Influence of carbide coatings on the oxidation behavior of carbon fibers

Abstract

The oxidation behavior of reactive carbon fibers, such as ex-Pan T300, is studied in oxidizing environments by means of TGA and tensile strength measurements. Different carbide coatings are investigated with a view to fiber protection during an air exposition at high temperature. Thin, continuous, and uniform films of TiC, SiC, B 4C, or B 4C SiC multilayers are obtained by an RCVD process. Complementary experiments are performed on bulk graphite substrates coated by the same process. The carbide thicknesses do not exceed 60 nm for the T300 fibers and 60 μm for the bulk substrates. The presence of such coatings is found able to protect the carbonaceous materials against oxygen and air oxidation for extended periods to temperatures of at least 600 °C. However, SiC and TiC coatings that form silica and rutile films during oxidation offer only limited protection due to the diffusion of oxygen along microcracks and grain boundaries of the oxides. B 4C and B 4C SiC multilayers give better protection both in a dry oxygen atmosphere and in room air. Boron oxide and borosilicate have a glassy structure, and they act as a diffusion barrier, providing protection by inhibition of oxygen diffusion, thereby slowing down the carbon gasification. The mechanical characteristics of the fibers protected by a boron-silicon carbide coating remain constant after a half hour of oxidation at 600 °C.