Monte Carlo simulations of texture interface formation within pyrocarbon matrix of carbon/carbon composites

Based on the Particle-Filler model (P-F) and Langmuir-Hinshelwood mechanism of pyrocarbon deposition, a multistep heterogeneous reaction kinetic mechanism including adsorption/desorption/dehydrogenation reactions was proposed, by which continuous deposition of pyrocarbon on surface of the carbon fiber in the preparation of carbon/carbon composites and the formation process of texture interface were theoretically modeled. Then a Gibbs ensemble Monte Carlo (MC) method was used to simulate the pyrocarbon texture formation in chemical vapor infiltration (CVI) process. This study shows that P-F bimolecular reaction happening on the surface of deposition will be restricted due to the fact that the adsorption of the light aromatic component in gas phase represented by C₆ is more likely to be suppressed, compared to the adsorption of the light linear hydrocarbons represented by C₂. With the change of the concentration ratio R of C₆ to C₂ component, the texture formation process of pyrocarbon exhibits bi-stability distribution leading to the formation of two different metastable phases, i.e. medium-textured (MT) and high-textured (HT) pyrocarbons. And a sharp texture interface appears in the pyrocarbon of the carbon/carbon composites. The further simulation results indicate that the bistable transition of pyrocarbon textures is accompanied by a hysteresis cross domain, which is affected by the composition parameter R of gas phase, initial concentration of light linear hydrocarbons C₂ and deposition temperature T. CVI process parameters have to be selected outside the range of hysteresis cross domain to produce pyrocarbon with a pure composition.