Chemical vapor infiltration: Dispersed and graded depositions for ceramic composites



A structured-geometry model for dispersed and graded deposits was developed for chemical vapor infiltration of multiply-woven substrates of carbon filters. An earlier model was modified to allow for two reactants in the feed. The model predicts gas-phase concentration profiles in the voids of substrates and deposition amounts of two reactants as a function of time and location. Results are shown for feeding reactant gases simultaneously for dispersed deposition and periodically for the study of the graded deposition for a typical substrate. The variation in relative contents of reactants in the feed with time shows how the composition of the deposit varies. Porosities and changes of dimensions with time everywhere in the substrate are also predicted. This is an advantage of the structured-geometry model vs. a simplified homogeneous geometry for the substrate. Such a simplification may preclude predicting failure, such as delamination, which would require a local description of the composite structure.

Sensitivity to input parameters such as temperature, pressure, and reactant feed concentration is discussed, and two substrate geometries are compared.

The behavior of the system is predicted to be dominated by the times to fill gaps between filaments at the ply surfaces and the outermost space between plies. Furthermore, faster kinetics and slower reactant diffusion favor deposition of one material near the surface of a ply or the matrix and the other near the center of the ply or matrix. By manipulating feed rates of reactants, uniformity of material and overall porosity of the composites are predicted to be enhanced.