Study On Bonding Process And Mechanism Of Porous/Dense Si₃N₄ Ceramic Joints Using LZAS Glass-ceramic

In the military field,missiles,as an important branch,play an important role in the national defense industry.The missile radome plays an important role in protecting the navigation antenna and improving the reliability and aiming accuracy of the missile.It has important research and reality on the selection of materials.significance.Due to its excellent high temperature resistance,rain erosion resistance,high hardness and high strength,dense Si₃N₄ can be used as a radome head material.The porous Si₃N₄ is not only light in weight,low in density,but also has a low dielectric loss,good wave transmission performance,excellent high temperature resistance and other characteristics,and is suitable as a missile radome cover material.If the two can be reliably combined,it is expected to obtain a missile radome with excellent performance.In this paper,two kinds of Si₃N₄ ceramics were connected by using a glass-ceramic system with thermal expansion coefficient close to that of the base metal.The influence of solder composition on the microstructure and properties of the joint was explored,and the connection mechanism was explained.In this paper,a Li₂O-ZnO-Al₂O₃-SiO₂-B₂O₃（LZAS（B））solder and three different Li₂O-ZnO-Al₂O₃-SiO₂（LZAS1/LZAS2/LZAS3）solders were used in joining porous Si₃N₄ and dense Si₃N₄ ceramics.By evaluating the thermal expansion coefficient of the glass after crystallization,it was found that the thermal expansion coefficient of the whole glass can be changed within the range of 0.6×10-6⁷.0×10-6/°C.it’s close to the two Si₃N₄base materials which can reduce the residual stress inside the joint.In study of LZAS（B）solder found that B₂O₃-containing solder would have a large number of pores inside the joint,which was not conducive to the connection.In addition,the pre-oxidation of the dense Si₃N₄ base material improved the wettability of the glass solder.The wettability of LZAS solder to the base metal was studied.The results showed that as Li₂O decreased from 7.5 wt.%to 2.5 wt.%,the melting temperature of the solder increased gradually,and the wettability of the solder for dense Si₃N₄ decreased first and then increased.Wetting angles were 25°,40°,26°The wettability of the porous Si₃N₄ was gradually lowered,and the final wetting angles were 50,62,and 91.When using LZAS solder with different Li₂O content,it was found that Li₂O had the effect of lowering the melting temperature of glass ceramics,improving the fluidity of molten glass,strengthening the interaction with porous Si₃N₄,and changing the microstructure of welds.The porous Si₃N₄/dense Si₃N₄ was bonded by LZAS solder.The results showed that there were Li and Zn elements in the joint.The joints obtained after welding mainly had two kinds of microstructures:pure glass or spodumene plus residual glass phase.It is also found that there are two impregnated layers on one side of the porous Si₃N₄ ceramic in most of the joints,and the final joint structure is two:1.Dense Si₃N₄/glass/impregnated layer I/impregnated layer II/porous Si₃N₄;2.Dense Si₃N₄/glass+LiAl（SiO₃）₂/impregnated layer I/impregnated layer II/porous Si₃N₄.The joint process of different compositions of solder was explored.It was found that for LZAS1 solder,the joint shear strength at 1370°C for 10 min was the highest,which was 28.7 MPa,and the joint structure was dense Si₃N₄/glass+LiAl（SiO₃）₂/impregnated layer I/impregnated layer II./porous Si₃N₄;For LZAS2 solder,the highest shear strength at 1400°C for 10 min is 51.8MPa,and the joint structure is dense Si₃N₄/glass/impregnated layer I/impregnated layer II/porous Si₃N₄;For LZAS3 solder,the joint shear strength at 1460°C for 10 min is the highest,33.3 MPa,and the joint structure is dense Si₃N₄/glass/impregnated layer I/impregnated layer II/porous Si₃N₄.The high temperature shear and high temperature thermal shock tests were carried out on the LZAS2 solder.The structure shows that the joint obtained with LZAS2 solder has a shear strength of 53.7 MPa at 850°C and can withstand 4 thermal shocks（850°C air cooling）without failure,has good high temperature mechanical properties.