Chemical Supercritical Fluid Infiltration of Pyrocarbon with Thermal Gradients: Deposition Kinetics and Multiphysics Modeling

Abstract

The chemical supercritical fluid infiltration process is a recent variation of the chemical vapor infiltration (CVI) process that allows rapid and efficient manufacturing of ceramic-matrix composites (CMCs), albeit still needing optimization. This article proposes a quantitative assessment of the process dynamics through experiments and modeling. The kinetics of carbon deposition were determined through two sets of experiments: CVD on a single filament at pressures between 10 and 50 bar and infiltration at pressures ranging between 50 and 120 bar. The CVI experiments were conducted under important thermal gradients and were interpreted using a model-based reconstitution of these gradients. We found that (i) the kinetic law has to incorporate the potential effect of the reverse reaction (i.e., etching of C by H2); (ii) the activation energy and pre-exponential factor both decrease with pressure up to 50 bar, then remain roughly constant, and (iii) although the apparent activation energy is modest, a favorable situation occurs in which an infiltration front builds up and travels from the hottest to the coldest part of the preform due to the presence of sufficient heat flux. A numerical simulation of the process, based on the solution of momentum, heat, and mass balance equations, fed with appropriate laws for the effective transfer properties of the porous medium and their evolution with infiltration progress, was performed and validated by comparing the simulated and actual infiltration profiles.

How is it possible to get optimal infiltration fronts during chemical vapor infiltration with thermal gradients ?

“…Case #4 has the particularity of using very high-pressure methane as precursor: the conditions can be called supercritical because indeed the critical pressure and temperature of methane are overtaken, but the process is operated in a parameter region where the perfect gas law holds [88]. It is otherwise quite similar to case #2: a vertical carbon resistor and a fibrous preform inserted around it in a cylindrical geometry.…”

“…This process was first patented in 2006 [90] and described in a publication in 2014 [91]; a recent re-assessment of the process has been carried out [88], with a strong focus on thermal measurements. The detailed multi-physics model framework developed for the preceding cases has been adapted to the present one.…”