Research On Brazing Mechanism And Joint Properties Of C/C And SiC_f/SiC Composites

C/C composites have the advantages of high specific strength and low thermal expansion coefficient,which are important lightweight high-temperature structural materials.Si C_f/Si C composites have excellent high temperature mechanical properties and oxidation resistance,and have great significance in hot end components.To make full use of the performance advantages of the two composites and expand their possible applications,the two composites were brazed with high-temperature Si based braze alloys.A new method combining reactive melting infiltration and brazing was proposed,an infiltration structure was introduced into low-density C/C brazed joints,and a composite joint structure with gradient transition of thermal expansion coefficient was designed.The interfacial structures and forming mechanism between Si based braze alloys and C/C and Si C_f/Si C were investigated.The mechanical properties of C/C–Si C_f/Si C joints were optimized by regulating joint microstructure.A dissimilar brazed joint with excellent high-temperature resistance was obtained.First,the Si-14Ti braze alloy was selected to study the infiltration behavior of Si-based eutectic brazes into low-density C/C,and the infiltration model of melt in porous C/C was established.The physical relationship between infiltration height and pore size and reaction time was expounded.Si C formed between the braze alloy and C/C sealed the holes and realized controllable spreading of the braze.Si C and Si-Ti Si₂ infiltration structure in the holes strengthened the C/C.The infiltration process was then optimized.The deposited layer of C/C infiltrated at 1400℃showed dense microstructure which could achieve good sealing effect.The infiltrated C/C was brazed using Si-14Ti braze.The obtained C/C joint was composed of Si matrix reinforced by Ti Si₂.A dense Si C reaction layer formed between the seam and C/C.The joint microstructure evolution was realized by adjusting brazing parameters.A Ti Si₂ microarray in-situ enhanced C/C joint was obtained.When the holding time was 90 min,the joint showed best room temperature shear strength of 41 MPa.Pulled-out carbon fibers and Ti Si₂ micro-arrays strengthened the joint.The high-temperature shear strength at 1000℃of low-density C/C joint was about 46 MPa and fracture mainly occurred in the Si C reaction layer.The low-density C/C joint using Si-14Ti braze alloy had good mechanical performance,but the two-step process was complex and took too long time.Thus,the high-density C/C joints using Si-14Ti and Si-10Zr braze alloys were studied.Firstly,the microstructure of high-density and low-density C/C joints using Si-14Ti was compared.Due to less infiltration structure in high-density C/C joint,the shear strength at room temperature was only about 22 MPa.The wetting behavior of Si-10Zr braze alloy on C/C indicated that the braze showed good wettability.The contact angle can reach 22°.C/C–Si-Zr–C/C joints were composed of eutectic Si and Zr Si₂.A Si C layer formed between the seam and C/C.When temperature rose,the average area occupied by Zr Si₂decreased from 85μm² to 19μm².Si C layer gradually grew,while the brazing seam became narrow.The shear strength of joint was elevated to about 32 MPa.The interfacial microstructure between Si alloy and C/C composites was explored.TEM analysis indicated that Si C interfacial layer was composed of nan-Si C and micro-Si C.The nano-Si C layer includedα-Si C andβ-Si C,while micro-Si C layer mainly consisted ofβ-Si C.Nano-Si C had high-density stacking faults and grain boundaries,which improved C diffusion to form micro-Si C.The brazed joint of high-density C/C composites using Si based braze alloys showed reliable mechanical properties.Thus Si-14Ti and Si-10Zr braze alloys were applied to join the C/C and Si C_f/Si C.The eutectic alloy showed good wettability on Si C_f/Si C.The C/C–Si C_f/Si C joints were composed of eutectic microstructure.Si C layers formed at the interfaces on both sides,4～19μm thick.The melt permeated into both materials to form pinning structures,especially on the Si C_f/Si C side.The forming mechanism of interfacial reaction layer on the side of Si C_f/Si C compisites was revealed.Si alloy directly bonded to the Si C matrix and no reaction product existed at interface.A new coarsed Si C reaction layer formed besides Si C fibers.The new reaction layer formed mainly due to dissolution and reprecipitation of the Si C nano-grains in Si C fibers.A diffusion layer rich in Ti or Zr formed between the Si C reaction layer and Si C fibers on the Si C_f/Si C side.Through optimizing processing parameters,the C/C–Si-Zr–Si C_f/Si C joint poccessed better room-temperature（about 38 MPa）and high-temperature strength（about 45 MPa at 1000℃）,indicating that the joint had good high-temperature application prospect.