Research On Mechanism Of Brazing SiO₂-BN Ceramic And Nb With CNTs Reinforced TiNi Solder

SiO2-BN ceramic, known for its outstanding heat resistance, thermal shock stability,and corrosion resistance, is used in aerospace engine and antenna radome of missile. Nb,with high specific strength and thermostability, is also widely used in aerospace. Inpractical applications, it is necessary to bond SiO2–BN ceramic with Nb to take fulladvantage of its benefits.While joining the materials above using the method of brazing, the followingproblems still need to be solved: the residual stress caused by the difference of CET;poor high temperature mechanical properties; and poor wettability on the surface ofceramic. However traditional brazing alloys cannot satisfy the requirements mentionedabove. Therefore, in order to meet the requirments above, this dissertation aims todevelop a new type of Carbon Nanotubes (CNTs) reinforced TiNi brazing alloys whichprocesses low CET as well as good mechanical and high temperature properties, and toapply it in the brazing of SiO2-BN and Nb.The method of Plasma Enhanced Chemical Vapour Deposition (PECVD) was usedin the in-situ growth of CNTs on the surface of Ni-TiH2particles, which realized theuniform dispersity and structural integraty of CNTs. The following is the optimalparameters of the in-situ growth of CNTs, temperature:570℃; ratio of CH4and H2:40:10; total gas pressure:700Pa; radio-frequency power:175W. The CNTs/TiNicomposite brazing alloys prepared under the above paramaters had proper length anddensity and dispersed uniformily.Compare the interface of the joints brazed by TiNi and CNTs/TiNi brazing alloys,in the interface of joints brazed by CNTs/TiNi brazing alloy the diffusion length of Nbwas longer，and more TiNi-(Nb,Ti) euthectic emerged. Because of the good plasticy ofTiNi-(Nb,Ti) euthectic, the shear strength of joints brazed with1.5%vol.%CNTs/TiNibrazing alloy was83MPa,with the brazing parameters being1140℃/5min, which was a60%enhancement compared with the joints brazed with TiNi brazing alloy. Besides,both Nb and CNTs have excellent high temperature mechanical properties and CNTsprevent the growth of grain of the joints at high temperature, so SiO2-BN/Nb jointsbrazed by CNTs/TiNi brazing alloy showed outstanding high temperature mechanicalproperties. The shear strength of such joints at800℃eached49MPa，with a strengthretention being59%, which is much higher than that of the joints brazed by TiNibrazing alloy(20MPa).Analysis showed a good wettabilty of TiNi brazing alloy on the surface of CNTsand maintained so after high temperature annealing. At defects of CNTs, small amountsof C reacted with Ti and produced TiC. In the wetting system of CNTs/TiNi/SiO2-BN, CNTs acted the role of transmitting Ti atoms from low concentration area to highconsentration area, which boosted the reaction wetting on the surface of ceramic andimproved the wettability of TiNi brazing alloy.