Joining Process And Bonding Mechanism Of Si₃N₄/Nickel-based Superalloy Joints By Partial Transient Liquid Phase Bonding

Silicon nitride（Si₃N₄）ceramic is a light-weight ceramic with excellent hightemperature strength,creep-resistant,low thermal expansion,and excellent resistance to thermal shock,wear,and oxidation,which has played an important role in aerospace and energy fields.Nickel-based superalloys have also been widely applied as industrial gas turbine in aeronautics.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 Si₃N₄ ceramics to nickel-based superalloys during engineering application.Partial transient liquid phase bonding can produce a heatresistant joint at lower temperature than operational temperature,which has received more attention to join dissimilar materials.In this dissertation,Si₃N₄ ceramics have been joined to DZ483 superalloys by partial transient liquid phase（PTLP）bonding using Ti/Cu/Ni multi-interlayers and Ti/Au/Ni multi-interlayers,respectively.The effect of bonding parameters on microstructure and mechanical properties of joint has been investigated.The microstructure and composition,phase composition and mechanical properties of joint were systematically investigated by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,energy dispersive spectroscopy（EDS）,high resolution transmission electron microscopy（HRTEM）,selected area electron diffraction（SAED）,Vickers micro-indentation and three point bending tests.The mechanism of interfacial reaction between Si₃N₄ and DZ483 superalloy was also discussed.The results show that the Si₃N₄/DZ483 joint by PTLP bonding with Ti/Cu/Ni multiinterlayers has been identified as follows: Si₃N₄ ceramic/TiN layer /Ti₂ Ni /Cu-rich /CuNi solid solution/Cu-rich/CuTi₂/Ni₃Ti/Cr-rich/DZ483 superalloy.According to the TEM analysis,it was found that the TiN reaction layer was composed of two zones: one next to the Si₃N₄ ceramic with a thickness of about 0.4 μm and the other zone with grains of about 0.8 μm.The diffusion layer thickness increased and atoms diffused more adequately with the rising bonding temperature and time.With the joining temperature rising,the bending strength of the joining samples increased initially and then decreased.When the joining temperature was 1323 K,the bending strength of the joints reached 170 MPa.The high-temperature mechanical properties of Si₃N₄/DZ483 superalloy joint with Ti/Cu/Ni multi-interlayers were evaluated by a three point bending test.Hightemperature flexural tests yielded the average strength values of the joints at 873 K and 1073 K as 125 MPa and 95 MPa,respectively.With increasing test temperature,the flexural strength of the joint decreased slightly.However,the average strength of the joints decreased sharply when the temperature reached 1273 K.The oxidation resistance of the joints was evaluated by bending strength testing at room temperature after annealing treatment of the joints in air for 100 h at different temperatures.The flexural strength reached 128 MPa after the joint was annealed in air for 100 h at 1073 K.The joint has good high-temperature performance.However,the fracture of the joints occurred after annealed in air for 100 h at 1173 K due to the oxidation of the filler alloy.In addition,the high-temperature fracture mechanism of the joints was discussed.The fractures of the joints at room temperature exhibited a brittle fracture characteristic,while plastic deformation of the interlayers occurred at elevated temperatures.The Si₃N₄/DZ483 joint by PTLP bonding with Ti/Au/Ni multi-interlayers consists of following: Si₃N₄ ceramic/TiN layer/Au-rich/Ni-rich/Ni interlayer/Au-rich/TiO/Ni₃ Ti layer/Cr-rich/DZ483 superalloy.According to the study of bonding parameters,it can be found that the TiN reaction layer was thickened and the mechanical properties of the joint was improved with bonding temperature or holding time increasing in certain ranges.However,when the bonding temperature is too high or the holding time is too long,Ni solid solution transformed to an intermetallic compound of TiNi₃,which leads to the decrease of mechanical properties of joints.Therefore,the average bending strength of Si₃N₄/DZ483 joint bonded at 1323 K for 60 min reached the maximum values of 211 MPa at room temperature.When test temperature increased to 1173 K,the average bending strength of 91 MPa was obtained.The bending strength reached 85 MPa when the joint was annealed in air for 100 h at 1173 K.Moreover,the nanoscale wavy interface was observed at the Si₃N₄/TiN interface,which was thought to provide high mechanical strength.Investigation of kinetics associated with Si₃N₄/DZ483 joint PTLP bonding process with Ti/Au/Ni multi-interlayers demonstrated that the growth of interface reaction layer obeyed the parabola law governed by the diffusion of participating elements.The activation energy of TiN and Ni₃ Ti reaction layer formation of the joint is 283.8 kJ/mol and 186.7 kJ/mol,respectively,which is based on the dynamics calculation.The crystallographic orientations between the Si₃N₄ ceramic and the Ni interlayer was analyzed by HRTEM and SAED.There is no specific crystal orientation relationship between TiN and Ni.The TiN crystal shared a coherent orientation relationship with the Si₃N₄ ceramic,which offered high mechanical strength at the interface.