Study On Process And Mechanism Of Surface Activation And Brazing Of SiC Ceramics

Silicon carbide（SiC）ceramics are widely used in aerospace,nuclear energy,machinery,petroleum,optics,integrated circuits and other fields due to their excellent characteristics such as high temperature strength,good wear resistance,good thermal stability,small thermal expansion coefficient（CTE）,high hardness and excellent thermal shock resistance.SiC ceramics are also joined by brazing to broaden application,and active elements（Ti,Cr,Zr,V,et al.）play a decisive role in the wetting and brazing processes of ceramics.Good brazing joints can be obtained by active brazing,however,composition segregation of filler and excessive intermetallic compounds in the joints are also existed.In addition,the existence of the original surface adsorption layer,oxide layer and the microstructure of substrate have important influence on the wetting of filler on ceramics as well as the interfacical microstructure and performance of the subsequent brazing joint.Therefore,surface activated brazing method is proposed to realize the joining of SiC ceramics.Microstructure and shear strength of SiC ceramics joints brazed by Ag Cu Ti filler directly and surface activation（active film deposition and ion bombardment）,respectively,were analyzed in detail.When brazed by Ag Cu Ti directly,the interfacial reaction layer was stratified and the mean shear strength was 17.9MPa.After deposited by active Ti or Cr film,SiC ceramics brazed joints can be successfully achieved using inactive Ag Cu filler with the shear strength of 26.1MPa and 29.6MPa.When SiC ceramics were pre-treated by ion bombardment of 1×10¹⁷/cm² at 50ke V,the interfacial stratification of brazed joint was eliminated and the microstructure was optimized as SiC/Ti₅Si₃+Ti C mixed layer/Ag（s,s）+Cu（s,s）containing Ti Cu/Ti₅Si₃+Ti C mixed layer/SiC,the shear strength of 30.9MPa was obtained.HRTEM analysis showed that the lattice mismatch rate of SiC/Ti₅Si₃ and SiC/Ti C interface in the joint pre-treated by ion bombardment is 0.27%and 0.31%,respectively,which are semi-coherent interfaces with low strain energy.The lattice mismatch rate of Ti C/Ti₅Si₃ interface was 6.7%,which is lower than that of Ti C/Ti₅Si₃ interface in original SiC brazing joint,indicating that ion bombardment could effectively improve the interfacial joining quality of the brazing joint.These results showed that surface activated brazing of SiC ceramics can effectively improve joint strength,and ion bombardment activation showed greater potention.The effects of Ar ion bombardment parameters（fluence and bias voltage）on surface microstructure and properties of SiC ceramics were studied.At 60ke V,with the increase of fluence from 1×10¹⁴/cm² to 1×10¹⁷/cm²,the process from SiC_x（x<1）phase to amorphous layer occurs on the SiC ceramics surface.With bias voltage increasing,the surface amorphous layer thickness increase,while the surface roughness of SiC decreases.Under 50ke V and 1×10¹⁷/cm²,the hardness and elastic modulus of SiC surface are decreased from 21.3GPa and 388.9GPa for original SiC to 11.5GPa and277.8GPa,respectively,while the surface energy increases from 37m J/m² for the original state to 42.5 m J/m².Ar ion bombardment can cause debonded of Si-C bond on SiC surface,and SiC_x（x<1）and C interstitial atoms and vacancies（C Frenkel pairs）are first formed.With continuous ion bombardment,Si-C bond of SiC_x（x<1）breaks continually,then large numbers of Si and C Frenkel pairs and a part of antisite defects are formed,the formation of amorphous layer in SiC surface are appeared.The wetting behavior of Ag Cu Ti filler on SiC ceramics by ion bombardment was studied.The initial contact angle of the filler on substrates are decreased due to the increasing surface energy of SiC by ion bombardment.Ion bombardment accelerates the interfacial reaction of Ti and SiC,which shortens the incubation period significantly,however,more time used in subsequent diffusion and spreading stage to achieve equilibrium state is needed,which is resulted from that diffusion of Ti and melts is hindered by amorphous layer induced by ion bombardment.The contact angles are between 12～14°or 10～12°at varies fluence or bias voltage.With incrase of ion bombardment,the metallurgical bonding quality of SiC/Ag Cu Ti interface is improved gradually.The effect of bias voltage of ion bombardment on brazing of SiC ceramics using Ag Cu Ti filler was studied,and the mechanism of ion bombardment assisted brazing of SiC ceramics was clarified.With the increase of bias voltage,the interfacial stratification gradually disappeared and the thickness of the interfacical reaction layer decreased first and then increased slightly.At 60ke V,some microcracks are appeared at reaction layer.With the increase of bias voltage,the shear strength of the joint increased first and then decreased.The interfacial reaction layer is formed by one-step chemical reaction when SiC was pre-treated by ion bombardment:6Ti+3（Si）+（C）=Ti C+Ti₅Si₃,?G_T=-787.3+22.1×10^-3T（k J?mol）.