| The ZrC-SiC composite is an important family member of the ultra high temperature ceramics (UHTCs) by introducing SiC as a secondary phase into the ZrC ceramic. The superior thermal and mechanical properties of ZrC-SiC composite make it applicable in extreme environment, such as the conical cap, leading edge and cover plate of hypersonic flight vehicles. However, it is difficult to obtain large scale or complex ceramic components due to their brittleness and limited preparation techniques. Therefore, it is necessary to join ZrC-SiC composite to itself or to metals during its application. In this situation, transient liquid phase bonding was applied to the joining between ZrC-SiC composite and Nb with Ti-Ni composite interlayers. The characteristic interfacial reaction was successfully controlled by optimization of the Ti-Ni interlayers. Meanwhile, a designed Mo interlayer was used to release the residual stress and thus improve the mechanical property of the ZrC-SiC/Nb joint.The typical interfacial reaction between ZrC-SiC composite and Ti-Ni alloy was analyzed by changing the chemical compositions of the Ti-Ni composite interlays, which is a critical factor in the microstructure evolution of the ZrC-SiC/Nb joint. The interfacial reaction mechanisms of Ti/ZrC-SiC system and Ni/ZrC-SiC system were studied using thermodynamic analysis and advanced characterization methods, such as transmission electron microscopy, high resolution transmission electron microscopy and selected aera electron diffraction. During the joining process, the interaction between Ti and ZrC-SiC is in fact the growth process of TiC compound. The interaction between Ti and ZrC-SiC leads to the formation of TiC layer on the interface. When the ZrC-SiC composite was joined with Ni-rich Ti-Ni liquid, the interfacial reaction focused on the Ni-SiC system, corresponding reaction products were Ni2Si and C. Furthermore, small quantities of TiC particles were formed near the interface. The violent interaction between Ni and ZrC-SiC composite was proven to be hamful to the joint. Especially, the lamellar graphite formed in the joint decreased its mechanical property.Thermaldynamic calculations were used to analyze the atomic activity in Ti-Ni system. Correspondingly, the transform of Ti-Ni/ZrC-SiC interfacial reaction was related to the change in chemical compositions of Ti-Ni liquid. Two typical interfacial reactions were predicted and observed in the Ti-Ni/ZrC-SiC reaction system. They were the Ti/ZrC-SiC interaction and the Ni/ZrC-SiC interaction, respectively. Based on the research results, the activity of Ni is close to zero when its content in the Ti-Ni liquid is less than40at.%. Instead, Ti possesses high activity in the liquid alloy and tends to react with the composite. Both Ti and Ni show low activity when the content of Ni rangs from40at.%to60at.%. The critical mole fraction for Ni in the Ti-Ni system is0.52. Exceeding the critical point, Ni takes part in the interfacial reaction. With increasing in Ni content, the Ni/ZrC-SiC interaction increases and becomes the the primary process when the content of Ni exceeds60at.%. Corresponding reaction products are Ni2Si and C. In fact, the Ti-Ni/ZrC-SiC interfacial reaction depends on the activity of Ti or Ni, which is the outward manifestation of the Ti-Ni chemical compositions.The chemical compositions of the Ti-Ni interlayers and corresponding joining parameters were optimized based on the thermaldynamic calculation results, combining with microstructure analysis and mechanical property test. For the ZrC-SiC joint bonded with Ti-Ni interlayers, the interfacial structure can be described as ZrC-SiC/TiC/Ti2Ni+TiNi+TiNi3+TiC/TiC/ZrC-SiC. The maximal shear strength of the joint reached172MPa when bonded with Ti-40Ni alloy at1150°C for10min. In the joining between ZrC-SiC and Nb, however, the dissolution of Nb consumed a portion of Ti and thus decreased its activity near the ceramic. Therefore, the chemical composition of the liquid was adjusted to Ti-32Ni. Sound joints were obtained when joined with Ti-32Ni alloy at1050°C for10min. The shear strength of the bonded ZrC-SiC/Nb joints reached141MPa at room temperature and82MPa at800°C.The Mo interlayer was designed to release the residual stress of the ZrC-SiC/Nb joint. The residual stress and its distribution were simulated by FEM at first. The simulation results were verified by shear test of the joints. The Mo interlayer not only decreased the stress but also shifted its concentration point. When a100μm interlayer was used, the equivalent Von-Mises stress of the joint decreased from320MPa to296MPa. As a result, the shear strength of the ZrC-SiC/100μmMo/Nb joint reached166MPa at room temperature and90MPa at800°C, which were18%and10%higher than that of the ZrC-SiC/Nb joint. Particularlly, this type of interlayer was proven to be favorable in the ceramic-metal joining system. |
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