Examination of strength-controlling factors in C/C composites using bundle composites

C/C composites unidirectionally reinforced by one fiber bundle (the bundle C/C composites) were fabricated and tested in tension. Through the comparison between the tensile strength of the bundle C/C composite and that of the laminated one, effects of lamination on the tensile strength of the C/C composites were examined. Results suggested that defects in the laminae, such as transverse cracks, have no effects on the tensile strength of the C/C composites. This means the strengthcontrolling factors are included in the bundle. As one of the factors in the bundle, the effect of bonding strength of fiber/matrix interface was examined by using the fiber bundles with different surface treatments. In addition to surface oxidized and non-oxidized fiber bundles, an attempt was made to form porous carbon coating layers around the fibers to reduce the bonding strength intentionally. Results suggested that the bonding strength of the fiber/matrix interface, which was evaluated by bundle pushout tests, have significant effect on the tensile strength of C/C composites. It was also shown that the porous coating layers were effective in improving the tensile strength of C/C composites.