Research On The Mechanism Of Wetting And Low-temperature Bonding Of Graphite And Ceramics By Sn-Ti

Graphite possesses good thermal shock resistance and lubricity and high thermal conductivity,etc.Ceramics have good mechanical properties and high dielectric constant.They are widely used in industry.In order to fully exploit their perfprmance,the joining of them are usually conducted.Brazing is a commonly used method to join graphite and ceramics.Currently,the mostly used filler are Ni-,Ag-and Cu-based alloy,these high-temperature alloys usually result in joint cracking.Low-temperature bonding is one of the methods to solve this problem.For low-temperature bonding,Sn-based alloy is the mostly used filler.However,it mainly applied to solder metals to metals.To realize low-temperature bonding,the fillers must have a proper wettability on graphite and ceramic.In this work,graphite is studied as an example.Firstly,the wetting interface characteristics are studied by first-principle calculation to investigate the intrinsics for active element promotes wetting.Then the wetting tests are conducted to study the spreading behavior of Sn-based alloy on graphite.Based on the wetting researches,the premetallization-assissted low-temperature bonding of graphite is conducted.Then the low-temperature bonded and brazed joints are compared.Finally,the premetallization-assissted low-temperature bonding method is conducted to SiC and ZrO2 to investigate its suitable for ceramic.In the reactive spreading process,three interfaces,Sn/graphite,Sn-(Ti)/graphite and Sn/TiC,are appeared in turn.First-principle calculations are conducted to analyze the interfacial characteristics of them.The calculated results indicate that Sn block is mainly composed of metal bonds and partial covalent bond.In the same atomic layer of graphite,strong covalent bond exists between C atom.Also graphite possesses weak metallicity.The interaction of Sn/graphite interface is very weak.As active Ti adsorbs,the work of separation increases from-0.62J/m2 to-0.33J/m2(Sn-(Ti)/graphite interface and ionoc bond forms between Ti and C.Furthermore,once active Ti reacts with graphite forming Sn/TiC interface,the work of separation imporves to 0.44?1.59J/m2,which indicates effective bonding.When Sn and C are in top position(top-C),the Sn/TiC interface obtains the highest adhesion and the distance between Sn and C is 2.30A.Ionic bond forms between Sn and C atom.The wetting behavior of Sn-Ti alloy on graphite is investigated and the wettability and spreading kinetics are analyzed.The influences of Ti and Cr on spreading behavior are compared and the enhancement of adsorbtion of actives at interface on spreading is explored.Ti reacts graphite forming TiCx,which reduces the solid/liquid interfacial energy.The wettability of Sn-based alloy on graphite is improved with the contact angle decreases from?138° to 7°.The adhesion between Sn and TiCx is consistent with the calculated work of separation.As temperature rises,the stoichiometry of TiCx decreases,corresponds a stronger metallicity,inducing a lower contact angle.When more Ti introduced into alloy,more Ti6Sn5 remains in melt,whereas the thickness of interfacial layer does not increase.Excessive Ti6Sn5 intermetallics lowers the fluidity of melt inhibiting the spreading.Cr improves the wettability of Sn on graphite by its reactions with graphite forming Cr3C2 and Cr7C3 layer.The more metallic Cr7C3 has a higher proportion in the interfacial layer enhancing the spreading.When Cr content reaches 5%,the lowest contact angle of 20° is obtained.All Cr participate in the interfacial reactions,the thickness of reactive layer increases remarkable,as more Cr is added.The activities of Ti and Cr in Sn melt are calculated to be 10-5?10-3 and 10?102,respectively,and higher values corresponds to higher temperature.Compared to Ti,at the same temperature,Cr has a more violent reaction with graphite resulting in a faster spreading.The activation energies for spreading of Sn-Ti/graphite and Sn-Cr/graphite system are 182.66kJ/mol and 271kJ/mol,both values are lower than the C-C bond energy.The analyses of interfacial structure and spreading kinetics of Sn-Ti/AlN system reveal that both interfacial reactions forming new pahses and adsorbtion of actives at interface promote spreading.The adsorbtion behavior enhances spreading and lowers the activation energy for spreading.Moreover,adsorbtion of Ti is more positive than that of Cr to promote spreading.Sn-based alloys are used to braze and metallize graphite,and graphite/graphite joints are bonded at low temperature(250?).The joining properties of Sn-based alloy and mechanical properties of joints are studied.Due to the lacking of Ti in unite contacting area,joints with continuous brazing seam are brazed by Sn-based alloy with Ti content over 3%.The typical interfacial structure of brazed joints is graphite/TiCx layer/?-Sn(Ti6Sn5+Sn-Ag+Sn-Cu)/TiCx layer/graphite.Higher Ti content results in more Ti6Sn5 in brazing seam.More melt filler flows out at elevated temperature narrowing the brazing seam.Metallized graphites are bonded together at 250?/1min in air,which has a same interfacial micro structure to brazed joint.Moreover,the shear strength of bonded joints is comparable to that of brazed ones,the values locate in 17.6MPa?20.6MPa and all joints fracture in graphite.It indicates that premetallizing graphite by Sn-based alloy is a reliable method to obtain comparable graphite/graphite joint at low temperature.The wetting and joining properties of Sn-Ti/ZrO2 and Sn-Ti/SiC system are studied.The wetting behavior of Sn-Ti on ZrO2 and interfacial analyses reveal that interfacial structures decides the final contact angle.Ti deprives O from ZrO2 and reacts with it forming Ti2O3,which decreases contact angle from 144° to 42°.The contact angle further decreases to 22° when Ti11.31Sn3O10 covers Ti2O3 layer at 4%Ti.The thicker interfacial layer increases the diffusion distance of O slowing the spreading.Reliable ZrO2/ZrO2 joints are successfully brazed and bonded following metallization by Sn-Ti.Ti content changes the interfacial layer and the amount of Ti6Sn5 in joint seam,whereas,it has no influences on the shear strength(22?28MPa).All of the joints fracture in joint seam.The wetting behavior and microstructure analyses indicate that Ti reacts with SiC producing TiC and free Si,then Ti5Si3 covers on TiC layer by the reaction between Ti and Si.The interfacial layer lower solid/liquid interfacial energy decreasing contact angle to 20°.The spreading of Sn-Ti on SiC is firstly limited by interfacial reactions then transfers diffusion-limited stage.In the brazed SiC/SiC joints by Sn-Ti,only TiC layer forms at interface.Incontinuous Ti5Si3 layer grows on TiC in the low-temperature bonded joints.The shear strength of joints ranges from 27 to 32MPa.The joints fracture along seam in ductile mode.