Comparative studies of the attack of pyrolytic and isotropic graphite by atomic and molecular oxygen at high temperatures.

Abstract

Direct experimental determinations are reported of the kinetics of O-atom attack of pyrolytic graphite at surface temperatures up to 2100°K. Comparisons with data obtained under identical conditions for lower-density isotropic graphites reveal that in atomic oxygen pyrolytic graphite is not much more oxidation-resistant than less dense iso tropic graphites. For basal-plane pyrolytic graphite at 2000°K (3140°F) and 3 X 10~ torr oxygen pressure, dissociation causes a 50-fold enhancement in carbon atom removal per gas collision, with higher enhancements being indicated at higher oxygen pressures. It is concluded that nondiffusioncontrolled ablation rate predictions for re-entry vehicle nose caps, slender cones, and leading edges must account for an important O-atom contributon. Moreover, decisions on the relative reactivities of graphites or chars based on observations in undissociated air, or in the absence of accurate O-atom simulation, should be viewed with extreme caution.