Crystal-structure-controlled tribological behavior ofcarbon-graphite seal materials in partial pressures of helium andhydrogen. I. Specimen characterization and fundamentalconsiderations

Abstract

Three commercially available carbon-graphite cryogenic materials(EC-5, M234 and P5N) that are used as seals in rocket turbopumpassemblies have been characterized as part of a larger study toelucidate their tribological characteristics in helium andhydrogen. This report (part I) documents the results ofstructural/chemical characterization that included XRD, Ramanspectroscopy, and AES/EELS. Outgasing studies were also performedusing TGA-FTIR. The data indicate that the EC-5 consists of wellgraphitized carbon with three-dimensional order and a nominalcrystallite size (L_c) of 50 nm. The materialalso contained turbostratic carbon with a d-spacing of0.340 nm and a crystallite size of 12 to 14 nm. The LiF-doped P5Ncontains a mixture of turbostratic carbons which show onlytwo-dimensional order, and has smaller crystallite sizes thanthose in EC-5. The M234 contains significant amounts of metallicsilver; the carbon is mostly amorphous with only a minorcrystalline (turbostratic?) component. EC-5 was found to beessentially outgasing-free, while heating of the turbostraticand/or amorphous materials yielded polar gases (e.g., CO andCO₂) and, in the case of the M234, low molecularweight hydrocarbons. The relationship between thecrystallographic structure of these carbons and theirtribological performance is explored in parts II and III.