Sensitivity of the Deposition Rate to the Surface Reaction Parameters during Chemical Vapor Infiltration of SiC Matrix Composites

Chemical vapor infiltration (CVI) is typically used to fabricate high-purity matrix composites for extreme environments. In CVI, a gas-phase precursor is injected into a chamber, which diffuses and reacts on the surface of a fiber preform. The deposition quality and rate depend upon the surface reactions, which can be controlled in different ways, such as higher operating temperature, thermal-gradient CVI, forced-flow CVI, etc. A computational model is desirable for parametric studies, but such an approach should accurately describe transport, energy interactions, interface dynamics, and surface reactions occurring during CVI. In this study, we employ the Direct Simulation Monte Carlo (DSMC) technique to simulate the flow around microscale fibers. DSMC is a robust and efficient tool for solving rarefied gas dynamics using a molecular kinetic scheme with a particle-based probabilistic approach. It allows examining fundamental aspects of molecular motion, intermolecular collisions, and chemical reactions. The sticking coefficient of the chemical species participating in the surface reactions affects the deposition during CVI. It depends upon parameters such as the preform structure, surface coverage, and operating conditions. It is usually determined empirically from experiments. In this study, we examine the sensitivity of the deposition rate of SiC to the sticking coefficients of chemical species in fiber preforms of different sizes (1 – 10 μm) and at a range of operating conditions.