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In a classical chemical vapor infiltration (CVI) process, the competing effects of chemical kinetics and reagent gas transport lead to non‐uniform depositions such that outer layers of a preform densify faster leaving the core highly porous. Currently, CVI must be performed at a sufficiently low temperature to achieve good densification quality which leads to high processing time and cost. Volumetric heating of the preform, especially through microwaves, can create temperature inversion such that the core is hotter than the outer surface and potentially, overcome the challenges associated with isothermal CVI. Direct numerical simulations (DNS) of densification under various such temperature distributions indicate that microwave heating in CVI processing can lead to better (uniform) densification of porous preforms. The role of key parameters describing the temperature distributions on the densification behavior is investigated. Strategic temporal control of the temperature distribution shows that processing times can be reduced by almost half while maintaining a good densification quality similar to that of low‐temperature isothermal processing. Inside‐out densification due to the inverted temperature profile is a key distinguishing characteristic of microwave assisted CVI. [ABSTRACT FROM AUTHOR]

Interface‐resolved direct numerical simulations (DNSs) of chemical vapor infiltration (CVI) have been performed over a range of furnace‐operating conditions (Thiele moduli) and for practical woven preform geometries. A level‐set method is used to resolve the geometry of the initial preform at tow scale. The interface between the vapor and solid phase is then evolved in time through the entire CVI densification cycle, fully resolving the time‐varying topology between the two phases. In contrast to previous level‐set methods for CVI simulation, the physical reaction and diffusion processes govern the level‐set movement in the current approach. The surface deposition kinetics is described by the usual one‐step model. In this paper, the DNS data are used to study the evolving porosity, surface‐to‐volume ratio, and flow infiltration properties (permeability and effective diffusivities). Comparisons are made to popularly‐assumed structure functions and the standard, Kozeny–Carmen porous media model commonly employed in modeled CFD simulations of CVI. The virtual DNS experiments reveal a Thiele modulus and preform geometry (fabric layup) dependence which the existing microstructural and infiltration models are not able to describe throughout the entire densification process. The DNS‐based, woven geometry‐specific correlations can be applied directly to mean‐field, furnace‐scale CFD simulations. [ABSTRACT FROM AUTHOR]

The densification of additively manufactured porous preforms by chemical vapor infiltration (CVI) is studied using pore‐resolved simulations and experiments. Experimentally, 3D printed silicon carbide (SiC) preforms are subject to CVI synthesis using methyltrichlorosilane (MTS) precursor to obtain high purity SiC/SiC composites. Optical images of the cross sections of the processed preforms are analyzed to obtain the spatial porosity distribution. The numerical method is based on a level set formulation to capture the spatial distribution and time evolution of the pore scale microstructural characteristics. The coupled transport and kinetic effects are represented using a dimensionless Thiele modulus. Simulations are initialized using representative synthetic preform geometries comprising of packed particles based on the size distribution of the powder used for 3D printing. The simulation results are validated against the experimental observations in terms of total density and the distribution of residual porosity. The densification characteristics, porosity classification, concentration profiles, and structure functions are analyzed as functions of processing temperature and Thiele modulus. [ABSTRACT FROM AUTHOR]