Technical And Mechanism Study On Vacuum Brazing Of SiC Ceramic

Due to the properties of high-specific stiffness, the low-thermal expansion property, the radiation resistant of space partical and especially excellent heat shock-resisting, SiC ceramic is the first choice of high performance speculum. High quality joining of SiC ceramic was successfully realized by using TiZrNiCu. Optimal brazing parameters, as well as the bonding mechanism were identified. The effect of brazing parameters on the microstructure was also discussed, and the fracture path was defined, finally calculated the interfacial thermodynamics and investigated the residual stress distribution of joints . The results showed that the microstructure of the joints was SiC/TiC/Ti₅Si₃+Zr₂Si /Zr(s,s)/Ti(s.s)+Ti₂(Cu,Ni)/(Ti,Zr)(Ni,Cu) when brazing temperature was 1250K and holding 10 minutes. When the temperature was or above 1350K, compound TiSi2 in the braze center apeared. When the holding time was or above 30min, a lot of TiC apeared in the brazinrg seam center.The shear stress was tested to evaluate the mechanical property of the joints. The results showed the maximum shear stress was 110MPa, when the brazing temperature was 1250K and holding time was 10 minutes. As hoisting the the brazing temperature and prolonging the holding time, the shear stress both climbed up then declined. At this time, the fracture took place in the TiC layer and all the fracture were brittle. Using models of activity coefficients for ternary solid state alloy, calculated the activity coefficients of Ti in the ternary solid state alloy and the Gibbs free energy change of the interfacial reactions. In the brazing condition, Ti5Si3 was the most stable silicides . Ti5Si4, TiSi, and Ti3Si wouldn't appear in the interface. When the brazing temperature was 1350K , TiSi2 was likely to emerge in the interface. According to analyzing the interfacial microstructure, joint formation was divided into the following stages: physical contact of the brazed surfaces, melt of TiZrNiCu and the appearance of reaction products at ceramic side; diffusion of TiZrNiCu alloy to the ceramic side, width increase of reaction layer at ceramic side and homogeneousness of liquid filler; stop of reaction at ceramic side ,formation of hypoeutectic microstructure Ti(s.s)+Ti₂(Cu,Ni)+Ti₂(Ni,Cu), solidification of intermetallic compounds on joint center. Applying the method of inclining brazing gap, the optimal brazing gap.was ascertained. When the brazing gap was between 30μm and 50μm, the shear stress was maximum at 115MPa. Proper brazing gap could help the element Si diffusion in time , which reacted with the elements of Ti and Zr ,and producted the compounds of TixSiy, ZrxSiy. These small quantity of compounds which were small in shape and formed in the centre of the brzing gap, could strengthen the mechanical properties. Therefore, excellent joint was obtained.and the braze will be a homogeneous solid solution which could minish the creat of intermetallic compound . Finally, optimal braze gap made the braze joint has excellent mechanical property.The residual stress distribution of brazed ceramic was simulated by MARC numerical simulation. The results showed that different sizes of brzaing metal had the same distribution situation of the postweld residual stress of ceramic liner, the peak of residual stress appeared in the interface between the ceramic and the brzaing metal. Taking all the factors into consideration, the optimum size of brzaing matal were 37×74 mm². In such case, the maxmum residual stress decreased 38% compared with the size of brzaing matal were 75×150 mm².