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All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

Microstructural Constraints on the Formation of Diamond in Ureilites, the Case from the Kenna Meteorite

The origin of diamonds in ureilitic meteorites, a major family of ultramafic achondrites, has long been debated, with various formation mechanisms inferred, including: 1) shock conversion of preexisting graphite due to a hypervelocity impact on the ureilite parent body; 2) vapor deposition during condensation of the solar nebula; or 3) static transformation in the mantle of a large planetesimal (>1000 km in diameter). The Kenna ureilite is a 10.9 kg stone that was found in Kansas in 1972 and is dominantly composed of olivine and pigeonite (68% and 22% modal abundance, respectively), together with minor metal and carbon-rich regions. Within the carbon-rich domains researchers have identified microdiamonds up to ~10 µm in size. Due to the proximity of the extremely hard microdiamonds with soft graphite and moderately hard silicates, achieving an appropriately flat surface for electron backscatter diffraction (EBSD) analysis has proved challenging. We have, therefore, employed a Hitachi ArBlade 5000 broad beam Ar ion milling system to prepare a sample with an adequate surface uniformity. The ion milling dramatically improved indexing of the EBSD patterns and facilitated high-resolution microstructural analysis of the carbon-rich domains. The carbon-bearing regions occur as tabular domains between polygonal olivine and pigeonite, and are composed of aggregates of micrometer-scale diamond with intermediate graphite, oxides and metal grains. Within the microdiamonds, {111} twins are abundant, many of which form ragged lamellae consistent with mechanical twinning. The microdiamond assemblages appear to pseudomorph tabular graphite habits, which together with the abundant deformation twins, supports a shock transformation mechanism for microdiamond formation within the Kenna meteorite. To compliment the EBSD microstructural analyses, coordinated laser Raman, focused ion beam/transmission electron microscopy imaging and diffraction data, and NanoSIMS N and C tomographic isotopic imaging will be collected. These data will provide further constraints on the formation of diamonds within ureilites.

Timmons M Erickson↗
NASA NTRSDate not supplied

Coordinated Microstructural and Isotopic Characterization of Diamond Inclusions within the Kenna Ureilite

Introduction: The origin of diamonds in ureilitic meteorites, a major family of ultramafic achondrites, has long been debated, with various formation mechanisms inferred, including: 1) shock conversion of preexisting graphite due to a hypervelocity impact on the ureilite parent body [e.g., 1]; 2) vapor deposition during condensation of the solar nebula [e.g., 2]; or 3) static transformation in the mantle of a large planetesimal (>1000 km in diameter) [e.g., 3]. The Kenna ureilite is a 10.9 kg stone that was found in Kansas in 1972 and is dominantly composed of olivine and pigeonite (68% and 22% modal abundance, respectively), together with minor metal and carbon-rich regions. Within the carbon-rich domains researchers have identified microdiamonds up to ~10 µm in size [4]. Methods: Due to the proximity of the extremely hard microdiamonds with soft graphite and moderately hard silicates, achieving an appropriately flat surface for electron backscatter diffraction (EBSD) analysis has proven challenging. We have, therefore, employed a Hitachi ArBlade 5000 broad beam Ar ion milling system to prepare a sample with an adequately uniform surface flatness across the various minerals. The ion milling dramatically im-proved indexing of the EBSD patterns and facilitated high-resolution microstructural analysis of the carbon-rich domains. After microstructural EBSD analyses using an Oxford Symmetry detector we collected coordinated hy-perspectral cathodoluminescence (CL) analyses using a Gatan Monarc system mounted on a JEOL 7900F field emission (FE) scanning electron microscope at NASA Johnson Space Center. A focused ion beam (FIB) foil was extracted from a region of interest and analyzed using a JEOL 2500 FE scanning transmission electron microscope (STEM). In situ carbon and nitrogen isotopic analyses were collected from diamond-bearing domains using a Cameca NanoSIMS 50L at NASA JSC. Results: The carbon rich regions occur as tabular domains between polygonal olivine and pigeonite, and are composed of aggregates of micrometer-scale diamond with intermediate graphite, oxides and metal grains. Within the microdiamonds, {111} twins are abundant, many of which form ragged lamellae consistent with mechanical twinning. The microdiamond assemblages appear to pseudomorph tabular graphite habits, which together with the abundant deformation twins, support a shock transformation mechanism for microdiamond formation within the Kenna meteorite. Further supporting this observation, the diamond laths show close alignment of <111> and <110> between domains, consistent with a solid-state transformation from graphite along <0001> and <1-210>, respec-tively [5]. A FIB foil was extracted from a region with mechanical twins, and high resolution STEM analyses reveal cross-cutting veins of graphite and oriented kamacite crystals together with additional sets of {111} nano-twins formed in at least three orientations. Isotopic analyses of C and N reveal sector zoning patterns [cf. 3] that are unre-lated to the diamond substructure identified by EBSD yet consistent with hyperspectral CL images. Conclusions: These data provide new constraints on microdiamond formation within ureilites. The microdia-monds exhibit orientation variants defined by aligned <111> and <110> axes consistent with inherited crystallo-graphic orientations from transformation of a single hexagonal graphite parent crystal [5]. Crosscutting the micro-diamonds are veins of graphite and kamacite, together with additional {111} nanotwins. Taken together these data suggest that microdiamonds from ureilites formed from a solid state, diffusionless transformation from graphite, likely associated with a major shock event, which was subject to subsequent reversion either during decompression or a later deformation event. The N and C isotopic sector zoning is unrelated to diamond microstructures and likely reflects inheritance from the parent graphite crystal.

Ureilite↗
NASA NTRS2006

Interfacial Strength and Physical Properties of Functionalized Graphene - Epoxy Nanocomposites

The toughness and coefficient of thermal expansion of a series of functionalized graphene sheet - epoxy nanocomposites are investigated. Functionalized graphene sheets are produced by splitting graphite oxide into single graphene sheets through a rapid thermal expansion process. These graphene sheets contain approx. 10% oxygen due to the presence of hydroxide, epoxide, and carboxyl functional groups which assist in chemical bond formation with the epoxy matrix. Intrinsic surface functionality is used to graft alkyl amine chains on the graphene sheets, and the addition of excess hardener insures covalent bonding between the epoxide matrix and graphene sheets. Considerable improvement in the epoxy dimensional stability is obtained. An increase in nanocomposite toughness is observed in some cases.

Miller, Sandi G.↗
NASA NTRS1972

Improved oxidation-resistant carbon and graphite materials

A variety of silicon-carbide treated carbon and graphite products were prepared, and oxidation and strength properties determined. Carbon and graphite materials used in the preliminary study include fiber, tape, felt, foam, bulk, and laminar configurations. The silicon-carbide treatment used in preparing the oxidation resistant specimens employs either vacuum or ultrasonic impregnation of various silicone-silicon mixtures into the matrix material. The impregnation phase is followed by controlled thermal reaction at which time the silicon carbide is synthesized and takes the form of an integral protective boundary layer. Thickness of the layer ranges from a fraction of a mil to 10 mils, depending on the matrix material and processing parameters. Oxidation resistance of all the silicon-carbide-treated specimens investigated was found to be significantly improved. Mechanical strengths of the laminar composites were also found to be somewhat improved. At room temperature the specific strength of the silicon-carbide treated fibers and tapes was degraded to some extent; however, their relative strengths were significantly increased at elevated temperatures in oxidizing atmosphere.

Muir, H. M.↗
NASA NTRS1985

Thermo-oxidative stability of graphite fiber/PMR-15 polyimide composites at 350 deg C

A series of graphite fiber/PMR-15 polyimide composites were subjected to isothermal aging at 350 C in flowing air (100 cc/min and 1000 cc/min) over a 520 hr time period. The graphite fibers were analyzed by ISS/SIMS techniques before composite fabrication. Fibers exposed at the surface of the composite due to the isothermal aging process were also analyzed by the ISS/SIMS method. Component and composite weight less studies were also conducted for similarly exposed materials. Optical micrograph investigations of composites to follow the progress of the thermo-oxidative process were also conducted. Flexural and interlaminar shear strengths of the imaged and aged composites were measured. The relationship of component and composite properties as they relate to the thermo-oxidative behavior of the materials was discussed.

Scola, D. A.↗
NASA NTRS1989

CdO Pretreatment For Graphite Lubricant Films

Lubrication of rubbing steel surfaces with graphite improved by sputtering cadmium oxide onto surfaces, according to report. Lubricating films consisting of mixtures of cadmium oxide and graphite did not perform as well as films of graphite alone on surfaces pretreated with cadmium oxide. Primary beneficial effect obtained by sputtering pretreatment with cadmium oxide, which apparently improves bond between metallic substrate and graphite.

Fusaro, Robert L.↗
NASA NTRS1986

Sputtered cadmium oxide as a surface pretreatment for graphite solid lubricant films

Sputtered films of cadmium oxide were applied to sand blasted AISI 440C HT stainless steel disks as a surface pretreatment for the application of rubbed graphite films. Mixtures of cadmium oxide and graphite were applied to the nonpretreated sandblasted metal and evaluated. The results were compared to graphite films applied to other commercially available surface pretreatments. It is found that sputtered CdO pretreated surfaces increase the endurance lives of the graphite films and decrease the counterface steady state wear rate of the pins almost an order of magnitude compared to commercially available pretreatments. The CdO additions in general improved the tribological properties of graphite. The greatest benefit occurred when it was applied to the substrate rather than mixing it with the graphite and that sputtered films of CdO perform much better than rubbed CdO films.

Fusaro, R. L.↗
NASA NTRS1987

Sputtered cadmium oxide as a surface pretreatment for graphite solid-lubricant films

Sputtered films of cadmium oxide were applied to sand blasted AISI 440C HT stainless steel disks as a surface pretreatment for the application of rubbed graphite films. Mixtures of cadmium oxide and graphite were applied to the nonpretreated sandblasted metal and evaluated. The results were compared to graphite films applied to other commercially available surface pretreatments. It is found that sputtered CdO pretreated surfaces increase the endurance lives of the graphite films and decrease the counterface steady state wear rate of the pins almost an order of magnitude compared to commercially available pretreatments. The CdO additions in general improved the tribological properties of graphite. The greatest benefit occurred when it was applied to the substrate rather than mixing it with the graphite and that sputtered films of CdO perform much better than rubbed CdO films.

Fusaro, Robert L.↗
NASA NTRS1961

Investigation of Oxidation-Resistant Coatings on Graphite and Molybdenum in Two Arc-Powered Facilities

A number of leading-edge specimens of ATJ graphite and 0.5-percent titanium-molybdenum alloy were tested in a 6-inch subsonic low-pressure arc-powered tunnel and a 1,500-kw subsonic arc jet to determine the effectiveness of several refractory coatings in preventing the oxidation of these materials. The results indicate that the siliconized coating for the ATJ graphite and the W-2 and Durak MG coatings for the titanium-molybdenum alloy provide adequate protection at temperatures up to 3,000 F for the durations of the tests in these facilities, approximately 70 seconds in the arc tunnel and approximately 10 minutes in the arc jet. Weight losses (less than one-half of 1 percent) experienced by a few of the coated specimens indicate that tests of longer duration in these environments may prove deleterious and that the stability of these coatings may be depreciated under the low-pressure environmental conditions encountered during reentry.

Peters, Roger W.↗
NASA NTRS1997

Space Environment Stability and Physical Properties of New Materials for Space Power and Commercial Applications

Useful and informative results were obtained on virtually all materials investigated. For example, the stability of ITO-based arc-proof transparent coatings was greatly improved by substitution of silicon oxide for magnesium fluoride as a dopant. Research on 'air-doped' ITO films has yielded new insight into their conduction mechanism which will help in further development of these coatings. Some air-doped films were found to be extremely pressure sensitive. This work may lead to improved, low-cost gas sensors and vacuum gauges. Work on another promising transparent arc-proof coating (titanium oxide) was initiated in collaboration with industry. Graphite oxide-like materials were synthesized and tested for possible use in high energy-density batteries and other applications. We also started a high-priority project to find the cause of unexpected environmental damage to the exterior of the Hubble Space Telescope (HST) discovered on a recent Shuttle mission. Materials were characterized before and after exposure to soft x-rays and other threats in ground-based simulators.

Hambourger, Paul D.↗
NASA NTRS2005

Graphite Sheet Coating for Improved Thermal Oxidative Stability of Carbon Fiber Reinforced/PMR-15 Composites

Expanded graphite was compressed into graphite sheets and used as a coating for carbon fiber reinforced PMR-15 composites. BET analysis of the graphite indicated an increase in graphite pore size on compression, however the material was proven to be an effective barrier to oxygen when prepegged with PMR-15 resin. Oxygen permeability of the PMR-15/graphite was an order of magnitude lower than the compressed graphite sheet. By providing a barrier to oxygen permeation, the rate of oxidative degradation of PMR-15 was decreased. As a result, the composite thermo-oxidative stability increased by up to 25%. The addition of a graphite sheet as a top ply on the composites yielded little change in the material's flexural strength or interlaminar shear strength.

Campbell, Sandi↗
NASA NTRS1975

Wear and friction of oxidation-resistant mechanical carbon graphites at 650 C in air

Studies were conducted to determine the friction and wear properties of experimental carbon-graphites. Hemispherically tipped carbon-graphite rider specimens were tested in sliding contact with rotating Inconel X-750 disks in air. A surface speed of 1.33 m/sec, a load of 500 g, and a specimen temperature of 650 C were used. Results indicate: (1) hardness is not a major factor in determining friction and wear under the conditions of these studies. (2) Friction and wear as low as or lower than those observed for a good commercial seal material were attained with some of the experimental materials studied. (3) The inclusion of boron carbide (as an oxidation inhibitor) has a strong influence on wear rate. (4) Phosphate treatment reduces the friction coefficient when boron carbide is not present in the base material.

Allen, G. P.↗