Intermediate Depletion in SiC Chemical Vapor Infiltration: Coupled Pyrolysis, Surface Deposition, and Porous Transport

Chemical vapor infiltration is a reactive porous-deposition process in which precursor transport, gas-phase pyrolysis, heterogeneous surface deposition, and reactant depletion occur simultaneously. In this work, detailed MTS gas-phase pyrolysis and heterogeneous surface kinetics are coupled with a 1D PFR/Darcy porous-preform model to study how operating conditions control deposition-relevant intermediate species during SiC CVI. The model is used to examine temperature, pressure, and velocity effects through the preform thickness. The results show that coupled homogeneous and heterogeneous chemistry creates a position-dependent intermediate pool, so deposition cannot be represented by one fixed global reaction over the full operating range. Instances of a single step rate expression calibrated from experiments are outlined to realize this limitation. Higher temperature and pressure increase inlet deposition, but they also increase early intermediate consumption and downstream depletion. Higher inlet velocity helps carry deposition-relevant intermediates farther into the preform and supports more sustained through-thickness deposition; however, SiC yield can be very low. We define quantitative measures to realize the trade-off between uniformity of depositions and effective precursor utilization. Within the adopted heterogeneous surface mechanism, downstream deposition loss is associated with depletion of the silicon-side SiCl2/SiCl2* pathway, while CH2* remains the main carbon-side surface intermediate. Effective one-step Arrhenius models remain range-dependent approximations because apparent kinetics change with intermediate formation, transport, and surface consumption.