Preparation Of Carbon/Carbon-graphite Composites And Its Properties

As the development of high-speed train, there are more stringent requirementsabout pantograph slider materials. Considering the protection of the catenary and thepresence of high-temperature arc, carbon materials is the best choice for slide material.But the disadvantages of carbon materials are its poor mechanical properties and weakelectrical conductivity. In this paper, carbon fibers are used to improve the mechanicalproperties while sub-micron graphites are added to reduce the resistivity of carbon strip.Different amounts of graphite have been added to carbon fiber felts by ultrasonic ingraphite suspension. The graphite suspension dispersion medium, suspensionconcentration, ultrasonic time, and the preform porosity play a role on the graphiteaddition amount. More graphites are added to carbon fiber felt when dispersed inalcohol than in water because alcohol is easier to wet carbon fibers than water. It is goodfor graphite addition when increasing the concentration of suspension. But when theconcentration is too large, it is bad for graphite addition because pores of the fiberpreform are easier to be blocked. The denser the preform, the more difficult for graphiteaddition. As high as15vol%graphite has been added to carbon fiber felt, which mainlyexist between the layers of the preform and a few in the fiber bundle.Carbon/carbon-graphite composites with different amount of graphite have beenprepared by pitch impregnation-caronization method. The pressure, temperature ofimpregnation and the existence of graphite have an effect on impregnation. The graphitein preform interlayers prohibits the pitch impregnation. The more the graphite, the moredifficult for impregnation. When the pressure is up to3MPa, the carbon fiber felt withgraphite can be completely impregnated. As the composites become denser, it is moredifficult to impregnate, then higher temperature is needed. During the liquid phase ofpitch carbonization, the gases produced will extrude the pich, which greatly reduce thedensification efficiency. When the preform are carbonized in pitch, there is a hotisostatic pressure formed by liquid pitch, which significantly inhibit the outflow of pitchin the prefrom. When the initial carbonization pressure reaches5MPa, it throughlyinhibits the pitch outflow. After6times of densification, composites reach a density of~1.7g/cm~3.Under polarized light microscopy, the texture of carbon matrix in carbon/carboncomposites is domains. For carbon/carbon-graphite composites, the carbon matrixes aredomains and mosaics because graphite prohibits the coalesce of mesophase during pitchcarbonization. Under SEM, the interface of carbon fiber and domains matrix looks likeshell plate structure which is weak, while the interface of carbon fiber and mosaics matrix have a good bonding. There are lots of fibers pull-out on the fracture surface ofall composites, implicating a pseudo-plastic fracture behavior. The graphite improvesthe interface bonding of carbon fiber and carbon matrix. Thus, the interface debondingand fiber pull-out need more load, resulting higher mechanical properties. When theamount of graphite is up to7.6vol%, the bending strength increases from128±12MPato188±3MPa, compressive strength increases from155±3MPa to190±5MPa,impact toughness increases from1.7J/cm~2to2.6J/cm~2. Meanwhile, as the amount ofgraphite increases, the resistence of composites decreases. The more the graphite, thebetter the conductivity. When the amount reaches7.6vol%, the resistance of compositesreduces from30.3μΩ·m to25.6μΩ·m.