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The effects of heat treatment temperature, HTT, on the tensile strength (u) of PAN-based carbon-fiber-reinforced carbon matrix composites (C–Cs) were examined. In order to understand the mechanisms yielding variation of the tensile strength, the distribution of fiber strength and the interfacial strength between the fiber and the matrix were also determined as a function of HTT. The us of the C–Cs and the PAN-based-fiber was found to decrease with increasing HTT. The degradations of the fiber strength proceeded with distortion of the fiber cross section and reduction of the fiber diameter. The interfacial strength slightly decreased with increasing HTT. The tensile strength of C–Cs is generally enhanced by the reduction of the interfacial bonding strength. Thus, the interfacial change is not a main source of the strength degradation. Finally, a comparison of u between two types of C–Cs, showed that the only difference was reinforcing fibers and led to the conclusion that the primary factor yielding the degradation of C–C strength by HTT was the fiber degradation.

The tensile fracture stress and strain of carbon fiber-reinforced carbon matrix composites (C–Cs) were examined as functions of the bulk density. When the density increased, the interfacial strength of the C–Cs monotonically increased, and the tensile fracture strain decreased. In contrast, the tensile fracture stress was improved and degraded in the regions of density lower and higher than 1.6 g/cm3, respectively. Two tensile fracture mechanisms of the examined C–Cs were identified with the transition at the density of 1.6 g/cm3. In the low-density region, load transfer capability across fiber–matrix interfaces was shown to have an important role, and in the high-density region, stress concentrations at matrix-crack tips were presumed to be a major factor for the tensile fracture of C–Cs. This suggests that the most important interfacial property for tensile fracture is not interfacial sliding but debonding stress.

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