| Silicon nitride ceramics which possess superior mechanical properties,excellent dieletric properites and good corrosion resistance, have been one ofpredominant missile radome materials. In the practical application of Si3N4ceramic,the assembly of missile radome with other parts requires the joining of Si3N4ceramic radome to a metallic holder. Invar alloy with a small coefficient of thermalexpansion (CTE) is commonly used for metal connecting ring. As the adhesivebonded joints are poor in mechanical properties and ageing resistance, as well as theadditional weight added by mechanical bonding, Ag-Cu-Ti active filler alloy wasapplied to join Si3N4ceramic to Invar alloy and the effect of brazing parameters andfiller alloy’s thickness on microstructure and mechanical properties wereinvestigated. According to the dissolution mechanism of Invar alloy, Cu interlayerwas applied to adjust interficial microstructure and improve the mechanicalproperties of Si3N4/Invar joint. In addition, the formation mechanism of the brazedjoints was analyzed.Si3N4ceramic was brazed to Invar alloy using Ag-30Cu-2Ti filler alloy. Thetypical microstructure of Si3N4/Invar joint was as follows: Invar alloy/Fe2Ti+AlNi2Ti+Ag(s,s)+Cu(s,s)/Fe2Ti+TiN/Si3N4ceramic. Brazing parameters andfiller alloy’s thickness had significant impact on the formation and distribution ofintermetallic compounds and the thickness of reaction layer at the brazing/Si3N4ceramic interface. While raising the brazing temperature, increasing the holdingtime or the filler alloy’s thickness, the intermetallic compounds and the thickness ofreaction layer would increase. Meanwhile, the joint strength would first increaseand then decrease. The maximum joint strength (206MPa) was obtained while theSi3N4ceramic and Invar alloy were brazed with a thickness of80μm Ag-30Cu-2Tiat850℃for10min.Aiming at completely restraining the generation of Fe2Ti+AlNi2Ti intermetalliccompounds and improving the capability of relieving residual stress, Ag-Cu-Ti/Cu/Ag-Cu composite interlayer was designed to join Si3N4to Invar. The typicalmicrostructure of the brazed joints was identified as: Invar alloy/Cu(s,s)+Ag(s,s)/Cu(s,s)/Ag(s,s)+Cu(s,s)/Cu-Ti+Ti5Si3+TiN/Si3N4ceramic. The influence of the Cu-foil thickness on the interface microstructure and mechanical properties of jointswas investigated. The result showed that the shear strengths of the joints obtainedwith Ag-Cu-Ti/Cu/Ag-Cu composite interlayers with different Cu foil thicknesswere higher than that of the joints brazed with Ag-30Cu-2Ti foil filler. In addition, the highest joint strength (236MPa) was received by using80μm thick Cu foil.When brazing Si3N4ceramic and Invar alloy with the composite interlayer, theinhibition mechanism of Fe2Ti+AlNi2Ti intermetallic compounds was investigated,which could be divided into two parts. On the one hand, the order of the foils playedan important role, and the Ag-Cu eutectic foil melt completely before Ag-Cu-Ti foilcould prevent effectively the Ag-Cu-Ti filler alloy from spreading to the side ofInvar alloy along the surface of Cu foil. On the other hand, the Cu foil played asignificant role in separating the melt Ag-Cu alloy and Ag-Cu-Ti alloy so that Tidiffused to the ceramic side in a single direction. As a result, the formation ofintermetallic compounds was inhibited completely. |
