EFFECT OF FIBER-MATRIX CO-CARBONIZATION ON CARBON-CARBON COMPOSITE PORE MORPHOLOGY

Carbon-carbon composites (CCCs) are essential components in aerospace and defense industries because of their unparalleled ability to maintain exceptional thermomechanical properties at extreme temperatures. Widespread application beyond these industries is not economically practical due to the excessive cost and time associated with processing these materials. CCCs predominantly begin as carbon fiber-reinforced polymers (CFRPs) which are pyrolyzed to convert the matrix precursor to a carbon-rich one. As the matrix is pyrolyzed, the inert carbon fibers prevent shrinkage in plane of the fabric which promotes stress-induced cracking within the material. Densification of these cracks is the bottleneck that drives up processing cost and time. It is hypothesized here that CCCs can be produced with advantageous pore morphologies by replacing carbon fibers with polymer fibers that exhibit similar thermomechanical behavior as the matrix during pyrolysis. Previous co-carbonization studies relied on the same fiber–matrix system, characterized by dissimilar thermomechanical behavior during pyrolysis, and did not consider alternative pairings that could better complement each other. In this study, thermomechanical analysis (TMA) was used to characterize poly(p-phenylene-2,6- benzobisoxazole) (PBO) fibers, and a benzoxazine-based polymer was selected to match their shrinkage behavior. A PBO fiber–reinforced polymer composite was fabricated via resin transfer molding (RTM), and its thermomechanical behavior was characterized using TMA in both inplane and transverse orientations relative to the fabric. The in-plane response primarily reflects fiber reinforcement, whereas the transverse response is strongly influenced by the polymer matrix. Thermal degradation of PBO generates a larger volume of volatile gases, which enlarges and interconnects cracks, as confirmed by optical microscopy. The resulting pore network exhibits higher permeability, enabling more efficient densification compared to CFRP-based systems. Heat treatment of the fiber has shown that manipulating the fiber shrinkage enables tuning of the pore morphology in CCCs, offering a pathway to more controlled and efficient densification.