Kinetics of Carbon/Carbon Composite Oxidation and Inhibition by Site Blockage

Carbon-Carbon composites exhibit excellent mechanical properties, especially at high temperatures. Unfortunately oxidation is a serious problem for these materials. Carbon materials can adsorb oxygen and react, forming carbon oxides at temperature as low as 400'_aC, limiting their use in high temperature oxidative environments. Acid treatments, in particular phosphoric acid, have been used with some success to prevent oxidation. We studied the inhibiting effect of phosphoric acid using powder samples, 75 åÁV 125 micrometers, from a carbon brake composite. The samples were impregnated by incipient wetness with an aqueous solution of ortho-phosphoric acid. The samples were heat-treated and oxidized using temperature programming method (TPO) under dry airflow. Results show that the activation energy for all of the samples are similar, inferring that phosphoric acid blocks oxidation sites without changing the chemical properties of the material; moreover, results with different concentrations of phosphoric acid show that the inhibiting effect is seen in all temperatures åÁV between 600'_aC and 900'_aC. Results with larger particles show a better performance of the phosphoric acid treated samples, suggesting a mass transfer related control of the oxidation rate. Studies are being conducted to determine if the inhibitors form a film slowing down oxidation of carbon composite materials.