Hermetic SiC composite tubes

A process was developed to fabricate hermetic, fiber-reinforced SiC composite tubes. Filament-wound preforms were densified with SiC from CH{sub 3}SiCl{sub 3} by an isothermal CVI process at reduced pressure. SiC composite tubes were pressure tested to 20 MPa helium at room temperature and 15 MPa helium at 1,000 C. Leak rates were <8 {times} 10{sup {minus}5} cm{sup 3}/s. Samples were tested under vacuum at room temperature to the detection limit of the instrument. Leak rates were <2 {times} 10{sup {minus}11} cm{sup 3}/s under these conditions. Preforms with a braided fiber architecture of Nicalon fiber infiltrated under similar conditions did not result in thin-walled hermetic structures.