| In the aerospace field,dense Si3N4 ceramic materials can replace metal materials applied to the rain eroding head of the missile radome due to its high strength,corrosion resistance,high temperature resistance,and excellent dielectric properties,thereby reducing the dielectric loss of the radome.Porous silicon nitride ceramics can be used in missile radome enclosures due to their low dielectric loss,high temperature resistance,excellent thermal shock resistance,and ease of processing.Therefore,the joint between the porous Si3N4 ceramic and the dense silicon nitride ceramic is realized to the integrated application of the structure and function of the Si3N4 ceramic,and has broad application prospects.This paper selects the glass-ceramic which has excellent dielectric properties to achieve the joint of porous/dense Si3N4 ceramics.It explores the basic properties of the glass-ceramics solder itself and the influence of different solder compositions and jointing process parameters on the microstructure and properties of the joints.The joints of the organizational transformation and the bonding mechanism.For the joint of porous/dense Si3N4 ceramic base materials,CaO-Al2O3-SiO2(CAS)glass-ceramics were designed for bonding.Through the study of the basic properties of the CAS glass-ceramic system,it had been found that by adding Li2O,the melting temperature of the system can drop below 1200?,and the glass solder can be melted and prepared at relatively low temperatures.All of the solders in this system had good crystallization ability.Among them,the type of precipitated phases can be adjusted between spodumene(LiAlSi2O6),anorthite(Ca Al2Si2O8)and wollastonite(CaSiO3)through the change of composition.The coefficient of thermal expansion of the crystallized glass block was measured and it was found that the coefficient of thermal expansion can be adjusted between 3.5-8×10-6/?.Only 9.4% difference between the optimal thermal expansion coefficient and the coefficient of thermal expansion of the dense Si3N4(3.2×10-6/?).They can achieve good heat matching.By studying the wetting behavior of CAS glass solders with different compositions on dense Si3N4,it was found that glass solders with TiO2 have excellent wettability with a wetting angle of 14° due to the formation of Ti N reaction layer on the dense Si3N4 surface.At the same time,it was found that the Li2 O content also affects the wettability.When the Li2 O content was reduced from 5wt.% to 3wt.%,the wetting angle also increases from 45° to 51°.We used different composition of CAS-based glass-ceramics to joint porous Si3N4 and dense Si3N4 ceramics,it was found that the solder had an infiltrated layer on the porous Si3N4 side.When using eutectic CAS1 solder added with TiO2,a layer of Ti N was formed at the bonding interface.In the layer,only spodumene with a low coefficient of thermal expansion(CTE=0.9×10-6/?)was precipitated in the middle of the weld,and the highest shear strength was only 10 MPa.When using near-eutectic20 CaO-18 Al2O3-57 SiO2-5Li2O(CAS2)and 20 Ca O-18 Al2O3-59 SiO2-3Li2O(CAS3)with Li2 O contents of 5wt.% and 3wt.%,it was found that the mainly precipitation phases of CAS2 solder were spodumene,anorthite and wollastonite,and CAS3 glass solder only precipitated anorthite.They can obtained good joint organization.By adjusting the bonding temperature,cooling rate and adding heat treatment process parameters,CAS2 can reach the optimum shear strength of 45 MPa at the bonding temperature of 1100? and cooling rate of 5?/min.CAS3 solder can be bonded at the temperature of 1200?,the best shear strength is 52 MPa under the heat treatment process of 950? for 1h,and the high temperature shear strength of the joint at 850? can reach 59 MPa,Which reached the strength of the porous Si3N4 ceramics itself. |
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