The effects of atomic oxygen on silicon-carbon systems in extreme environments

Abstract

Vehicles traveling at hypersonic speeds require thermal protection systems (TPSs) that can withstand the extreme temperatures and reactive atomic oxygen species present in these environments. Ultra-high temperature ceramics are candidate TPSs, and many of them contain silicon carbide, allowing them to resist chemical attack by forming a protective oxide-containing layer, called passive oxidation. At very high temperatures, however, the layer will decompose, subjecting the material to ablation from reaction with O-atoms, called active oxidation, through a process called the passive-to-active oxidation transition (PAT). We have conducted molecular beam-surface scattering experiments to investigate the interactions of O-atoms with SiC at high temperatures, which revealed that with a lower fluence of O-atoms above the PAT, the SiC surface undergoes graphitization, while a sufficiently higher fluence of O-atoms promotes active oxidation. Analysis of the oxide layer decomposition revealed a decomposition process that initiated at the oxide-SiC interface. These insights will be useful for the development of more accurate predictive models, but they also aided understanding of the ablation of silicone-coated heat shields for atmospheric entry applications. For these applications, phenolic impregnated carbon ablator (PICA), a material composed of a carbon fiber network (FiberForm) and a resole phenolic resin stable against high heat convection and conduction, is used. Silicone is sprayed onto PICA to reduce dust, but the silicone can also form an oxide layer, which, like on SiC, will resist O-atom attack until it decomposes at very high temperatures, exposing the underlying material to reactive O-atoms. We conducted additional experiments in which a beam of atomic oxygen was directed at silicone-coated and uncoated samples of PICA as well as FiberForm, which revealed high nonreactive O-atom product scattering when the oxide layer was present, while with the decomposition of the oxide, product scattering resembled O-atom scattering from the underlying substrate. Additional studies probed the oxidation layer that is formed on pure silicone during O-atom bombardment, which revealed a three orders of magnitude reduction in erosion yield compared to that of Kapton H, a polyimide. This new data on PICA and FiberForm has been provided to NASA Ames for their development of an ablation model.