Oxidation behavior of carbon/carbon composites with/without CVD ceramic coatings

The poor oxidation resistance of C/C composites has greatly limited their application to a relatively low temperature or a non-oxidizing environment. Before the potential of C/C composites can be fully realized, either their oxidation resistance must be improved or effective protective systems must be developed. These two goals cannot be reached without a better understanding of the underlying oxidation or protection mechanisms.; The C/C composites in the present study were manufactured by carbonizing carbon fabric-phenolic resin composites at 1000{dollar}spcirc{dollar}C and infiltrating with petroleum pitch, followed by pressure carbonization at 10 KSI and 600{dollar}spcirc{dollar}C, and pressureless carbonization at 1000{dollar}spcirc{dollar}C. The samples were then coated with {dollar}rm Bsb4C, SiC/Sisb3Nsb4, or Bsb4C/SiC/Sisb3Nsb4{dollar} by the chemical vapor deposition (CVD) technique to improve their oxidation resistance. Thermodynamic calculations were conducted using the computer program, SOLGASMIX, to predict the deposition phases under various deposition conditions (input gas concentration, temperature, and pressure). The application of B{dollar}sb4{dollar}C coating provided good oxidation protection for C/C composites up to a temperature of 1000{dollar}spcirc{dollar}C. A {dollar}rm SiC/Sisb3Nsb4{dollar} coating increased the oxidation onset temperature from 500{dollar}spcirc{dollar}C to 600{dollar}spcirc{dollar}C and reduced the oxidation rate by more than one order of magnitude. A {dollar}rm Bsb4C/SiC/Sisb3Nsb4{dollar} coating reduced the oxidation rates of the C/C composites by two orders of magnitude up to 1400{dollar}spcirc{dollar}C.; The oxidation of the C/C composite without a coating was found to be controlled by the rate of C-O{dollar}sb2{dollar} reaction at temperatures below 650{dollar}spcirc{dollar}C and controlled by diffusion of the reactant (O{dollar}sb2){dollar} or product (CO) through a gas boundary layer at temperatures higher than 850{dollar}spcirc{dollar}C. The oxidation of B{dollar}sb4{dollar}C-coated C/C composites was controlled by the chemical reaction rate at temperatures below 700{dollar}spcirc{dollar}C while above 800{dollar}spcirc{dollar}C the rate-limiting steps could not be determined. For {dollar}rm SiC/Sisb3Nsb4{dollar}-coated C/C composites, the rate-limiting step was the chemical reaction at temperatures below 900{dollar}spcirc{dollar}C and diffusion through the coating cracks at temperatures above 1000{dollar}spcirc{dollar}C. The cracks, which formed due to thermal expansion coefficient mismatch between coating and substrate, decreased in width during re-heating in the temperature region reduced the oxidation rate when the rate-limiting step was diffusion through the cracks. For {dollar}rm Bsb4C/SiC/Sisb3Nsb4{dollar}-coated C/C composites, oxidation was limited by chemical reaction at temperatures below 700{dollar}spcirc{dollar}C and by diffusion through liquid boron oxide at temperatures above 800{dollar}spcirc{dollar}C.