Oxidation behavior of carbon-carbon composites

Abstract

This study investigates the oxidation mechanisms of carbon-carbon composite materials of the type used for aircraft brakes. The rate-controlling steps for oxidation at temperatures ranging between 450 and 750°C were determined by measuring activation energies of composite samples in bulk form and in ground form and by measuring oxidation rates as a function of reaction gas flow. Below 650°C, the two forms of carbon samples showed considerably different activation energies, which suggests that in bulk samples, oxidation is controlled by diffusion through pores, and that as grinding reduces pore length, activation energy increases. At 750°C, the ratio of oxidation rates was the same as the square root of the ratio of reactant flow rates. It was therefore concluded that the dominant oxidation mechanism at 750°C is diffusion of oxygen through a film of gaseous reaction products at the sample surface. The oxidation mechanism of carbon-carbon composite samples at temperatures between 450 and 650°C is primarily controlled by the diffusion of oxygen into pores. Between 650 and 750°C, the rate-controlling mechanism undergoes a transition from diffusion through pores to diffusion through stagnant gas film.