Fast Cvi Preparation Of Tablet C / C Composite Material,

Carbon/carbon(C/C) composites are materials which associate excellent thermal and mechanical properties with low density. Their high strength, toughness, stiffness and their thermal shock resistance due to high thermal conductivity and low thermal expansion are maintained up to very high temperature. Thus, C/C composites are widely used in aerospace and military fields.In this experiment, an improved pressure-gradient method was developed in order to make carbon fiber-reinforced carbon composite plates specially. C/C composite plates with needle-punched felt as preforms were prepared by pressure-gradient chemical vapor infiltration using propylene as source gas and nitrogen as carrier gas. The mixed gas pushed by pressure-gradient penetrates through the preform along the thickness orientation, which reduces the resident time and enhances the rate of densification. The result show that follows:(1) After series of experiments, the average density of C/C composite plates (300mmv×300mm×15mm) could be up to 1.57 g·cm-3 at optimal condition.(2) There was a critical temperature of 1313 K in the selective temperature interval in terms of the change rate of densification. When the temperature was below 1313 K, the rate of densification increased rapidly with temperature rising. However, it increased obviously slowly when the temperature is above 1313 K. In addition, the pressure of deposition which has a crucial effect on the rate of densification is also an important parameter in Pressure-difference Thermal-gradient CVI. It's investigated that the rate of densification could increase from 0.0076 g·cm-·h-1 to 0.0184 g·cm-3·h-1 when the pressure changed from 2.0 kPa to 3.0 kPa.(3) The polarized optical micrographs showed that the microstructure of pyrolytic carbon prepared by CVI in C/C composite plates mostly is smooth laminar. Only on the edge of some samples, it appeared some different characteristics. The Raman spectroscopy displayed that the structures of pyrocarbon are different in the different parts of samples and in the different location of the same pyrocarbon ring. The structure of pyrocarbon in fibre cloth was affected larger than the radomly chopped fibres by the desposition temperature.(4) Based on the Mercury-injection test results, it can be seen that pores with large size (>60μm) were filled rapidly during CVI process; as proceeding of CVI, density of the composite increased continuously, however, the amount of pores with about 20nm in size increased remarkably.(5) Three-point bending tests showed that bending strength and flexural modulus of plate C/C composites are 122MPa and 17.5GPa for the density of 1.50g·cm-3. The C/C composites plate have an excellent mechanical property, Compared with other processes of fabrication. The fracture surfaces were analyzed by Scannig electron microscope. The composites with various densities all exhibit a peudo-plastic fracture behavior due to moderate interface intensity of pyrolytic carbon and carbon fibers.