| C/C composite has many excellent characteristics such as high strength,light density,resistance to ablation,good impact.It is a vey important composite in the aerospace field.It is difficult to process it into a complex component,which is limited in practical application.Nb has excellent high temperature performance,and its density is the smallest in the refractory metals.The weldability of Nb is also very good.Therefore,the brazing of C/C composite with Nb to the nozzle is an ideal choice,which can significantly increase its thrust-weight ratio and reduce the consumption of energy.The purpose of this thesis was to improve the surface roughness of C/C composite,enhance the wettability of brazing filler metal,and change the structure of TiC reaction layer in the joints by the growing of carbon nanotubes(CNTs)on the surface of C/C composite to improve the bonding rate of joints.In this thesis,CNTs were grown on the surface of C/C composite by PECVD and the best method of preparating catalyst was the spin coating.The optimal parameters were summarized as follows: 0.10 mol/L for Fe(NO3)3 isopropanol solution concentration,750 ℃ for the growth temperature,10 minutes for holding time,200 W for the radio frequency(RF)power,10 sccm: 40 sccm for the gas flow ratio of the H2 and the CH4,850 Pa for the gas pressure.The results of the Raman spectra and TEM showed that CNTs under the optimum process parameters were multi-walled carbon nanotubes(MCNTs).The number of its walls was about 15 layers.The average diameter of CNTs was 18.9 nm and the length was about 2.5 μm.C/C and Nb were brazed with the CuTi brazing filler.The typical interface was: C/C/TiC/TiCu+TiCu4+Cu(s,s)/TiCu+TiCu4+Cu(s,s)+Nb(s,s)/Nb.The energy spectrum analysis results showed that many Cu-Ti compounds formed in the brazing seam brazed with the CuTi brazing filler.When the brazing temperature was 950 ℃,the holding time was 10 minutes,the joint strength was the highst.It was up to 41 MPa.The strength of joint before the growth of CNTs was 17 MPa,and it was increased by 141%.With the increasing of brazing temperature or the prolonging of holding time,the thickness of TiC reaction layer and the continuous TiC reaction layer were becoming thicker,and brittle compounds such as TiCu were also increasing.The shear experimental results showed that the shear strength of joint increased first and then decreased.When the brazing temperature was 890 ℃,the joint bonding rate was low.The fracture position was mainly at TiC reaction layer and a small amount was at C/C composite material.With the brazing temperature rising to 950 ℃,TiC reaction layer and the content of other brittle phases such as TiCu were moderate.The fracture position was mainly at the C/C composite side of the fracture.When the temperature rising to 1000 ℃,the content of TiC reaction layer and the content of other brittle phases such as TiCu were more,and the joint had excessive residual stress,so the brazing joint was broken on the C/C composite side.The enhancement mechanism of CNTs mainly includes two aspects: to enhance the wetting and change the TiC reaction layer.When the temperature was 950 ℃ and the holding time was 10 minutes,after the growth of CNTs on the surface of C/C composite,the wetting angle of the brazing filler metal was reduced from 37 ° to 15 °.After the CNTs were grown on the surface of C/C composite,the continuous TiCu and TiC reaction layers on the surface of C/C composite became thinner only 1 μm,and adjacent to the thickness of the dispersed TiCu reation layer was reduced by 50%,and the thickness of the dispersed TiCu reation layer was less than 10 μm.The particle distribution was more concentrated.Moreover,the strengthening effect of the fine grain significantly enhanced the joint strength.At the same time,the reaction phase grain is nanocrystalline at the side of C/C composite.The joint strength increased significantly because of its fine grain strengthening effect.Above all significantly enhanced the joint strength. |
PDF Full Text Request