Design Of Lmas Glass-Ceramic And Micromechanisms For Joining Porous And Dense Si₃N₄ Ceramics

Silicon nitride（Si₃N₄）ceramics are one of the most important optional materials for radome due to their high strength,good fracture toughness,high temperature resistance and excellent dielectric properties.Porous Si₃N₄ with low density and excellent wave-transparent performance can be used as radome body,and dense Si₃N₄ceramic can be used as a rain corrosion resistance outer layer.To join porous with dense Si₃N₄ shows important engineering significance.Traditional glue bonding method is adopted.the adhesive materials are easy to age,resulting in the component falling off at high temperature.In this paper,a novel glass filler was proposed to join porous with dense Si₃N₄,and the high temperature resistant crystal phases precipitated from glass form the glass-ceramic interlayer,which can realize the high temperature reliable service of the joint.Based on the analysis of joint stress relief and interface structure,Li₂O-Mg O-Al₂O₃-Si O₂ glass was designed and prepared.The crystallization mechanism of glass,the interaction between glass and ceramics,and the microstructure evolution mechanism of glass-ceramic in the joint were deeply analyzed,providing a new idea for the joining of ceramics.In order to ease the stress of the joint,finite element simulation was used to analyze the influence of coefficient of thermal expansion（CTE）,elastic modulus and thickness of the glass-ceramic interlayer on the thermal stress of joint.The regulation range of CTE of glass-ceramic interlayer was proposed to be 3.5～4.5×10^-6/°C.Based on the above simulation results,the three compositions of Li₂O-Mg O-Al₂O₃-Si O₂glass was designed and prepared,and effects of B₂O₃content and Mg O/Al₂O₃ratio on glass properties were studied,respectively.The results show that B₂O₃ is beneficial to reduce the melting temperature of glass,but because B₂O₃ is a glass network forming body,it has the effect of inhibiting the crystallization of glass.The residual glass phase content is more than 20%,which is not conducive to the improvement of high-temperature resistance.Then B₂O₃-free glass fillers were developed.The LAS+MS ofβ-Li Al Si₂O₆ and Mg₂Si O₄ and LAS+MA ofβ-Li Al Si₂O₆and Mg Al₂O₄ were prepared.As Mg O cannot form the glass network,it can break the glass network and make it tend to crystallize.At the same time,the accumulation of negative ions by Mg²⁺promotes the crystallization tendency.Therefore,the increase of Mg O/Al₂O₃ ratio can improve the crystallization rate of glass.The optimized glass-ceramic CTE shows a good thermal match with substrates.The wettability of LMAS-B₂O₃ glass is poor,and the interface bonding between LMAS-B₂O₃ and the substrate is low through sessile drop tests.The wettability of glass on dense Si₃N₄ is improved significantly by the pre-oxidation treatment.LAS+MS and LAS+MA glass can be spread on both porous and dense Si₃N₄ surface,with excellent wettability.The liquid glass can infiltrate into porous Si₃N₄.The infiltration behavior of liquid glass to porous Si₃N₄ is in accordance with the capillary force driving infiltration mechanism,and the thickness of infiltration layer is proportional to the square root of time.The microstructure analysis shows thatβ-Li Al Si₂O₆/Mg₂Si O₄ andβ-Li Al Si₂O₆/Mg Al₂O₄ are precipitated in LAS+MS and LAS+MA glass-ceramic interlayers,respectively.A dense infiltration layer is formed on the side of porous Si₃N₄.The infiltration layer not only plays a role as a transition layer,but also enhances the porous Si₃N₄ and improves the strength of the joint.TEM results show that the bonding is realized by the interaction between glass and the intergranular glassy phase of dense Si₃N₄.The optimal joint was obtained by using LAS+MA solder at 1340°C for 10min.The shear strength of the joint was 84 MPa at room temperature and 87MPa at 850°C,respectively.The finite element method was used to simulate the distribution of residual stress in the actual joint.The joint stress is mainly concentrated at the angle between the interface edge of glass-ceramics and dense Si₃N₄.The infiltration layer plays a role of gradient transition,which effectively eases the stress of porous Si₃N₄ and is beneficial to the reliability of the joint.Raman spectroscopy was used to conduct nondestructive testing of the microregion residual stress near the interface of dense Si₃N₄,and the results show that the residual tensile stress of LAS+MS and LAS+MA glass-ceramics is formed within 30μm microregion near the dense Si₃N₄ interface,which is consistent with the change trend of finite element simulation results.