Atmospheric pressure chemical vapor infiltration of a titanium carbide interphase coating on carbon fiber

Abstract

Thin film coatings of titanium carbide (TiC) were deposited onto carbon fibers via atmospheric pressure chemical vapor infiltration (CVI) using TiCl4, CH4, and H2 gases. Three CVI parameters were optimized to obtain high quality coatings: temperature, total gas flow rate, and H2/CH4 flow ratio. Scanning electron microscopy was used to assess the quality and morphology of the coatings, including the roughness, uniformity, fiber coverage, and fiber adhesion. The temperature was optimized to 900 °C, which yielded complete fiber coverage with minimal fiber bridging and good adhesion. Minimizing the total gas flow rate to 37.5 standard cubic centimeters per minute reduced the roughness of the TiC coating. Finally, maximizing the ratio of H2/CH4 to 4:1 was optimal for all coating qualities. Selected area electron diffraction confirmed the presence of crystalline TiC. A combination of transmission electron microscopy and fiber tow tensile testing revealed that the outer layer of carbon fiber becomes damaged by the CVI process, causing loss of strength. To circumvent this surface degradation, it is necessary to protect the fibers with a pyrolytic carbon interphase coating before depositing TiC. This method of depositing TiC onto carbon fibers proves to be competitive in terms of coating quality, deposition temperature, reaction time, and fiber strength compared to other TiC deposition methods including reactive-CVI and molten salt synthesis.