Chemical Vapor Infiltration of Silicon Carbide

Process conditions for CVI of SiC vary considerably. For isothermal infiltration of Nicalon 2D cloth preforms 4 x 5 X 50 mm in size and containing 40 to 50 vol.% fiber, Fitzer and Gadow [122] recommend an infiltration temperature of 1050°C, a total pressure of 3 kPa, and a time of 14 days. A yield (efficiency with which the MTS was utilized) of 60% was quoted but it may not have pertained to this exact set of process conditions. Deposition rates increase with temperature and pressure for isothermal processing [122, 123]. 

Oak Ridge National Laboratory and Georgia Tech have extensively developed the thermal gradient-forced flow process for infiltration of SiC fiber (primarily Nicalon cloth) preforms with SiC [37, 124-133]. The process has been demonstrated for disks up to 2 diameter x thick ( 5 x 2,5 cm) and for hollow tubes. Two basic processes show promise. In one, the gas flows upward through the preform the process is terminated when the back pressure reaches 10 psig (= 69 KPa). The second process permits radial flow of the reagents when the hot surface of the preform seals. With either process, infiltration times (= 20 h) are much shorter than for the isothermal process. 

Amercom, Inc., and 3M have made small and large [(— 8 diameter x 6 long 20 X 180 cm)] tubes. Both Nicalon SiC fiber and Nextel fiber have been used with an SiC matrix. 3M Company prepares a product called Siconex which consists of Nextel fibers in a CVI SiC matrix. 

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