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At least 793 records · Page 44

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NASA NTRS1986

Slip casting and extruding shapes of rhenium with metal oxide additives. 1: Feasibility demonstration

The feasibility of fabricating small rhenium parts with metal oxide additives by means of slip casting and extrusion techniques is described. The metal oxides, ZrO2 and HfO2 were stabilized into the cubic phase with Y2O3. Additions of metal oxide to the rhenium of up to 15 weight percent were used. Tubes of 17 mm diameter with 0.5 mm walls were slip cast by adapting current ceramic oxide techniques. A complete cast double conical nozzle demonstrated the ability to meet shapes and tolerances. Extrusion of meter long tubing lengths of 3.9 mm o.d. x 2.3 mm i.d. final dimension is documented. Sintering schedules are presented to produce better than 95% of theoretical density parts. Finished machining was found possible were requried by electric discharge machining and diamond grinding.

Barr, F. A.↗

NASA NTRS1985

Reaction of metals in lower earth orbit during Space Shuttle flight 41-G

The effects of ambient space environment on metals were studied by exposing specimens of Cu, Ag, Au, Ni, Cr, Al, Pt, and Pd on flight 41-G (STS-17). Data obtained by ellipsometry (ELL), Rutherford backscattering (RBS), and proton-induced X-ray emission (PIXE) before and after flight are summarized. Although the effects of space environment were most pronounced for silver, there were significant changes in the surface properties of the majority of the other metals. The surface optical constants proved to be the most sensitive measure of surface changes. These changes are attributed to the interaction of the metals with atomic oxygen.

Fromhold, A. T., Jr.↗

NASA NTRS1985

Refractory amorphous metallic (W/0.6/ Re/0.4/)76B24 coatings on steel substrates

Refractory metallic coatings of (W/0.6/ Re/0.4/)76B24 (WReB) have been deposited onto glass, quartz, and heat-treated AISI 52100 bearing steel substrates by dc magnetron sputtering. As-deposited WReB films are amorphous, as shown by their diffuse X-ray diffraction patterns; chemically homogeneous, according to secondary ion mass spectrometry (SIMS) analysis; and they exhibit a very high (approximately 1000 C) crystallization temperature. Adhesion strength of these coatings on heat-treated AISI 52100 steel is in excess of approximately 20,000 psi and they possess high microhardness (approximately 2400 HV50). Unlubricated wear resistance of such hard and adherent amorphous metallic coatings on AISI 52100 steel is studied using the pin-on-disc method under various loading conditions. Amorphous metallic WReB coatings, about 4 microns thick, exhibit an improvement of more than two and a half orders of magnitude in the unlubricated wear resistance over that of the uncoated AISI 52100 steel.

Thakoor, A. P.↗

NASA NTRS1984

Mechanical-contact-induced transformation from the amorphous to the partially crystalline state in metallic glass

Friction and wear tests were conducted with 3.2- and 6.4-millimeter-diameter aluminum oxide spheres sliding, in reciprocating motion, on a Fe67Co18B14Si1 metallic foil. Crystallites with a size range of 10 to 150 nanometers were produced on the wear surface of the amorphous alloy. A strong interaction between transition metals and metalloids such as silicon and boron results in strong segregation during repeated sliding, provides preferential transition metal-metalloid clustering in the amorphous alloy, and subsequently produces the diffused honeycomb structure formed by dark grey bands and primary crystals, that is, alpha-Fe in the matrix. Large plastic flow occurs on an amorphous alloy surface with sliding and the flow film of the alloy transfers to the aluminum oxide pin surface. Multiple slip bands due to shear deformation are observed on the side of the wear track. Two distinct types of wear debris were observed as a result of sliding: an alloy wear debris, and/or powdery-whiskery oxide debris.

Miyoshi, K.↗

NASA NTRS1985

Metallic glass as a temperature sensor during ion plating

The temperature of the interface and/or a superficial layer of a substrate during ion plating was investigated using a metallic glass of the composition Fe67Co18B14Si1 as the substrate and as the temperature sensor. Transmission electron microscopy and diffraction studies determined the microstructure of the ion-plated gold film and the substrate. Results indicate that crystallization occurs not only in the film, but also in the substrate. The grain size of crystals formed during ion plating was 6 to 60 nm in the gold film and 8 to 100 nm in the substrate at a depth of 10 to 15 micrometers from the ion-plated interface. The temperature rise of the substrate during ion plating was approximately 500 C. Discontinuous changes in metallurgical microstructure, and physical, chemical, and mechanical properties during the amorphous to crystalline transition in metallic glasses make metallic glasses extremely useful materials for temperature sensor applications in coating processes.

Miyoshi, K.↗

NASA NTRS1985

Film and interstitial formation of metals in plasma-sprayed ceramics

A method is described to electrodeposit noble metals such as platinum and ordinary metals such as copper on and within plasma-sprayed ceramic materials and ceramic fiber materials. Low-density ceramic fiber bodies were vacuum impregnated with plating solution and attached to an electrode. Light micrographs illustrating the density and location of deposited materials are presented and discussed. Voids in the plasma-sprayed ceramic were filled with deposits that vary from spherical to lens-shaped circular and have particle size corresponding to the full range of void size. Multiple coatings of ceramic and metal can be sequenced.

Hendricks, R. C.↗

NASA NTRS1985

On 3d bonding in the transition metal trimers - The electronic structure of equilateral triangle Ca3, Sc3, Sc3(+), and Ti3(+)

It is pointed out that transition metals and transition metal (TM) compounds are currently of considerable interest because of their relevance to catalysis and to materials science problems such as hydrogen embrittlement and crack propagation in metals. The present paper is concerned with complete active space Self-Consistent Field (SCF) externally contracted configuration interaction (CASSCF/CCI) calculations for the low-lying states of Sc3 and Sc3(+). A comparison is conducted regarding the bonding in the Ca3, Sc3, and Cu3 molecules. This comparison makes it possible to predict general trends for the TM trimers. Attention is given to the qualitative features of the bonding in the TM trimers, the basis sets and other technical details of the calculations, the calculated results for Sc3 and Sc3(+), and conclusions from this work.

Walch, S. P.↗

NASA NTRS1987

Characteristics and Applications of Metal Hydrides

Report discusses engineering principles of uses of metal hydrides in spacecraft. Metal hydrides absorb, store, pump, compress, and expand hydrogen gas. Additionally, they release or absorb sizeable amounts of heat as they form and decompose - property adapted for thermal-energy management or for propulsion. Describes efforts to: Identify heat sources and sinks suitable for driving metal hydride thermal cycles in spacecraft; develop concepts for hydride subsystems employing available heating and cooling methods; and produce data base on estimated sizes, masses, and performances of hydride devices for spacecraft.

Egan, G. J.↗

NASA NTRS1987

Fatigue-Testing Apparatus for Metal Matrix Composites

Thermal and mechanical load cycling simulated in realistic fatigue tests. Efficient joining of metal matrix composite components to supporting structures is major concern facing users of these materials. Lewis Research Center designed and developed two thermal/mechancical fatigue test facilities, one to test metal matrix composite specimens and another to test compostite/metal attachment bond joints. Thermal/mechanical fatigue facility designed for testing metal matrix composites permits specimen-temperature excursions with controlled heating and loading rates. Second facility designed to test composite/metal attachment bond joints and to permit heating to maximum temperature of 1,400oC (760oC) within 10 min and cooling to 300oF (150oC) within 3 min. Facility has unique capabilities not found in other laboratories.

Westfall, Leonard J.↗

NASA NTRS1986

Development of AC impedance methods for evaluating corroding metal surfaces and coatings

In an effort to investigate metal surface corrosion and the breakdown of metal protective coatings the AC Impedance Method was applied to zinc chromate primer coated 2219-T87 aluminum. The model 368-1 AC Impedance Measurement System recently acquired by the MSFC Corrosion Research Branch was used to monitor changing properties of coated aluminum disks immersed in 3.5% NaCl buffered at ph 5.5 over three to four weeks. The DC polarization resistance runs were performed on the same samples. The corrosion system can be represented by an electronic analog called an equivalent circuit that consists of transistors and capacitors in specific arrangements. This equivalent circuit parallels the impedance behavior of the corrosion system during a frequency scan. Values for resistances and capacities that can be assigned in the equivalent circuit following a least squares analysis of the data describe changes that occur on the corroding metal surface and in the protective coating. A suitable equivalent circuit was determined that predicts the correct Bode phase and magnitude for the experimental sample. The DC corrosion current density data are related to equivalent circuit element parameters.

Knockemus, Ward↗

NASA NTRS1987

Mechanical strength and tribological behavior of ion-beam deposited boron nitride films on non-metallic substrates

An investigation was conducted to examine the mechanical strength and tribological properties of boron nitride (BN) films ion-beam deposited on silicon (Si), fused silica (SiO2), gallium arsenide (GaAs), and indium phosphide (InP) substrates in sliding contact with a diamond pin under a load. The results of the investigation indicate that BN films on nonmetallic substrates, like metal films on metallic substrates, deform elastically and plastically in the interfacial region when in contact with a diamond pin. However, unlike metal films and substrates, BN films on nonmetallic substrates can fracture when they are critically loaded. Not only does the yield pressure (hardness) of Si and SiO2 substrates increase by a factor of 2 in the presence of a BN film, but the critical load needed to fracture increases as well. The presence of films on the brittle substrates can arrest crack formation. The BN film reduces adhesion and friction in the sliding contact. BN adheres to Si and SiO2 and forms a good quality film, while it adheres poorly to GaAs and InP. The interfacial adhesive strengths were 1 GPa for a BN film on Si and appreciably higher than 1 GPa for a BN film on SiO2.

Miyoshi, Kazuhisa↗

NASA NTRS1987

Strong metal-support interactions

It has been demonstrated that synergistic metal-support effects can occur which markedly enhance specific activity and alter selectivity in certain reactions. Because of the presence of such effects in certain reactions conducted under reducing conditions (that is, under H2 pressure), but not others, the creation of unique sites at the metal-support interface seems to be the best model at the present time to explain this behavior. The postulation of these sites, which are specific for a certain reactant such as CO, provides an effective explanation for the higher methanation rates that have been reported over some catalysts. The creation of these sites in the adlineation zone is facilitated by hydrogen spillover from the metal surface, and this same process can also enhance the reduction of many oxide supports. Although oxygen spillover is much less probable due to its higher heat of adsorption, it is much less well understood and the possibility of rate enhancements in CO oxidation caused by special interface sites cannot be discounted at the present time. Consequently, this seems to be an important area of future research.

Vannice, M. Albert↗

NASA NTRS1986

High-pressure metallization of FeO and implications for the earth's core

The phase diagram of FeO has been experimentally determined to pressures of 155 GPa and temperatures of 4000 K using shock-wave and diamond-cell techniques. A metallic phase of FeO is observed at pressures greater than 70 GPa and temperatures exceeding 1000 K. The metallization of FeO at high pressures implies that oxygen can be present as the light alloying element of the earth's outer core, in accord with the geochemical predictions of Ringwood (1977 and 1979). The high pressures necessary for this metallization suggest that the core has acquired its composition well after the initial stages of the earth's accretion. Direct experimental observations at elevated pressures and temperatures indicate that core-forming alloy can react chemically with oxides such as those forming the mantle. The core and mantle may never have reached complete chemical equilibrium, however. If this is the case, the core-mantle boundary is likely to be a zone of active chemical reactions.

Knittle, Elise↗

NASA NTRS1987

Mixed Cu-simple metal dimers and trimers - CuLi, CuLi2, CuNa, CuK, CuBe, CuBe2, Cu2Be, CuAl, and CuAl2

Theoretical studies of selected diatomic and triatomic molecules containing copper and the simple metals Li, Na, K, Be, and Al are presented, with emphasis on elucidating the nature of the bonding in mixed transition metal-simple metal systems. Large Gaussian basis sets are used in the diatomic calculations, and are used to calibrate the triatomic calculations, in which somewhat smaller Gaussian basis sets are employed. Electron correlation is incorporated using both the single-reference singles plus doubles configuration interaction and coupled pair functional methods. It is found that alkali atoms form very polar sigma bonds with copper, and that the ionicity increases with the inclusion of higher excitations because they improve the electron affinity of copper, which in turn allows a larger negative charge on copper. Aluminum is found to form stronger bonds than beryllium, since it does not have to undergo sp hybridization. Some of the trimers bond by forming three-center three-electron bonds. These multicenter bonds are quite strong even when compared to the two-electron bonds in the dimers or to other bonding mechanisms in the trimers.

Bauschlicher, Charles W., Jr.↗

NASA NTRS1987

Slip casting and extruding shapes of rhemium with metal oxide additives. Part 2: Development of grain stabilized rhenium parts for resistojets

The adaptation of the powdered particle process used for pure metal oxides to the coprocessing of rhenium oxides suitable to produce pure miniature resistojet hardware has been successful. Both slip casting and extrusion processes were used. The metal oxide ZrO2 was stabilized into the cubic phase with Y2O3, for use as a potentially grain stabilizing additive to rhenium. Straight meter long tubing in two sizes are reported. Tubing suitable for resistojet ohmic heater use of fully fired dimensions of nominally 3.8 mm o.d. x 2.2 mm i.d.. and 1.26 mm o.d. x .45 mm i.d. with 0, 0.5, 1.0 and 5.0% zirconia additives were produced for further study. Photomicrographs of these are discussed. The addition of the metal oxide zirconia to rhenium resulted in more dense and less porous parts. The additions of phase stabilized zirconia most likely act as a sintering aid. Tubes of varying diameter were slip cast which were representative of miniature pressure cases.

Barr, Francis A.↗

NASA NTRS1988

Noncontact temperature measurement: Requirements and applications for metals and alloys research

Temperature measurement is an essential capability for almost all areas of metals and alloys research. In the microgravity environment many of the science priorities that have been identified for metals and alloys also require noncontact temperature measurement capability. For example, in order to exploit the full potential of containerless processing, it is critical to have available a suitable noncontact temperature measurement system. This system is needed to track continuously the thermal history, including melt undercooling and rapid recalescence, of relatively small metal spheres during free-fall motion in drop tube systems. During containerless processing with levitation-based equipment, accurate noncontact temperature measurement is required to monitor one or more quasi-static samples with sufficient spatial and thermal resolution to follow the progress of solidification fronts originating in undercooled melts. In crystal growth, thermal migration, coarsening and other experiments high resolution thermal maps would be a valuable asset in the understanding and modeling of solidification processes, fluid flows and microstructure development. The science and applications requirements place several constraints on the spatial resolution, response time and accuracy of suitable instrumentation.

Perepezko, J. H.↗

NASA NTRS1987

Mechanical strength and tribological behavior of ion-beam-deposited boron nitride films on non-metallic substrates

Miyoshi, Kazuhisa↗

NASA NTRS1988

Color-magnitude diagrams for six metal-rich, low-latitude globular clusters

Colors and magnitudes for stars on CCD frames for six metal-rich, low-latitude, previously unstudied globular clusters and one well-studied, metal-rich cluster (47 Tuc) have been derived and color-magnitude diagrams have been constructed. The photometry for stars in 47 Tuc are in good agreement with previous studies, while the V magnitudes of the horizontal-branch stars in the six program clusters do not agree with estimates based on secondary methods. The distances to these clusters are different from prior estimates. Redding values are derived for each program cluster. The horizontal branches of the program clusters all appear to lie entirely redwards of the red edge of the instability strip, as is normal for their metallicities.

Armandroff, Taft E.↗