Microstructure And Mechanical Properties Of Silicon Carbide Joints Diffusion Bonded By In-situ Formed Ceramic Composite Interlayer

In this paper,a series of composite interlayer materials were prepared based on in-situ reaction under vacuum conditions to further diffusion-bond SiC ceramics.Respectively,reasonable composite interlayer materials were designed and appropriate bonding technique was adopted during the process of diffusion bonding.To further explore the diffusion bonding mechanism,the relationship between the microstructure and mechanical properties of the joints was analyzed.Some composite ceramic interlayers with similar physical and chemical properties and structure to SiC matrix were designed and fabricated to improve the bonding strength of joints.The number of cracks in the interlayer was reduced by decreasing the residual stress caused by the difference of thermal expansion coefficient between the bonding layer and SiC matrix.TiC_x-SiC composites were successfully fabricated to bond SiC ceramics by reactive SPS technique at 1300? with the mole ratio of Ti:Si:C=1.x:3:2(x=0,0.1,0.2,0.3).But at this time,there were many defects in the joints and the reliability of the joints was poor.Ti₃SiC₂ was used to bond SiC ceramics in the range of 1200-1600?.With the increasing temperature,the interlayer hardness increased gradually,while the shear strength of SiC joint increased first and then decreases.When the temperature rised to1500?,some Ti₃SiC₂ phases decomposed to form brittle and hard TiC_x phase,which increased the hardness of the interlayer.At this time,Ti₃SiC₂-TiC_x composite bonding layer was formed,and the highest shear strength(82.4 MPa)of the joint was obtained.However,as the temperature continued to rise to 1600?,Ti₃SiC₂ was completely decomposed.The bonding layer was mainly composed of TiC_x phase,and the hardness of the interlayer continued to increase.Without Ti₃SiC₂ phase,the residual stress can not be effectively alleviated,resulting in a large number of crack defects in the joint,which led to a significant reduction in the joint strength.Compared with neat Ti₃SiC₂or TiC_x phases,the Ti₃SiC₂-TiC_x composites possessed higher bonding strength.At 1400?,hard reinforcing phases(Ti B₂,TiC and SiC)were formed by in-situ reaction of B₄C and Ti₃SiC₂.Particle reinforced interlayer composites were prepared and SiC ceramics were joined at the same time.Ti B₂,TiC and SiC were distributed uniformly in Ti₃SiC₂ matrix,which improved the mechanical properties of the joint layer material.By changing the amount of B₄C,the microstructure of bonding layer was adjusted,and the thermal expansion coefficient of the connecting layer can be reduced,so as to achieve better bonding strength.With increasing B₄C content,the hardness of the interlayer increased gradually,while the shear strength of SiC joint increased first and then decreased.In the range of 5-15 wt.%B₄C,higher bonding strength of SiC joints can be obtained.When 7.5 wt.%B₄C was added,the maximum shear strength of the joint was 112.5 MPa,which was 53.9%higher than that without B₄C.SiC-Ti B₂ composites were successfully prepared to further bond SiC ceramics in the range of 1200-1700? by using Ti₃SiC₂-B₄C-Si system.With the increase of SPS temperature,the bonding strength of SiC ceramic joints first increased and then decreased,and the maximum shear strength(128.0 MPa)was obtained at 1600?.At1600?,the intermediate layer of the composite is composed of 48.7 vol%SiC and51.3 vol%Ti B₂ phases.The thermal expansion coefficient of the composite interlayer was similar to that of SiC matrix,so that better bonding strength can be achieved.Ti₃SiC₂-TiC_x composites were prepared by in-situ reaction of TiH₂-Si-C system in the range of 1150-1450?,and SiC ceramics were joined at the same time.With the increase of temperature,the hardness of the interlayer material first decreased and then increased,while the shear strength of SiC joint first increased and then decreased.In the range of 1250-1350?,the defects in the middle layer are less and the interface with SiC base metal is better.For 1350? joints,Ti₃SiC₂-TiC_x composite bonding layer was formed,and the maximum shear strength(80.5 MPa)of the joints was obtained.Ti₃SiC₂-SiC,Ti₃SiC₂-SiC-TiC and TiC-SiC composites were prepared using TiH₂-Si-C-Al system at 1300-1600?,and the bonding of SiC ceramics was acheved at the sam time.With the increase of SPS temperature,the bonding strength of SiC ceramic joints first increased and then decreased,and the maximum shear strength(124.9 MPa)was obtained at 1450?.The fabricated Ti₃SiC₂-SiC composite possessed higher hardness and damage resistance than neat Ti₃SiC₂ material.