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At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Development of a process for producing ribbon shaped boron filaments

A ribbon-shaped boron filament with a tensile strength of 138 KN/sq cm (200 ksi) was investigated. The investigation was carried out using both carbon and tungsten as substrate materials. No satisfactory results were obtained utilizing uncoated, copper-plated or silicon carbide coated tungsten ribbon substrates. Carbon ribbon substrates were prepared by pyrolysis of stretched polyimide tape. A severe deposition gradient occurred in all dc reactor experiments due to convective cooling at the edge of the substrate that resulted in a weak, nonuniform filament. RF heating, however, completely eliminated this gradient problem and a 90 cm (35.5 in.) long boron ribbon was produced at .85 cm/sec (100 ft/hr) using a frequency of 40.68 megahertz. This boron ribbon, however, exhibited a light-bulb effect during deposition to 59 KN/sq cm (85.7 ksi) were obtained from this ribbon. Several silicon carbide ribbon shaped filaments were also produced during this investigation by decomposition of CH3SiHCl2 onto carbon substrates. Contrary to the boron work, silicon carbide ribbon was readily prepared in dc static reactors. A very uniform and smooth deposition was obtained with strengths up to 71.1 KN/sq cm (103.5 ksi).

Basche, M.↗
NASA NTRS1974

Compatibility of refractory materials for nuclear reactor poison control systems

Metal-clad poison rods have been considered for the control system of an advanced space power reactor concept studied at the NASA Lewis Research Center. Such control rods may be required to operate at temperatures of about 140O C. Selected poison materials (including boron carbide and the diborides of zirconium, hafnium, and tantalum) were subjected to 1000-hour screening tests in contact with candidate refractory metal cladding materials (including tungsten and alloys of tantalum, niobium, and molybdenum) to assess the compatibility of these materials combinations at the temperatures of interest. Zirconium and hafnium diborides were compatible with refractory metals at 1400 C, but boron carbide and tantalum diboride reacted with the refractory metals at this temperature. Zirconium diboride also showed promise as a reaction barrier between boron carbide and tungsten.

Sinclair, J. H.↗
NASA NTRS1973

Carbon chemistry of the Apollo 15 and 16 deep drill cores

The carbon chemistry of the Apollo 15 and 16 deep drill cores is a function of the surface exposure plus the chemical and mineralogical composition of the individual samples. The depth profiles of carbide and methane yields in the Apollo 15 core show a general decline with depth and correlate with the solar wind noble gas content, percentage agglutinates, track densities, and metallic iron. All horizons examined were exposed for a considerable time on the lunar surface. The Apollo 16 core samples show that chemical and mineralogical composition plays an important role in determining the nature of carbide-like material present in the fines. The higher aluminum and calcium contents and lower iron contents of highlands material result in carbide-like material yielding less CD4 and more C2D2 (deuteroacetylene) upon DF acid dissolution.

Wszolek, P. C.↗
NASA NTRS1977

Vacuum-assisted impregnation of materials

Vacuum-assisted liquid-impregnation treatment for silicon-carbide-reinforced carbon-carbon systems improves resistance of silicon-carbide coating to attack by oxygen at elevated temperatures. Porosity reduction treatment might also be applied to sintering silicon-carbide bodies to increase their density and to make them more resistant to hot gases for use in jet engines.

Rogers, D. C.↗
NASA NTRS1978

Advanced ceramic material for high temperature turbine tip seals

Ceramic material systems are being considered for potential use as turbine blade tip gas path seals at temperatures up to 1370 1/4 C. Silicon carbide and silicon nitride structures were selected for study since an initial analysis of the problem gave these materials the greatest potential for development into a successful materials system. Segments of silicon nitride and silicon carbide materials over a range of densities, processed by various methods, a honeycomb structure of silicon nitride and ceramic blade tip inserts fabricated from both materials by hot pressing were tested singly and in combination. The evaluations included wear under simulated engine blade tip rub conditions, thermal stability, impact resistance, machinability, hot gas erosion and feasibility of fabrication into engine components. The silicon nitride honeycomb and low-density silicon carbide using a selected grain size distribution gave the most promising results as rub-tolerant shroud liners. Ceramic blade tip inserts made from hot-pressed silicon nitride gave excellent test results. Their behavior closely simulated metal tips. Wear was similar to that of metals but reduced by a factor of six.

Solomon, N. G.↗
NASA NTRS1979

Study of high resistance inorganic coatings on graphite fibers

Coatings made of boron, silicon carbide, silica, and silica-like materials were studied to determine their ability to increase resistance of graphite fibers. The most promising results were attained by chemical vapor depositing silicon carbide on graphite fiber followed by oxidation, and drawing graphite fiber through ethyl silicate followed by appropriate heat treatments. In the silicon carbide coating studies, no degradation of the graphite fibers was observed and resistance values as high as three orders of magnitude higher than that of the uncoated fiber was attained. The strength of a composite fabricated from the coated fiber had a strength which compared favorably with those of composites prepared from uncoated fiber. For the silica-like coated fiber prepared by drawing the graphite fiber through an ethyl silicate solution followed by heating, coated fiber resistances about an order of magnitude greater than that of the uncoated fiber were attained. Composites prepared using these fibers had flexural strengths comparable with those prepared using uncoated fibers, but the shear strengths were lower.

Galasso, F. S.↗
NASA NTRS1979

The relation of the yield stress of high-pressure anvils to the pressure attained at yielding and the ultimate attainable pressure

A sensitive microprofilometer was used to determine the onset of yielding in the anvils of a supported opposed anvil device for the case of 3% cobalt-cemented tungsten carbide as the anvil material. In addition, it is shown how the commencement of yielding in boron carbide pistons, the yield strength being known, can be used to obtain the transition pressure to a conducting phase in gallium phosphide. The transition pressures of bismuth and gallium phosphide are obtained and it is found that these transitions are extremely close to the maximum attainable pressure in, respectively, a maraging steel and a 3% cobalt-cemented tungsten carbide.

Panda, P. C.↗
NASA NTRS1979

Primordial retention of carbon by the terrestrial planets

The thermodynamics of graphite, carbide, and carbonate formation in the presence of a solar-composition gas is examined, including the feasibility of producing solid solutions of carbon and carbides in metallic iron-nickel alloy. Gas-phase composition and graphite activities for a very wide range of nebular pressures and temperatures are calculated by a computer program that considers several hundred compounds, particularly all the important gaseous species containing H, C, O, N, S, Cl, P, and F. The results obtained indicate that CH4 and CO would be the dominant carbon compounds under all likely circumstances of temperature and pressure in the solar nebula, that graphite may have been the most abundant carbon species in certain cases, and that direct precipitation of graphite, carbides, or carbonates would have been impossible in the solar nebula. Carbon retention by the terrestrial planets is also investigated.

Lewis, J. S.↗
NASA NTRS1979

Feasibility of SiC composite structures for 1644 deg gas turbine seal applications

The feasibility of silicon carbide composite structures was evaluated for 1644 K gas turbine seal applications. The silicon carbide composites evaluated consisted of Si/SiC Silcomp (Trademark) - and sintered silicon carbide as substrates, both with attached surface layers containing BN as an additive. A total of twenty-eight candidates with variations in substrate type and density, and layer chemistry, density, microstructure, and thickness were evaluated for abradability, cold particle erosion resistance, static oxidation resistance, ballistic impact resistance, and fabricability. The BN-free layers with variations in density and pore size were later added for evaluation. The most promising candidates were evaluated for Mach 1.0 gas oxidation/erosion resistance from 1477 K to 1644 K. The as-fabricated rub layers did not perform satisfactorily in the gas oxidation/erosion tests. However, preoxidation was found to be beneficial in improving the hot gas erosion resistance. Overall, the laboratory and rig test evaluations show that material properties are suitable for 1477 K gas turbine seal applications.

Darolia, R.↗
NASA NTRS1982

Refractories Keep Silicon Crystals Pure

Formation of carbon monoxide gas is prevented by a linear of refractory material free of elemental carbon. For pressures above about 4 torr, silicon carbide can be used as refractory liner. The problem of carbide contamination can arise in crystal growth of any material that forms a carbide more stable than carbon monoxide. Prevention in such cases is possible by using noncarbon refractories in place of graphite.

Schmid, F.↗
NASA NTRS1982

Surface Seal for Carbon Parts

Surface pores in parts made of graphite or reinforced-carbon/ carbon materials are sealed by a silicon carbide-based coating. Coating inhibits subsurface oxidation and lengthens part life. Starting material for coating is graphite felt, which is converted to silicon carbide felt by processing it according to a prescribed time/temperature schedule. Converted felt is pulverized in a ball mill and resulting powder is mixed with an equal weight of black silicon carbide powder. Powder mixture is combined with an equal weight of adhesive to form a paste.

Shuford, D. M.↗
NASA NTRS1984

Oxidation-Resistant Slurry Coating for Carbon-Based Materials

New process uses paint sprayer and vacuum furnace to produce silicon carbide outer layer. In cross section of silicon and silicon carbide reaction zone, top layer of silicon adheres to silicon carbide layer. Crystals prominent on melted top surface of slurry coating. Process especially useful in coating repair.

Smialek, J. L.↗
NASA NTRS1982

Elevated temperature properties of aligned ferrous eutectics

Iron base alloys containing aluminum and chromium together with smaller amounts of yttrium and silicon are of interest for high temperature applications. Strengthening an inherently weak but oxidation resistant solid solution matrix with aligned in situ chromium carbides represents an attractive approach to achieving both surface stability and creep resistance at elevated temperatures. Aligned microstructures were produced in alloys of approximately 30 wt % (Cr + Mn), about 3 wt % C and the balance Fe consisting of a gamma matrix and the hexagonal carbide (Cr, Mn, Fe)7C3. The tensile and stress rupture strength to 2000 F of aligned Fe-20 w/t % Cr-10 wt % Mn-3.2 wt % C measured parallel to the carbide reinforcement exceeded those of the strongest iron-nickel superalloys, e.g., CRM-6D developed by Chrysler for automotive turbine application. The cyclic oxidation and sulfidation response of these alloys at elevated temperatures can be markedly improved by aluminum additions.

Lemkey, F. D.↗
NASA NTRS1982

Development of Fe-Mn-Al-X-C alloys

Development of a low cost Cr-free, iron-base alloy for aerospace applications involves both element substitution and enhancement of microstructural strengthening. When Mn is substituted for Ni and Al or Si is substituted for Cr, large changes occur in the mechanical and thermal stability of austenite in FeMnAlC alloys. The in situ strength of MC or M2C (M = Ti, V, Hf, Ta, or Mo) in FeMnAlC alloys was determined. The high temperature tensile strength depends more on the distribution of carbides than the carbide composition. Precipitation of a high volume percent-ordered phase was achieved in Fe2OMnlONi6Al6Ti (lC) alloys. As case, these alloys have a homogeneous austenitic structure. After solutioning at 1100 C for 5 hr followed by aging at 600 C for 16 hr, gamma prime or a perovskite carbide is precipitated. Overaging occurs at 900 C where eta is precipitated.

Schuon, S. R.↗
NASA NTRS1984

Electrokinetic potential and stability of SiC suspensions in electrolyte solutions

The dependence of zeta potential on pH of an aqueous Si carbide suspension was studied by an electroosmosis method. The effect of pH on the Si carbide electrical double layer is attributed to the participation of H and OH ions in the surface dissociation reactions of weak acid OH groups on an oxidized carbide surface. Maximum stability of the suspensions was observed at maximum negative zeta potentials.

Eremenko, B. V.↗
NASA NTRS1984

Chemical feasibility of lithium as a matrix for structural composites

The chemical compatibility of lithium with tows of carbon and aramid fibers and silicon carbide and boron monofilaments was investigated by encapsulating the fibers in liquid lithium and also by sintering. The lithium did not readily wet the various fibers. In particular, very little lithium infiltration into the carbon and aramid tows was achieved and the strength of the tows was seriously degraded. The strength of the boron and silicon carbide monofilaments, however, was not affected by the liquid lithium. Therefore lithium is not feasible as a matrix for carbon and aramid fibers, but a composite containing boron or silicon carbide fibers in a lithium matrix may be feasible for specialized applications.

Swann, R. T.↗
NASA NTRS1987

Thermal stability of the nickel-base superalloy B-1900 + Hf with tantalum variations

The microstructure of the solutionized and aged nickel-base superalloy B-1900 + Hf was examined after additional aging at 982 C for 72, 250, and 1000 hours. Alloy compositions that were examined contained the normal 1.34 at. pct (4.3 wt pct) Ta as well as 0.67 at. pct and zero Ta levels. The gamma-prime phase agglomerated, became platelike in morphology, and decreased in volume fraction for all three alloys throughout the aging treatments. Changes which occurred in the gamma and gamma-prime phase compositions were nearly complete after 72 hours of aging while changes in the MC carbide composition continued throughout the aging. Blocky M6C carbides precipitated along the grain boundaries of all three alloys in the first 72 hours of aging. In addition, an acicular form of this Mo/Cr/Ni-rich carbide developed in the intragranular regions of the Ta-containing alloys.

Harmon, B. S.↗
NASA NTRS1986

Characterization of precipitates in a niobium-zirconium-carbon alloy

A niobium alloy with 1 percent zirconium and 0.063 percent carbon by weight was investigated in the as-rolled and annealed conditions, and after high-temperature (1350 and 1400 K) exposure with and without an applied stress. In the as-rolled and annealed conditions, large metastable carbides were observed in addition to a regular distribution of small particles. During the high-temperature exposure, the majority of the large carbides were dissolved and a more stable carbide phase formed. This finely dispersed phase had a composition determined to be approximately 70 percent ZrC and approximately 30 percent NbC and showed some evidence of an orientation relationship with the matrix. The precipitates appeared to coarsen slightly after approximately 5000 hr exposure in the presence of an applied stress resulted in a decrease in the size and in the interparticle spacing of the stable precipitates. However, the composition of the precipitate phase and its ability to pin dislocations were not affected by the temperature or stress conditions.

Grobstein, T. L.↗