and fracture toughness.It laid the foundation for the subsequent work on the joining of composite ceramic themselves and with metal.Two kinds of Ti-Ni composite interlayers were designed to study the interface reaction characteristics between the interlayer and TTS composite ceramic.The mechanism of the interfacial reaction between the Ti-Ni composite interlayer and the TTS composite ceramic was analyzed by SEM,TEM and HRTEM combined with thermodynamic calculations.The results showed that the interface reactions were dominated by Ni of the Ti-Ni interlayer in the Ti-83Ni/TTS system.It was noted that Ni mainly reacted with Si C to form graphite and Ni-Si compounds,among which the formed Ni₂Si was the most probable and stable.Additionally,Ni also could be reacted with Ti B₂ to form Ni₃B.When Ni was in contact with Ti B₂ and Si C components at the same time,it preferentially reacted with Si C component.While in the Ti-24Ni/TTS system,the interfacial reactions were dominated by Ti of the Ti-Ni interlayer.Ti would react with Si C and Ti B₂ to produce Ti C,Ti₅Si₃ and Ti B,respectively.C atoms would diffuse into the aggregated Ti to generate new Ti C at the interface between Ti C and Ti.When the TTS composite ceramic is bonded by the Ti-Ni system,the interfacial reaction dominated by Ti is better.The composite ceramic side formed a continuous reaction layer,and the main reaction products showed excellent performance.Ti-24Ni interlayer was applied to join TTS composite ceramic and metal Ta.Compared with TTS/Ti-24Ni/TTS system,there were three characteristics in the TTS/Ta system due to the introduction of Ta.First,the mismatch between thermal expansion coefficients of TTS composite ceramic and metal resulted in residual stresses formed in the joint,and the failure of the joint at the room temperature occurred mainly on the TTS composite ceramic side.The second was that Ta dissolved into the liquid Ti-Ni alloy,and diffused toward the TTS composite ceramic side driven by the concentration gradient.Ta atoms replaced a small portion of Ti atoms in Ti B,forming the cluster of Ti B and（Ti,Ta）B.The enthalpy of formation of（Ti,Ta）B was calculated using first-principles calculations,and the results showed that the formation enthalpy of（Ti,Ta）B with B27-type structure became low with the increase of Ta content,which indicated（Ti,Ta）B is more stable than Ti B.Third,a large amount of Ti atoms gathered on the Ta substrate side and generated a biphasic structure ofα-Ti+β-（Ti,Ta）due to the strong affinity of Ta andβ-Ti.The mechanical performance of TTS/Ta joint significantly decreased under high temperature（800°C）service environment because of the formedα-Ti+β-（Ti,Ta）.Based on the failure mechanism of the TTS/Ta joint at room and high temperature,Ni/Ti-24Ni composite interlayer was designed to join TTS composite ceramic and metal Ta.Attributed to the addition of Ni barrier layer,the formation ofα-Ti+β-（Ti,Ta）on the metal side was prevented,and Ta-Ni compounds with high melting point were formed.On the TTS composite ceramic side,the interfacial reactions were still dominated by the active element Ti,which reacted with the ceramic to form Ti C and（Ti,Ta）B,and the brittle Ti₂Ni compound was replaced by Ti Ni at the center of the joint.The maximum shear strength of the joint at room temperature reached 183 MPa,and its shear strengths at 600°C,and 800°C were 146 MPa,and 125 MPa,respectively.The gradient interface of composite ceramic was designed to join metal Ta.The TTS was the main ceramic,and Ti B₂-20Ti C-5Si C（vol.%）with low Si C content and high Ti B₂ content was prepared as the interfacial ceramic involved in the interfacial reactions.Ni/Ti-24Ni composite interlayer was used to join gradient composite ceramic and metal Ta.Fine（Ti,Ta）B was formed in the reaction layer on the ceramic side due to the increased content of Ti B₂ in the composite ceramic,which improved the interfacial bonding strength on the composite ceramic side.On the other hand,the thermal expansion coefficient of the Ti B₂-20Ti C-5Si C composite ceramic was larger than that of the TTS composite ceramic,which could reduce the mismatch between the composite ceramic and the Ta and then enhance the mechanical properties of the joint.The maximum shear strength of the joint at room temperature reached 212 MPa and it was 153 MPa and 133 MPa at 600°C and 800°C,respectively.Compared with the joint of initial TTS composite ceramic and metal Ta,the room temperature shear strength of the joint of gradient composite ceramic and Ta was improved by 16%.