Correlation of the permeability and porosity development of carbon/carbon composites during pyrolysis

A cost-effective approach to manufacture Carbon/Carbon composites is to infiltrate carbon fiber preforms with a polymer precursor and pyrolyze it to form the carbon matrix. During pyrolysis, the microstructure changes due to decomposition reactions creating a high degree of porosity consisting of interconnected network of voids and cracks. To densify, the pyrolyzed composite is infiltrated with polymer precursor to fill the porous network. The cycle of pyrolysis followed by infiltration is repeated until the porosity is minimized and the desired density is achieved. The permeability, porosity and viscosity of the polymer govern the infusion pressure and time for the re-infiltration steps.In this work, the permeability of benzoxazine-based Carbon/Carbon composites was measured for different degradation levels with three distinctly different pyrolyzing schedules. The samples were characterized systematically at increasing pyrolyzing temperatures until full conversion of the precursor into carbon matrix was achieved. The porosity of the material was evaluated by X-ray micro-computed tomography and by pycnometry. The through the thickness permeability of the composite was measured via a pulse-decay experiment after each heating step in the pyrolysis cycle. In this experiment pressure decay with time is recorded as air is evacuated from the cavity through the connected pathways of the porous network within the sample. The experimental correlation between the permeability and interconnected porosity of the microstructure is established. It is shown that the pulse-decay test with air effectively characterizes the permeability of the pyrolyzed composite.