Interfacial Structure And Formation Mechanism Of Brazed Joint Of BN-SiO₂ Ceramic To Nb

Composed of crystalline h-BN and SiO₂,the BN-SiO₂ ceramic composite has been provided with both excellent performances,which is regarded as a desirable candidate in various aerospace applications.Nb has also been widely applied in aerospace due to its unique refractory characteristic.In practical application,it is always a frequent requirement of joining the BN-SiO₂ ceramic composite with Nb for the sake of acquire complementary advantages.In this study,the wettability of BN-SiO₂ ceramic composite is investigated and the brazability of BN-SiO₂ ceramic composite and Nb is discussed in detail.Furthermore,a composite interlayer with network architecture?CINRA?is designed in order to reduce the high residual stress in the brazed joint.The strengthening mechanism of the interlayer to the brazed joint has also well illuminated.The wetting behavior of molten Ti-Ni filler alloy on the BN-SiO₂ ceramic composite has been investigated by the sessile drop method to illustrate the role active elemental Ti plays during the wetting process.Results demonstrates that this wetting process falls into 3 stages as follows: rapid decreasing stage,sluggish spreading stage andfinal equilibrium stage.A good wettability of the Ti-Ni filler alloy on the BN-SiO₂ ceramic composite has been certified by observing the final contact angle of approximately 42°.Besides,the wetting interface in-between the TiNi/ BN-SiO₂ ceramic composite was studied by TEM,exhibiting the interfacial production of TiN,TiB,Ti₅Si₃ as well as Ti₃O₅.It indicates that TiN was the first reaction product during brazing which facilitate the wetting of Ti-Ni filler alloy on BN-SiO₂ ceramic composite.Moreover,In order to reveal the relationship between the final contact angle and the interfacial reaction products,the refined Rietveld of XRD pattern was taken.According to the refined Rietveld of XRD pattern of the reaction layer of the Ti-Ni/BN-SiO₂ ceramic composite,the wettability mainly depended on the final interfacial chemistry of the system instead of the intensity of the interfacial reactions.The BN-SiO₂ ceramic composite and Nb were brazed using Ti-Ni filler alloy.The reaction of the Ti and BN-SiO₂ ceramic was the key to the brazing process,and the reaction layers adjacent to the BN-SiO₂ ceramic composite were Ti₃O₅+Ti₅Si₃+TiB₂ layer and TiN layer.The activity of Ti in the braze filler was calculated using Miedema model.Through comparing the Ti chemical potential of the solid and liquid interface,the interfacial reaction mechanism was analyzed.The Ti chemical potential of filler alloy was 0.424.The effect of Nb dissolution on Ti element activity in Ti-Ni filler alloy was disscused.The activity of Ti was 0.778 using Koller model.The Ti chemical potential was higher than that of Ti-Ni filler alloy,and the interfacial reaction adjacent to the BN-SiO₂ ceramic composite was intensified.Accordingly,by using first-principle calculations,the h-BN/Ti and a-SiO₂/Ti interfaces are investigated at atomic and even electronic scale.Based on the simulation results,the Ti atom is more likely forming new interfacial phase with h-BN than SiO₂.A novel composite interlayer with network reinforcement architecture was designed by the SiC and Ti-Ni composite filler foils,with which Nb and BN-SiO₂ ceramic composite were successfully brazed in vacuum.During brazing,the filler alloy could infiltrate into the CINRA and react with SiC to form the network reinforcement architecture.When the brazing temperature was 1170°C and the holding time was 5min,the interfacial microstructure was BN-SiO₂ ceramic composite/Ti₃O₅+Ti₅Si₃+TiB₂+TiN/SiC+TiC+Ti₅Si₃/?Ti,Nb?-TiNi+TiNi+TiNi₂/Nb.With the brazing temperature increasing,the filler alloy infiltrates into the SiC sufficiently and reacts with the SiC intensively,which leads to the destruction of the nets structure of the SiC.Besides,the SNS cannot be infiltrated completely when the holding time is short.The maximum shear strength of 102 MPa can be obtained at 1170°C for 5min.The effect of interlayer parameters on the interfacial microstructure and mechanical properties of BN-SiO₂ ceramic composite/CINRA/Nb joints was further studied.When the porosity of CINRA is low,the filler alloy could not infiltrate due to the formation of closed pores.On the contrary,when the porosity of CINRA is large,the filler alloy could also not fill the CINRA pores due to the reduction of capillary force.Additionally,the filler alloy reacts with the CINRA intensively when its thickness is large,giving rise to the extreme degradation of the mechanical properties of the joint.When the porosity of the CINRA interlayer is 50% with the thickness of 2mm,the maximum shear strength could reach 102 MPa.The submodel of the CINRA interlayer has been established to further analyze the enhancement mechanism of the interlayer by relieving the residual stress in the joint.Results illustrate that the introduction of the CINRA interlayer changed the distribution of residual stress in the joint.In details,the residual stress distributes discontinuously within the CINRA skeleton and it mainly concentrates at the edge of the pores.However,the maximum residual stress in the submodel is lower.Compared with the directly brazed BN-SiO₂ ceramic composite/Nb joint,the maximum residual stress at the interface in-between BN-SiO₂ ceramic composite and the seam decreased was by 14% after using the CINRA interlayer.Eventually,the reinforcement mechanism of the CINRA interlayer to the joint has been studied.In essence,the main cracks could be transferred into multiple directions by the CINRA interlayer,which decentralized the principle stress,thereby impeded the crack growth.By analyzing the comparison,it is found that the minimum resistance is 7.43 MPa when the cracks grew along the CINRA skeleton,indicating where the cracks propagation could be easier occur along.This cracks propagation path possesses numerous twists and bends which could hinder the crack propagation,and accordingly be conductive to acquire considerable mechanical properties of the joint.