Electric Field-Assisted Joining Process Of C/SiC Composites And Its Mechanism

SiC ceramics have low density,excellent oxidation resistance,high and radiation damage resistance.Taking advantages of the excellent intrinsic properties of SiC,composites containing continuous carbon fibers and silicon carbide fibers composites（CMC-SiC）have been prepared to overcome the brittleness of ceramics.Thus,these composites have great potential for thermal and structural applications.However,application of these CMC-SiC components was limited in manufacturing technology because of their large size and complicated shapes.Therefore,the development of a fast/reliable joining technique is critical for the applications of CMC-SiC.Unlike the conventional joining technologies,the electric current field-assisted sintering technology（FAST）rely on the Joule heating effect,rendering it with high efficiency and energy saving characteristics and allowing high heating rates（several hundred ℃/min）and short holding times（a few minutes）.In addition,the electrical current used in the joining process accelerates self-diffusion and promotes ion migration through the joining interface.These features make FAST a promising method for joining CMC-SiC with enhanced reliability.In this dissertation,four kinds of interlayer systems（Ti-Si-C,Ti-Nb-Ti,Ni-Ti-Nb and Mo-W-Mo）are used to joining SiC-coated C/SiC composites via electric field assisted joining technology.The interfacial morphologies,phase evolution and mechanical properties of the joints were investigated.The influence of thermal residual stress,interfacial microstructure and interfacial reaction layer growth was associated with the reliability of joints.Finally,these results were used to propose a failure model for the joints.The main research contents and results are as follows:（1）C/SiC composites were successfully joined via the in-situ reaction of TiC-Ti-Siinterlayer.The effects of joining temperature and the interlayer thickness on the interfacial microstructure and reliability of joints were investigated in detail.By adjusting the joining temperature,the TiC-Ti-Siinterlayer transitioned,in-situ,from TiC-Ti-Sicompounds to Ti₃SiC₂grains without decomposition.Because of the plastic deformation behavior of Ti₃SiC₂ grains,the ability of the interlayer to inhibit crack propagation increased.For joints with different interlayer thickness,the distribution of thermal residual stress was calculated using finite element analysis,and the distribution was associated with the evolution of interlayer morphologies.When the C/SiC composites were joined at 1400 ℃,the thin SiC layer（～10μm）protected carbon fibers from erosion of the interlayer during the joining process and also ensured that the thermal residual stress was at a suitable threshold.Consequently,good interfacial bonding and a lower amount of structural defects caused the joints have higher shear strength（51±3.0 MPa）.（2）The Ti-Nb-Ti active interlayer was designed to investigate the interdiffusion behavior between Ti,Nb and SiC during the joining process,revealing the Kirkendall effect on the interfacial phases composition,interfacial microstructure and mechanical properties of the joints.Using thermodynamics and diffusion kinetic,the phase evolution was associated with the interdiffusion behavior.When joined at 1200 ℃,the apparent shear strength of the samples was as high as 61±6.0 MPa as a result of appropriate interface diffusion reaction.A thin continuous TiC layer was formed in-situ on the joining interface via fast interdiffusion during the joining process.This layer acted as a barrier hindering the diffusion of Siatoms from SiC to the interlayer.Additionally,the diffusion rate of Siis much higher than those of M in Mx Siy phase（M=Ti and Nb）.This led to vacancy condensation,and some micro voids formed at the joining interface by.The size of these micro defects increased rapidly with the joining temperature,significantly decreasing the interfacial bonding strength of the material（18±6MPa）.（3）The Ni-Ti-Nb gradient interlayer was designed to apply transient liquid phase method to join C/SiC under the electric field-assisted environment,and the mechanisms of low-melting Ni additive were investigated.The introduction of Ni increases the diffusion rate of Ti/Nb metal atoms,and this balances the diffusion flux of interfacial atoms,inhibiting the growth of Kirkendall voids.Dissolution of SiC into the interlayer and the reaction between dissociated Siatoms and Ni-Ti-Nb were found to be the dominant events during the joining process.A Ni/（Ti,Nb）C structure was formed on the interface via fast inter-diffusion.A Ni/CVD SiC finite element model was built to reveal the mechanism behind the formation of rough joining interface.Non-uniform dissolution of SiC（affected by stress）in the interlayer led to the in-situ formation of a rough joining interface,substantially enhancing the interfacial bonding strength of the composites.Finally,a reliable joint with a shear strength of 108±5 MPa and less micro-defects was obtained as a result of this good interfacial bonding.（4）The Mo-W-Mo interlayer was designed to release the release stress of C/SiC joints,and the diffusion mechanism of Mo/W and SiC under the electric field-assisted environment was studied.The mechanism behind the evolution of interfacial phases was revealed by diffusion kinetics and nano-indentation tests.The thermal residual stress distribution was simulated by using finite element analysis,and the results were correlated with the transformation of the interfacial morphologies.Appropriate interfacial reaction resulted in higher interfacial bonding strengths for joining temperatures（lower than 1600 ℃）.However,higher joining temperatures（1800 ℃）resulted in an excessive growth of interfacial species with high thermal expansion anisotropy（e.g.,Mo₅Si₃ and Mo₅Si₃C）.As a result,the thermal residual stress increase significantly and a large number of interfacial defects were generated,significantly decreasing the reliability of the joints.