Brazing Process And Interface Optimization Of C/C Composites And Nb

C/C composites have been widely used in aerospace field due to their low density,high strength at elevated temperatures,good ablation resistance and low coefficient of thermal expansion.Most components used in these areas are mainly made by metal or alloy,such as Nb,so C/C composites have to be connected with metal.At present,brazing,as one of the connection methods of C/C and metal,has been widely investigated.However,the release of residual stress in the joint is a problem.Therefore,the surface treatment method of C/C composite is used to assist the brazing process,reduce the residual stress in the joint and improve the joint strength.In this paper,active brazing C/C composites and Nb with AgCuTi was assisted by surface treatments,including pre-oxidation and in-situ growth of carbon nanotubes(CNTs).The surface states of C/C composites and their effects on shear strength of C/C,wettability and microstructure of brazed joint were analyzed by scanning electron microscopy(SEM),energy dispersive X-ray spectrometer(EDS),X-ray diffraction(XRD),transmission electron microscopy(TEM)and universal testing machine.The results showed that annular gaps with an adjustable size were uniformly constructed around the carbon fibers by pre-oxidation,and multi-wall CNTs were synthesized on the C/C surface and in the gap by CVD treatment.It was found that the contact angles on the original C/C and pre-oxidized C/C for 10 min were 25° and 31°,respectively,indicating the wettability became poor after pre-oxidation treatment.By growing CNTs,the wettability increased significantly,and the contact angle reduced to 9°.Through surface treatment,the interface structure of the brazed joint was optimized: the interface of the C/C side was changed from flat to infiltration interface and the zone of fine TiC particles was formed near this interface.Compared with original C/C,the shear strength of the joint brazed at 880°C for 10 min was increased from 29 MPa to 62 MPa after the surface treatment,and the fracture path changed from the TiC reaction layer to the infiltration zone.The influence of brazing parameters on the microstructure and shear strength of C/C-Nb brazed joint without and with pre-oxidation treatment was studied.It was found that the parameters affected the morphology of the brazed joint,the width of the brazing seam,the thickness of the reaction layer and the peak residual stress,thus affecting the shear strength of the joint.After optimizing the interface structure,the shear strength of joint has been improved,however,also affected by the brazing parameters.After testing,the maximum shear strength of C/C-Nb joints reached 29 MPa and 57 MPa,respectively,at 880°C for 10 min without and with pre-oxidation.By calculation and numerical simulation,the strengthening mechanism of joint includes the increased connection area,the increase rate of which is 374% when the temperature is preheated at 800°C for 5min;The pinning strengthening of the infiltrated Ag(s.s);The peak residual stress was decreased by infiltration zone as CTE transition zone,the peak residual stress was reduced from 231 MPa to 208 MPa after pre-oxidation;and the zone of TiC particles was used to alleviate the residual stress and strengthen the brazing seam.