Study On Construction Of Nanofiber-CF Multi-scale Preform And Its Strengthening And Toughening C/C Composites

During the work process of carbon/carbon(C/C)composite materials,the phenomenon of cracking,block collapse and interface debonding occurs frequently,which severely restricts its in-depth application in the aerospace field.The analysis shows that when the stress load of C/C composites reaches the stress limit of the carbon matrix,the carbon matrix began to crack and then produce destructive cracks,which will quickly transmitted to the carbon fiber(CF)result in CF be damaged,and finally lead to the sudden failure of the material.For the following reasons:Firstly,the carbon matrix lacks sub-micron and nano-scale reinforcement;Secondly,the bonding state of the carbon fiber/matrix interface is not conducive to the destructive cracks deflection at the interface,which cannot guarantee effectively the load will be transferred to the carrying CF,the strengthening and toughening mechanism is limited.This paper used Electrophoretic Deposition(EPD)to co-doped carbon nanotubes and silicon carbide nanowires(CNTs&SiCnws)on the CF surface to construct a nanofiber-CF multi-scale preforms,used chemical vapor infiltration(CVI)to load with high-texture pyrolytic carbon(HT-PyC)interface layer to optimize the carbon fiber/matrix interface to promote CNTs&SiCnws and CF cross-scale synergistic to strengthening and toughening C/C composites.By adjusting the CVI process time of 2 h,4 h and 6 h,and EPD process time of 1min,3 min and 5 min,to control the thickness of the HT-PyC interface layer and the content of CNTs&SiCnws.The effects of the content of CNTs&SiCnws and the thickness of HT-PyC interfacial layer on the matrix structure,microstructure,mechanical properties and oxidant properties of C/C composites were systematically studied.The relevant strengthening and toughening mechanism is elucidated.The main conclusions are as follows:(1)As the CVI time increases,the grooves on the surface of the CF basically disappear,and the orientation of the pyrolytic carbon interface layer is high,which is a typical HT-PyC.When the CVI time is 6 h,the adhesion structure between CF's appears;As the EPD time increases,the content of CNTs&SiCnws on the surface of the CF is increase,when the EPD time is 3min,the CNTs&SiCnws are distributed more uniformly,and there is no obvious agglomeration and shedding.(2)CNTs&SiCnws can induce the matrix PyC to transform from a single structure to a multi-layer structure.In the nanofiber-rich area,the nucleation and growth points are dense,which can induce the formation of isotropic(ISO)pyrolytic carbon,outside the nanofiber-rich area,the size of the crystals and grain boundaries is significantly larger than the nanofiber-rich area,the grain boundaries are cross-locked,and the PyC layer has higher optical activity,which is HT-PyC,and the overall present ISO-HT multi-layer structure.(3)An appropriate amount of CNTs&SiCnws can promote good crystal growth,reduce annular cracks,improve the order,integrity and graphitization of the PyC'structure,and improve the microstructure of the matrix.However,excessive CNTs&SiCnws and excessively thick HT-PyC interface layers do not significantly improve the microstructure of the matrix.(4)CNTs&SiCnws provide more nucleation points for the growth of PyC,but will hinder the flow and diffusion of hydrocarbon gas during the densification process,which will affect the permeability of PyC and cause the formation of closed-pore structure in the matrix,causing a slight chang in density and porosity of the C/C composite.(5)The introduction of CNTs&SiCnws and HT-PyC interface layer can realize the strengthening and toughening of C/C composites.For the bending performance,the strength of HT-PyC_4h-CNTs&SiCnws_3min-C/C is increased by 74.33%,and the failure mode is ductile fracture,and the improvement in strength and toughness is the most significant;For the shear performance,the strength of HT-PyC_2h-CNTs&SiCnws_3min-C/C is increased by nearly 100%,the fracture pseudoplasticity of HT-PyC_4h-CNTs&SiCnws_3min-C/C is the most obvious,and its strength improvement is not as good as the former;For out-of-plane compression performance,the impact is significantly less than bending performance and shearing performance,HT-PyC_4h-CNTs&SiCnws_1min-C/C has a more obvious improvement,and the compressive strength reaches 269.55 MPa,which is 24.63%higher than pure C/C.(6)CNTs&SiCnws can effectively reduce oxygen transmission channels such as carbon fiber/matrix interface debonding and matrix annular cracking,hindering the flow and diffusion of oxygen,and the interface layer can delay the oxidation of CF,thereby improving the oxidation resistance of C/C composites performance.For static oxidation performance,the weight loss rate of HT-PyC_6h-CNTs&SiCnws_1min-C/C sample is 11.28%,which is 64.46%lower than pure C/C;For thermal shock performance,CNTs&SiCnws will induce the carbon matrix to form a multi-layer structure and“closed-pore”structure,which is beneficial to resist structural damage caused by rapid changes in high and low temperatures,HT-PyC_4h-CNTs&SiCnws_3min-C/C has the best thermal shock resistance.