Reasearch On Bonding Process And Joining Mechanism Of Porous Si₃N₄/dense Si₃N₄ Joints Useing CAS-based Glass Ceramics

Porous Si₃N₄ ceramic is commonly used as the hood material of missile radomes due to its low dielectric constant,high temperature resistance,excellent thermal shock resistance and oxidation resistance.Dense Si₃N₄ ceramic has much better rain erosion resistance besides the advantages which porous Si₃N₄ ceramic has,and is a promising top material of the radome.The successful bonding of porous Si₃N₄ ceramics and dense Si₃N₄ceramics is of great significance to obtain a structure and function-integrated missile radome.In this work,CaO-Al₂O₃-SiO₂-Li₂O?CASL?glass ceramic solders with different Al₂O₃ content were used to join the two ceramics.Crystallization,wetting behavior and effects of bonding parameters on joints microstructure and mechanical properties were studied.Forming mechanism of the joints were also analyzed.20CaO-22Al₂O₃-56SiO₂-4Li₂O was firstly used.Crystallizing experiment results showed that spodumene?LiAlSi₂O₆?,calcite?CaAl₂Si₂O₈?and wollastonite?CaSiO₃?were the main phases precipitate from the glass ceramics.The solder has good wettability on dense Si₃N₄ was,the wetting angle reached 37°,but the wetting angle on porous Si₃N₄was 87°.Microstructure observation of the joints showed that the solder could infiltrate into porous Si₃N₄,but the welding seam is wide,and voids could also be found.The phenomenon indicates high viscosity of the liquid solder.The highest shear strength of the joints was only 30MPa after welding parameter optimization.To reduce the viscosity of the liquid solder,20CaO-20Al₂O₃-56SiO₂-4Li₂O and20CaO-18Al₂O₃-56SiO₂-4Li₂O solders which contain less Al₂O₃ content were used.With the decrease of Al₂O₃,the crystals precipitate from the glass ceramics remain the same.The coefficients of thermal expansion?CTE?of both glass ceramics without crystallizing process are much higher than that of the substrates,and the CTE of glass ceramics become lower when the crystals content increases after crystallization.With the reduction of Al₂O₃ content,its wetting ability porous Si₃N₄ was improved obviously,which is only 22°when using 20CaO-18Al₂O₃-56SiO₂-4Li₂O solder.The two substrates could be successfully bonded using the two CASL glass ceramic solders.When raising bonding temperature and increasing holding time,the width of the welding seam is reduced,the content of spodumene?LiAlSi₂O₆?also decreased,while calcite?CaAl₂Si₂O₈?and wollastonite?CaSiO₃?became the main phases.Shear strength of the joints firstly increased then decreased with increasing temperature and holding time.When the joints were welded at 1180?holding for 30min using 20CaO-20Al₂O₃-56SiO₂-4Li₂O solder,the highest shear strength of the joints at room temperature was obtained,which reached to 61MPa,and the shear strength at 800?could still reach to47MPa.Through microstructure analysis,it's considered that the bonding between porous Si₃N₄ and solders was formed by infiltration,while the bonding between dense Si₃N₄ and solders was formed by elements diffusion effect.