Study On Microstructure And Properties Of Brazed Joints Of ZrB₂-SiC Ceramic And TC4-TiBw Composite

Zr B₂-Si C（ZS）ceramics and TC4-Ti Bw（TTw）composites both possess low density,high specific strength and good performance at high temperatures.Thus,the performance of engines in the aerospace field can be greatly improved with the application of the joining structure of them..Active brazing has been the most efficient method for the bonding of ceramics and metals.However,the large mismatch in the coefficient of thermal expansion（CTE）between ceramics and metals results in large residual stresses in brittle ceramic substrates,which drastically reduces the strength of the dissimilar joints.Four methods were used in this study to reduce the residual stress:（i）Ag Cu ductile filler,（ii）optimized composite filler,（iii）skeleton of ceramic particles used for interlayer and（iv）gradient layer of CTE deposited on TTw substates.Ag Cu filler with low elastic modulus and yield strength was used to braze ZS and TTw for the release of the residual stress.When the joint was brazed at 820°C for 5 min,the Ti C/Ti₅Si₃double-layer interface layer and Ti（s,s）layer formed on the surface of Si C and Zr B₂in ZS,respectively.The atomic bonding at ZS/braze interface was crucial to join ZS and TTw.The brazing seam was mainly composed of Ag（s,s）,Cu（s,s）and some massive Ti-Cu phases consisting of Ti Cu₄and Cu（s,s）.Ti B whiskers were formed on the surface of Zr B₂when the brazing temperature increased to 910°C.As the brazing temperature further increased,the size and aspect ratio of Ti B whiskers increased,while the Ti C/Ti₅Si₃disappeared.The reaction at ZS/braze interface took place progressively with the increase of the brazing temperature inferred by the thermodynamics.The effects of the process parameters on the strength at ZS/braze interface were analyzed,and the sufficient interface strength could be obtained when the joint brazed at 820°C for 5 min.However,the strength of the joint was only 5.4 MPa,indicating that the key factor limiting the joint strength was residual stress.The composite filler could reduce the thermal expansion of brazing seam by adding low expansion phase particles（such as VI B group metal,non-metallic boride and carbide/nitride）to the Ag Cu filler,thus relieving the residual stress in joints.Herein,the low expansion phases commonly have high elastic modulus and yield strength,which would lead to the deterioration of the deformation ability of the brazing seam.In order to hinder the deterioration of deformation ability of brazing seam to a certain extent,Ag Cu+Nb B₂composite filler was designed.In the joint,the matrix of the brazing seam were composed of Ag（s,s）and Cu（s,s）.Only a thin layer of Nb B and a limited number of Ti B whiskers epitaxially grown along the（-11-1）_{Nb B}//（0-1-1）_{Ti B}and[-314]_{Nb B}//[12-2]_{Ti B}direction formed by in-situ reaction with Ti for that Nb B₂has high chemical stability.The hedgehog like Nb B₂-Ti B structure could also improve the strength at Nb B₂/braze interface acting as a low expansion phase.Because of the non-coherent crystal relationship between the matrix of brazing seam and the Nb B₂-Ti B structure,it was difficult to promote the nucleation and improve the microstructure refinement of the matrix of brazing seam.Nb B₂could prevent the deterioration of the deformation ability of the brazing seam in many ways.First of all,compared with VI B group metals,the consumption of Ti in the reaction with Nb B₂prevented the formation of Ti-Cu brittle intermetallic compounds.Secondly,carbide/nitride and non-metallic borides could react with Ti to form a large number of dispersed nano-type Ti C particles and Ti B whiskers,while the amount of the Ti B whiskers produced by in-situ reaction of Nb B₂was limited and they only concentrated on the surface of Nb B₂particles.Thus,the ability of hindering load transfer and dislocation movement was weakened,which could weaken the deterioration of brazing seam deformation ability.Thirdly,there was no obvious microstructure refinement in the brazing seam compared with carbide/nitride,which weakened the barrier of grain boundary to dislocation.When the joint was brazed by Ag Cu+2 wt.%Nb B₂at820°C for 5 min,the shear strength reached the maximum（36.7 MPa）.The joint fracture was located in ZS substrate,illustrating that the mechanical properties were still subject to residual stress.To further prevent the deterioration of the deformation ability of the brazing seam based on the decrease of thermal expansion gradient of the joint,the interlayer of immersion Si C ceramic skeleton（ISS）was used together with Ag Cu filler to braze ZS/TTw.During the brazing process,the interlayer of ISS was completely infiltrated and the framework structure of Si C particles was not damaged,accompanying the formation of the Si C particle aggregation layer.With ISS filled,the excess liquid filler formed Ag（s,s）and Cu（s,s）,which constructed the soft brazing seam layer adjacent to ZS and TTw substrate.Thus,the spatial distribution of Si C particles with low thermal expansion was controlled,and a soft/hard alternative joint structure was constructed.In this joint structure,Si C particles gathered in the region away from ZS,which decreased the thermal expansion gradient of the joint.The soft brazing seam adjacent to ZS could release residual stress through excellent deformation ability.The increase of brazing temperature promoted the infiltration of ISS,but the framework structure of Si C particles would be destroyed at high temperature.The thickness of the soft brazing seam and the consistency of the interface reaction between ISS/braze was controlled by changing the porosity of ISS.When the ISS porosity was 30 vol.%,the strength of the joint brazed at 820°C for 5 min was the highest（41.2 MPa）and the joint fractured in the Si C particle aggregation area,which indicated that the mechanical properties were limited by the low expansion aggregation area with insufficient bearing capacity for residual stress.To improve the strength/toughness of the low expansion phase aggregation area and keep low expansion phase away from ZS substrates,a stress relief gradient layer（SRGL）with low thermal expansion was prepared on the surface of TTw by cladding in vacuum furnace.Afterwards,the TTw deposited by the SRGL was brazed with ZS.Ti H₂+Cu+W₂B₅was adopted for the preparation of SRGL,Ti H₂+Cu was melted and then reacted with W₂B₅to form Ti B whiskers and W（s,s）nanoparticles.The high temperature promoted the density and generation of W（s,s）,while the formation of Ti B was hindered.When the W₂B₅content was 50 vol.%,the SRGL was prepared at 900°C for 10 min,the strength of the joint brazed with Ag Cu Ti filler at 820°C for 5 min reached 96.4MPa.And the brazing seam was mainly composed of Ag（s,s）and Cu（s,s）,which was conducive to the release of residual stress.SRGL with low expansion was far away from ZS,which also possessed high strength and toughness:Ti B whisker could refine SRGL matrix and carried out fine grain strengthening;various low-expansion phases produce a whisker-particle combination and a multi-scale combination of second phase dispersion strengthening.This study provided effective designing ideas to relax the residual stress in ZS/TTw joints,which also could be widely used in other ceramic/metal joints and promoted the engineering application of composite structures of ceramics and metals.