| Polyarylacetylene (PAA), a highly cross-linked aromatic polymer, has been investigated as a matrix precursor for carbon/carbon (C/C) composites. PAA has advantages over state-of-the-art phenolic resin systems because of its ease of processability, higher char yield, and lower pyrolysis shrinkage. Unidirectional PAA-derived C/C composites were fabricated in single and multi-tow configurations. Variables such as the processing heat-treatment temperature (HTT) (1100{dollar}spcirc{dollar}C, 1800{dollar}spcirc{dollar}C, 2150{dollar}spcirc{dollar}C, 2400{dollar}spcirc{dollar}C, 2750{dollar}spcirc{dollar}C), type of reinforcement used (T50, E35, E75, E105, E130, P55, P100, PX7, XN70), and matrix boron-dopant concentration were controlled in order to the study their effects on the microstructural and resultant mechanical behavior of these PAA-derived composites. The microstructural development of the matrix was examined at various processing stages using X-ray diffraction, Raman spectroscopy, optical, SEM, and TEM microscopy. Matrices heat-treated to 1100{dollar}spcirc{dollar}C and 1800{dollar}spcirc{dollar}C were amorphous. Upon heat-treatment to 2400{dollar}spcirc{dollar}C, localized graphitization was evident. Higher heat-treatment led to an increase in the degree and extent of graphitization emanating from the fiber/matrix interface. In-situ SEM flexural tests were also conducted in order to study the matrix crack propagation behavior and failure mechanisms of the undoped composites as a function of HTT. The structure of the matrix was shown to strongly influence the fracture behavior of the composite by affecting crack propagation. The tensile strength of the composites was dominated by the matrix zone ahead of the crack tip. The bond strength of resin matrix composite also strongly influenced the final mechanical behavior of composite. This bond strength was a function of the degree of localized matrix orientation, fiber modulus, fiber crystallite size, and processing HTT of the composite. The effect of carborane addition on the microstructural and mechanical properties of these composites was also examined. The graphitization of the matrix material was a function of both dopant concentration and HTT. The extent and degree of graphitization in the matrix was more controllable than the undoped composites. The mechanical behavior of the composites, however, was not only a function of the matrix microstructure, but was also dependant on the location of boron. |
