Study On The Microstructre And Properties Of C/C Composites Modified By Carbon Nanotubes

Due to the outstanding performance, C/C composites have developed from thermal protection materials into both thermal protection and structural materials which are widely applied in aerospace filed. However, during the preparing process, matrix annular cracking and poor fiber/matrix interface bonding always lead to the poor properties. Grafting carbon nanotubes(CNTs) in carbon fiber perform is an effective method to strengthen the adhesion between carbon fibers and pyrocarbon, to inhibit matrix defects and to improve the properties of C/C composites. The main way to prepare CNTs-carbon fiber multiscale prefrom is in-situ growth method, which has some disadvantages of high cost, process complexity and carbon fiber damage problems. To avoid these issues, in this work, a simple method with mild operating conditions and low-cost, i.e. electrophoretic deposition(EPD) was used to prepare multiscale hybrid prefrom which not only achieved the homogeneous deposition morphology of CNTs but also greatly shortened the time of preparation process. The effects of CNTs with different content on the densification rate, microstructures, oxidation and interlaminar shear properties of C/C composites have been studied and some main results have been obtained as follows:The influence of EPD time on the CNTs deposition was studied. Results show that EPD time has a great impact on CNTs content. More CNTs can be deposited in perform as EPD time increases. The optimal EPD time for CNTs uniformly depositoned on the surface and inner place of carbon felt is 60 s.The densification behaviors of C/C composites modified by CNTs were investigated and the results show that CNTs with different content can improve the densification rate of C/C composite. But as CNTs content increases, the increment shows an initial increase, followed by a decrease. When the EDP time is 60 s, the CNTs content of CNT-C/C is 3.43 wt%, the densification rate of CNT-C/C composite is the fastest due to the proper content and uniform distribution of CNTs.The effect of the CNTs content on the microstructure of C/C composites was also investigated. CNTs can change the deposition behavior of pyrocarbon,make the pyrocarbon grow around carbon fiber and CNTs simultaneously. Pyrocarbon around the carbon fiber shows smaller sizes and more ordered deposition because of the existence of CNTs. The structural integrity and the degree of graphitization of pyrocarbon are improved. The random orientation of CNTs makes the pyrocarbon in the CNTs enriched area shows ISO texture. Too much or too little CNTs show limited ability of inducing the ordered deposition of pyrocarbon.The oxidation behavior of performs and composites modified by CNTs was systematically researched. After introducing CNTs, the specific surface area and active sites increase, as a result, the starting temperature of CNTs modified perform decreases as the CNTs content increases. CNTs have an ability of inducing the ordered deposition of pyrocarbon and can reduce the number of defects, thus raising the starting temperature of C/C composite. During the isothermal oxidation test from 600 to 900 oC, the oxidation rate of CNT-C/C composite is much lower than that of C/C composite. The oxidation resistance of CNT-C/C composite is better than C/C composite since CNTs can act as connecting bridges within the matrix, strengthen the adhesion of matrix, thus inhibit the formation of rapid oxidation channels. But the degree of improvement first increased, then decreased. Among them, when the EDP time is 60 s, the CNTs content of CNT-C/C is 3.43 wt%, the antioxidant property of CNT-C/C composite is the best.The mechanism how CNTs influence the interlaminar shear properties of C/C composites was studied. CNTs can strengthen carbon fiber/matrix interfacial bonding and the cohesion of the matrix which makes the interlaminar shear strength and modulus of CNT-C/C composites were higher than that of pure C/C composites. When the EDP time is 60 s, the CNTs content of CNT-C/C is 3.43 wt% and the interlaminar shear strength and modulus of CNT-C/C composite reach the maximum, respectively 19.85 MPa and 675.85 MPa, which increase 79.89% and 204.85% than that of C/C composite. After introducing CNTs, the fracture mode of C/C composites changes from pseudo-plastic fracture to brittle fracture.