Preparation And Performance Study Of The Short Fiber C / C - Sic Composite

With the application background of braking materials and the purpose of cutting down the preparing cycle and cost, short carbon fibre reinforced carbon and silicon carbide (C/C-SiC) composites were manufactured through four steps: the dispersion of short carbon fiber and silicon powder by water suspension method; after polymer impregnation, compression moulding and carbonization to obtain the green body consisting of dispersed fibres and silicon; densification by multiple cycles of polymer impregnation and carbonization; generation of silicon carbide by means of in-situ reaction. The structure feature and graphitization of the composites have been studied using optical microscope, SEM, XRD and so on. Mechanical performance and tribological properties of composites are also tested, and the influencing factors of them are discussed. The conclusions can be drawn as the following:(1) The C/C-SiC composites have a layered structure with preferred orientation of short carbon fibres in the plane perpendicular with pressure, and only a few fibres are found in parallel direction. The silicon carbide is β -SiC with a uniform and isolated distribution, and some in the gaps between fibres and matrix.(2) Boron has catalytic effect of graphitization. With the increase of boron content, the degree of graphitization of composites arises. The cooperation of boron and silicon can speed the catalysis of boron. The degree of graphitization elevates with the addition of silicon, although the content of boron is cut down. The reaction of silicon and carbon will increases the activating effect of silicon on boron and then get a higher degree of graphitization, while the treatment of post-cure decreases the catalysis of stress graphitization.(3) The flexural strength of composites has a value of 30~60MPa, while flexural modulus of 8~12GPa, shear strength of 4~7MPa. The growth of fibre content raises the values of flexural strength and flexural modulus, but has no contribution on shear strength. With the increasingcontent of SiC, the mechanical performance declines. The flexural failure of C/C-SiC composites is brittle fracture. The micro-fracture mechanisms are shown as the pulled out, separated or brittle failure of the short carbon fibres, and the brittle fracture of carbon and silicon carbide matrixes.(4) The tribological properties of C/C-SiC composites are shown with coefficient of friction of 0.33, stability factor of 0.81, linear wear of composites of 17.8 u m each time, and linear wear of antithesis (30CrMoSiV steel) of 5.7 u m each time. Therefore the C/C-SiC composites can be applied in train, tank and car as braking materials with fine performance. With the increase of degree of graphitization of composites, the wear rate climbs up, so a temperate degree of graphitization may work well with better triboligical properties. The addition of braking pressure reduces the coefficient of friction of composites while increases the stability factor and wear rate of composites. The fiictional mechanism is the cooperation work of plough and adhesion with grain-abrasion primarily.