Reinforced carbon/carbon (RCC) has been used successfully as thermal protection on the space shuttle orbiter nose cap and wing leading edge for many missions. During reentry, this material is exposed to the harsh environment of high temperatures, rapid velocities, and reactive gases. RCC consists of a two-dimensional layup of carbon fiber fabric together with several layers of oxidation protection. The oxidation protection can be summarized as follows. A conversion coating of silicon carbide (SiC) is followed by infiltration of tetra-ethyl orthosilicate (TEOS), which decomposes to silicon dioxide (SiO2) on heating. Then, two outer coats of a sodium-silicate-based glass sealant are applied. In this study at the NASA Glenn Research Center, a series of controlled laboratory experiments was used in conjunction with observations of mission-exposed RCC to reveal the three major routes to degradation: oxidation of the carbon through cracks and fissures, sealant loss via vaporization, and silica/carbon reactions.
RCC oxidized in a furnace for 1 hr at 1100 °C.
Oxidation through cracks and fissures in RCC is a well-known issue. Despite the protection system, some oxidation is known to occur, particularly when the sodium silicate layer is depleted. The preceding image shows a typical cavity formed by the oxidation of RCC that has a SiC conversion coating with no glass sealants. The important features are (1) a clear path from the atmosphere to the carbon, which is the crack in this case, and (2) fibers exhibiting preferential attack along the edges. These features allow a pore created in oxidation to be distinguished from a pore created during processing. It is essential to make this distinction to assess any oxidation damage.
A two-stage diffusion model has been adapted to describe the oxidation of carbon through the cracks (ref. 1). Because of the thermodynamic incompatibility of carbon, gaseous carbon monoxide (CO(g)), and gaseous carbon dioxide (CO2(g)), the oxidation of carbon is treated is two steps:
C + CO2 = 2 CO
½ O2 + CO = CO2
The following illustration shows this schematically for a straight pore. Diffusive fluxes are derived for each region of the pore, and the consumption of carbon is calculated. Agreement between the observed values and the calculated values is within an order of magnitude. The disparity is likely due to the complex geometry of a crack in comparison to that of a straight pore.
Two-step diffusion-oxidation process for the oxidation of RCC through cracks. Ji is the flux (grams or moles/unit area - unit time) of species i; Xf, boundary of CO/CO2 and CO2/O2 region; L, length of pore; and X, diffusion direction.
A second important reaction is the vaporization loss of the sodium-silicate-based sealant. Sealant depletion is a well-known problem for mission-exposed RCC. Laboratory studies have provided data to establish an Arrhenius curve for sodium silicate vaporization in a boundary-layer-limited flux situation, illustrated in the following graph. Extension of these data to sealant loss for stagnation flow arcjet tests looks promising.
Arrhenius plot for the mass loss flux of silica-saturated sodium silicate normalized to velocity.
A third type of reaction is the interaction of TEOS-derived SiO2 with the carbon/carbon substrate. Processing involves infiltration of RCC with TEOS, using a vacuum to draw the TEOS through the thickness of the RCC. Necessarily, a small amount of TEOS remains in the carbon/carbon and is converted to SiO2. It is well-known that carbon reacts with SiO2 to form SiC (ref. 2). Laboratory experiments on carbon/carbon with TEOS-derived SiO2 show small regions of SiC developing in both vacuum and argon atmospheres. This is shown in the photomicrograph. These three types of reactions are critical in understanding and assessing the long-term performance of RCC in reentry environments.
RCC with TEOS-derived silica treated in argon for 15 min at 1600 ° C. The small bright regions around the fibers are SiC.
Find out more about the research of Glennís Durability and Protective Coatings Branch: http://www.grc.nasa.gov/WWW/EDB/
Dr. Nathan S. Jacobson, 216-433-5498, Nathan.S.Jacobson@nasa.gov
Authors: Dr. Nathan S. Jacobson, Dr. Donald Curry, and Neal Webster
Headquarters program office: Orbiter Project Office
Programs/Projects: Space Shuttle Orbiter, RTF
Last updated: October 16, 2006
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