KINETICS AND MECHANISM OF OXIDATION OF THE REINFORCED CARBON/CARBON ON THE SPACE SHUTTLE ORBITER

摘要

Reinforced carbon/carbon (RCC) protects the Space Shuttle Orbiter wing leading edge and nose cap from the heat of re-entry. The oxidation protection system is based on a SiC conversion coating. In this paper, fundamental laboratory oxidation studies from 600-1200℃ on RCC fabric, fabric and matrix, and SiC-protected carbon/carbon are discussed. Although conducted under laboratory conditions, these studies reproduce the morphologies observed under flight conditions. Oxidation below ～900℃ is reaction-controlled and is characterized by preferential attack of the matrix and thinning of the fibers. Further microscopy reveals that oxidation begins with preferential attack into the grooves of the fibers. At ～900℃, there is a transition to diffusion-controlled oxidation. This is characterized by more localized attack near the surface of the carbon/carbon. For SiC-coated RCC, cavities form at the base of cracks in the SiC. This morphology lends itself to a two-step diffusion model for carbon oxidation. Fluxes are considered in both the SiC channel and growing cavity. Experiments show good agreement with the model predictions. These studies show the criticality of a stable coating system with filled cracks to protect RCC.