CCCM SPECIFIC SURFACE ESTIMATION IN PROCESS OF LOW-TEMPERATURE OXIDATION

Abstract

The low-temperature oxidation process of carbon-carbon composite materials (CCCM) with a pyrocarbon matrix has been examined. Oxidation at temperatures of 450 to 700 °C is characterized by internal burnout of the carbon phase without a noticeable change in the experimental samples external volume. Oxidation resistance analysis of CCCM structural components was performed. CCCM and carbon fibers specific surface area were estimated by the low-temperature adsorption of nitrogen and krypton using the Brunauer-Emmett-Teller model and the theory of density functional model. Pore size distribution was calculated by the semi-empirical Horvath-Kawazoe method. A significant increase (about 10-15 times) in specific surface area of the composite material, together with a rise in free volume ~5%, was accompanied by total weight loss of about 5%. Specific surface area changes occur as a result of anisotropic etching of carbon fibers surface with the formation of micropores with 0.5–2.0 nm diameter range. Although macropores are formed mainly due to the oxidation of thermosetting binder pyrolysis residue, they do not contribute to the specific surface increase but solely provide access to micropores. Microporosity evolution leads to an increase of structural discontinuity degree and, ultimately, to the loss of matrix-filler contact boundary. As a result, an overall weakening of material mechanical characteristics is to be noted. Thus, oxidative degradation is closely related to the increase in void space. A follow-up study is on-going. In fact, an open question remains, namely whether the oxidative process occurs differently in the micropores of diameter less than 2 nm and how it may contribute to the oxidative stress resistance behavior of CCCM.