Fiber-matrix Interface Mechanical Properties of Carbon-Carbon Composites

Fiber-matrix interface mechanical properties are the key design point for the composites, however, methodology for evaluating interface properties of C/Cs are not still well established. In this study, capability of the fiber bundle push-out method was demonstrated as a measurement technique of fiber-matrix interfacial mechanical properties of various C/Cs. First, to carefully determine the experimental conditions to give proper interface mechanical properties, the influences of a radius of an indenter and the thickness of specimens on the measured results were examined. From the experimental result, interface debonding stress, τd, and sliding stress, τs, could be obtained as a constant value independent from specimen thickness as long as tested using the same indenter size and the proper specimen thicknesses of < 400 µm. However, absolute value of especially τd showed dependence of on indenter size. This was shown to be the limitation of this method that τd obtained from this method cannot be treated as a quantitative value. To demonstrate the capability of the method, the change of fiber-matrix interface mechanical properties of C/Cs on the type of fibers and final heat treatment temperature. τd decreased with the heat treatment temperature in both C/Cs with pitch based carbon fibers, K633 and K321 and C/Cs reinforced with a carbon fiber with higher heat treatment temperature gave lower τd at the same heat treatment temperature. Main reason for the degradation of τd up to 2273K was attributed to the enlargement of debonding area at fiber-matrix interface. Degradation of τd was enhanced at the temperature range of > 2273 K by the structure change of the interface from the graphitization of matrices. Finally, The fiber bundle push-out method was proved to be a strong tool to investigate fiber-matrix interface mechanical properties of C/Cs.