The study of alternating flow chemical vapor infiltration and a novel kinetics determination technique for the vapor deposition of silicon carbide via the decomposition of methyltrichlorosilane.

摘要

Ceramics matrix composites (CMC) combines the desirable characteristics of ceramics with good mechanical properties and are considered the most attractive material alternative for high temperature corrosive environments. Manufacturing CMC's is difficult due to the high melting temperature of the constitutive materials and the lack of material databases on these materials. Chemical vapor infiltration (CVI) is an attractive net-shape fabrication process for these materials and results in high quality composites.;A new type of chemical vapor infiltration is explored in this work. The Alternating Flow Chemical Vapor Infiltration (AFCVI) uses an isothermal reactor and forced flow to introduce the reactant gases into the interior of the preform. The gas flow is alternatively introduced from opposite sides of the preform to densify the composite. Composites fabricated using this process show improvement over existing isothermal processes and results in a very short cycle time. AFCVI does not, however, produce as uniformly densified parts as the forced flow thermal gradient. Initial modeling of this process emphasized the need for more accurate kinetics to accurately capture the deposition profile in the preform.;Kinetic parameters used in most CVI modeling are obtained from chemical vapor deposition (CVD) experiments. There are several fundamental differences between CVI and CVD that may affect the kinetics obtained and may limit the applicability of CVD kinetics in CVI. Namely the critical dimensions such as pore spacing and surface area to volume ratios are drastically different in these two systems. A new technique of determining the kinetic parameter specifically for CVI of SiC is developed and tested. This system consists of small 400 Um holes drilled in a solid graphite substrate. The results show excellent correlation at lower temperatures. Kinetic parameters obtained from CVD experiments severely over-predict the deposition thickness. The new parameters capture both the shape and absolute coating thickness in the CVI preform. The information obtained using this technique is unique to the kinetics preforms and gives CVI-specific knowledge.