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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Changes in surface chemistry of silicon carbide (0001) surface with temperature and their effect on friction

Friction studies were conducted with a silicon carbide (0001) surface contacting polycrystalline iron. The surface of silicon carbide was pretreated: (1) by bombarding it with argon ions for 30 minutes at a pressure of 1.3 pascals; (2) by heating it at 800 C for 3 hours in vacuum at a pressure of 10 to the minus eighth power pascal; or (3) by heating it at 1500 C for 3 hours in a vacuum of 10 to the minus eighth power pascal. Auger emission spectroscopy was used to determine the presence of silicon and carbon and the form of the carbon. The surfaces of silicon carbide bombarded with argon ions or preheated to 800 C revealed the main Si peak and a carbide type of C peak in the Auger spectra. The surfaces preheated to 1500 C revealed only a graphite type of C peak in the Auger spectra, and the Si peak had diminished to a barely perceptible amount. The surfaces of silicon carbide preheated to 800 C gave a 1.5 to 3 times higher coefficient of friction than did the surfaces of silicon carbide preheated to 1500 C. The coefficient of friction was lower in the 11(-2)0 direction than in the 10(-1)0 direction; that is, it was lower in the preferred crystallographic slip direction.

Miyoshi, K.↗
NASA NTRS1995

Low Cost Fabrication of Silicon Carbide Based Ceramics and Fiber Reinforced Composites

A low cost processing technique called reaction forming for the fabrication of near-net and complex shaped components of silicon carbide based ceramics and composites is presented. This process consists of the production of a microporous carbon preform and subsequent infiltration with liquid silicon or silicon-refractory metal alloys. The microporous preforms are made by the pyrolysis of a polymerized resin mixture with very good control of pore volume and pore size thereby yielding materials with tailorable microstructure and composition. Mechanical properties (elastic modulus, flexural strength, and fracture toughness) of reaction-formed silicon carbide ceramics are presented. This processing approach is suitable for various kinds of reinforcements such as whiskers, particulates, fibers (tows, weaves, and filaments), and 3-D architectures. This approach has also been used to fabricate continuous silicon carbide fiber reinforced ceramic composites (CFCC's) with silicon carbide based matrices. Strong and tough composites with tailorable matrix microstructure and composition have been obtained. Microstructure and thermomechanical properties of a silicon carbide (SCS-6) fiber reinforced reaction-formed silicon carbide matrix composites are discussed.

Singh, M.↗
NASA NTRS1996

Development and Processing of Nickel Aluminide-Carbide Alloys

With the upper temperature limit of the Ni-based superalloys attained, a new class of materials is required. Intermetallics appear as likely candidates because of their attractive physical properties. With a relatively low density, high thermal conductivity, excellent oxidation resistance, high melting point, and simple crystal structure, nickel aluminide (NiAl) appears to be a potential candidate. However, NiAl is limited in structural applications due to its low room temperature fracture toughness and poor elevated temperature strength. One approach to improving these properties has been through the application of eutectic composites. Researchers have shown that containerless directional solidification of NiAl-based eutectic alloys can provide improvement in both the creep strength and fracture toughness. Although these systems have shown improvements in the mechanical properties, the presence of refractory metals increases the density significantly in some alloys. Lower density systems, such as the carbides, nitrides, and borides, may provide NiAl-based eutectic structure. With little or no information available on these systems, experimental investigation is required. The objective of this research was to locate and develop NiAl-carbide eutectic alloys. Exploratory arc-melts were performed in NiAl-refractory metal-C systems. Refractory metal systems investigated included Co, Cr, Fe, Hf, Mo, Nb, Ta, Ti, W, and Zr. Systems containing carbides with excellent stability (i.e.,HfC, NbC, TaC, TiC, and ZrC) produced large blocky cubic carbides in an NiAl matrix. The carbides appeared to have formed in the liquid state and were randomly distributed throughout the polycrystalline NiAl. The Co, Cr, Fe, Mo, and W systems contained NiAl dendrites with a two-phase interdendritic microconstituent present. Of these systems, the NiAl-Mo-C system had the most promising microstructure for in-situ composites. Three processing techniques were used to evaluate the NiAl-Mo-C system: arc-melting, slow cooling, and containerless directional solidification. Arc-melting provided a wide range of compositions in an economical and simple fashion. The slow cooled ingots provided larger ingots and slower cooling rates than arc-melting. Directional solidification was used to produce in-situ composites consisting of NiAl reinforced with molybdenum carbides.

Newport, Timothy Scott↗
NASA NTRS1966

Fully Deuterated Aliphatic Hydrocarbons Obtained From Iron Carbide Treated with DCl and D2O

According to Oparin, Mendeleev thought that the origin of petroleum was the result of the hydrolysis of iron carbides by superheated steam under pressure from the deep interior of the Earth through geological formations where the metal carbides exist. As early as 1877, Mendeleev described the reaction leading to the synthesis of hydrocarbons according to the general equation 3Fe(sub m)C(sub n) + mH2O yields mFe3O4 + C(sub 3n)H(sub 8m). Other experimental studies on the production of hydrocarbons from cast iron have been reported. Because of the possibility that hydrocarbons may have been trapped within the carbon matrix of the cast iron, which usually has a high content of carbon, we have studied the reaction of pure iron carbide with deuterium chloride and deuterated water. This was done in order to distinguish any newly formed deuterated hydrocarbons from any possible impurities of trapped hydrocarbons. The experiments were carried out by simply allowing iron carbide to react with concentrated deuterium chloride in D2O. The volatile hydrocarbon fraction examined by gas chromatography-mass spectrometry (GC/MS), using a Finnigan 1020/OWA instrument. contained low molecular weight hydrocarbons in a range C3 to C7. Lower molecular weight hydrocarbons were not detected by GC/MS because the MS scanning mode was preset above mass 40 to exclude components of air. The identified hydrocarbons are similar to those obtained under prebiotic conditions using high frequency discharge. The hydrocarbons found in common were propane, butane, pentane, 3-methylpentane, hexane, and heptane. The percent yields decline with increasing carbon number (propane 11%, n-heptane 1%). Similar results have been obtained by the direct treatment of metal carbides by pulse laser vaporization mass spectrometry. These results show that the hydrolysis of iron carbides may have been a significant source of hydrocarbons on the primitive Earth. There appears to be a predominance of straight chain isomers, which may have been important in the prebiotic synthesis of membrane-forming compounds.

Marquez, C.↗
DOE OSTI2024

Additive manufacturing process for producing aluminum-boron carbide metal matrix composites

A method for additive manufacturing of a composite object containing a bonded network of boron carbide particles and aluminum occupying spaces between boron carbide particles, the method comprising: (i) producing a porous preform constructed of boron carbide by an additive manufacturing process in which particles of boron carbide are bonded together; and (ii) infiltrating molten aluminum, at a temperature of 1000-1400° C., into pores of said porous preform to produce said composite object constructed of boron carbide particles within an aluminum matrix, wherein the boron carbide is present in the composite object in an amount of 30-70 wt. %. The resulting composite material is also herein described.

Cramer, Corson L.↗
DOE OSTI2023

Predicting oxidation damage of ultra high-temperature carbide ceramics in extreme environments using machine learning

Determining the oxidation resistance of UHTC carbides in extreme environments is challenging theoretically and experimentally due to the high dimensional complexity of influencing variables and intricate testing setups. Herein we demonstrate the use of machine learning (ML) models trained with experimental literature data to predict the oxide thickness of UHTC carbides exposed to air based on composition, mean grain size, relative densification, holding time, and temperature. A multi-dimensional database with 76 occurrences is created containing experimental results of Hf, Zr, and Ta carbides plus additives. In this study, the preprocessed database is then used to train ML models to predict their oxidation behavior. The trained model predicts the oxidation damage in the form of an average oxide thickness in UHTC carbides with a Mean Absolute Error (MAE) of ±65.45 μm for samples in the testing set that developed thicknesses up to 1000 μm. The model successfully predicted oxidation damage for a recession rate lower than 60 μm/min. It is noticed that the ensemble method MAE is increased to ±134.34 μm while forecasting the oxidation of samples with a recession rate higher than the threshold. The unprecedented approach is a novel way to predict the damage through the oxidation of carbide compounds before processing for a smarter design with room for improvement.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Concurrent reaction-bonded joining and densification of additively manufactured silicon carbide by liquid silicon infiltration

Here, in this study, additive-manufactured silicon carbide preforms were joined and densified by reaction bonding via liquid silicon infiltration. The silicon carbide preforms were first printed by binder jetting additive manufacturing. To demonstrate concurrent joining and densification, two preforms with carbon or parchment papers at the interface were concurrently joined and infiltrated by liquid silicon. Results showed a robust interface with thicknesses ranging from 150 to 500 µm, depending on the paper type and the number of paper layers. High-energy synchrotron X-ray revealed that β-phase silicon carbide was formed inside the interface. Finally, two additively manufactured samples with complicated channel geometry were successfully joined. Energy dispersive spectroscopy of the interface of the channeled samples showed a consistent and robust joining. This concurrent approach of joining and densification enables efficiency improvement of fabricating silicon carbide parts with complicated geometries and widens geometry freedom for additive manufacturing of silicon carbide.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Hierarchically structured metal carbides as conductive fillers in thermo-responsive polymer nanocomposites for battery safety

Integrating thermo-responsive polymer switching materials (TRPS) into lithium-ion batteries (LIBs) has been recognized as one of the most effective strategies to prevent thermal runaway under various abuse scenarios. However, the current methods to obtain TRPS cannot satisfy different practical applications. Herein, we develop a versatile strategy for the preparation of various metal carbides (e.g., tungsten carbide, molybdenum carbide) with controllable hierarchical structure that is featured with surface protrusion structure, which is critical for high conductivity and rapid thermal response. Systematic studies of the phase and morphology evolutions by advanced characterizations illustrate that the reducing agent and reduction rate are critical for developing the specific morphology. In conclusion, by using the above-mentioned carbides as the conductive fillers of TRPS, the resulting TRPS with a specially controlled shape exhibits over 5 order of conductivity improvement compared with common carbides with particulate morphology, in addition to reversible shutdown performance and effective thermal abuse protections toward safe LIB operation.

25 ENERGY STORAGE↗
DOE OSTI2025

Irradiation-induced formation of G-phase precipitates and M 2 X carbides in self-ion irradiated HT-9

Ferritic-martensitic steels with high chromium content are a promising material group for advanced nuclear systems due to their high temperature strength and good irradiation tolerance. HT-9 is an optimized and often-studied alloy in this group, but additional studies are required on its radiation response under extreme conditions to be experienced in various types of nuclear reactors, especially with respect to phase stability under irradiation. Self-ion irradiation of HT-9 by 5 MeV Fe ions was used to simulate neutron-induced behavior reaching peak doses of 100 and 300 dpa at temperatures ranging from 450 to 550 °C. M 23 C 6 carbides that existed prior to irradiation were found to remain stable under all examined irradiation conditions. As irradiation progressed at 450 and 500 °C, however, formation of spherical-like G-phase precipitates and needle-like M 2 X carbides was observed. G-phase precipitates were found to be enriched in Ni, Si, and Mn, and show no interface segregation, whereas needle-like M 2 X carbides were rich in Cr and Mo and clearly displayed interface segregation of Ni and Si. M 2 X carbide formation is believed to be assisted by vacancies, while G-phase precipitation is thought to be assisted by interstitials. Finally, this difference in defect-mediated formation leads to a difference in distribution with depth. M 2 X carbides are distributed over shallower depths than that of G-phase precipitates, consistent with defect imbalance predictions that consider the influence of the injected interstitial effect.

atom probe tomography↗
DOE OSTI2022

High-entropy boride–carbide ceramics by sequential boro/carbothermal synthesis

We report a dual-phase high-entropy boride/carbide ceramic with fine grain size was synthesized by a sequential boro/carbothermal process. In the first step, a Hf-Nb-Ta-Ti-Zr-containing carbide was synthesized by carbothermal reduction of oxides followed by reaction of the carbide with B4C and ZrH2 to convert part of the carbide to boride. The resulting composition was ~29 vol% high-entropy boride with an average grain size of ~1.1 µm. Solid solution formation occurred at the densification temperature of 1900°C resulting in a relative density higher than 99%. The Vickers’ hardness was 26.5 ± 1.4 GPa. This is the first report of synthesizing dual-phase boride-carbide high-entropy ceramics from carbothermally synthesized, high-entropy carbide powders.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Performance of Carbide Alloy Compounds in Carbon Doped MoNbTaW

In this work, the performance of the carbon doped compositionally complex alloy (CCA) MoNbTaW was studied under ambient and high pressure and high temperature conditions. TaC and NbC carbides were formed when a large concentration of carbon was introduced while synthesizing the MoNbTaW alloy. Both FCC carbides and BCC CCA phases were detected in the sample compound at room temperature, in which the BCC phase was believed to have only refractory elements MoNbTaW while FCC carbide came from TaC and NbC. Carbides in the carbon doped MoNbTaW alloy were very stable since no phase transition was obtained even under 3.1 GPa and 870 °C by employing the resistor-heating diamond anvil cell (DAC) synchrotron X-ray diffraction technique. Via in situ examination, this study confirms the stability of carbides and MoNbTaW in the carbon doped CCA even under high pressure and high temperature.

36 MATERIALS SCIENCE↗
NASA NTRS1971

Compatibility of buffered uranium carbides with tungsten.

Results of compatibility tests between tungsten and hyperstoichiometric uranium carbide alloys run at 1800 C for 1000 and 2500 hours. These tests compared tungsten-buffered uranium carbide with tungsten-buffered uranium-zirconium carbide. The zirconium carbide addition appeared to widen the homogeneity range of the uranium carbide, making additional carbon available for reaction. Reaction layers could be formed by either of two diffusion paths, one producing UWC2, while the second resulted in the formation of W2C. UWC2 acts as a diffusion barrier for carbon and slows the growth of the reaction layer with time, while carbon diffusion is relatively rapid in W2C, allowing equilibrium to be reached in less than 2500 hours at a temperature of 1800 C.

Phillips, W. M.↗
NASA NTRS1976

Carbides in lunar soils and rocks

Carbides and the surrounding metal of eight particles from Apollo 14, 16, and 17 soils were analyzed for C, Fe, Ni, and Co with the electron microprobe. In addition, carbide-containing metal particles in two Apollo 17 rocks were studied. Carbide-metal assemblages were found in anorthositic hornfels, melt rock, and agglutinate, but not in any primary igneous rocks. Carbon measurements show that cohenite is the carbide phase present in all the samples. Cohenite exsolves from the carbon-rich metal phase during cooling of the lunar metal particle in the host rocks or soil fragments. Measurements of two-phase interface compositions indicate equilibration for carbides and metal during cooling below 600 C and possibly below 500 C.

Goldstein, J. I.↗
NASA NTRS1980

Anisotropic Tribological Properties of Silicon Carbide

The anisotropic friction, deformation and fracture behavior of single crystal silicon carbide surfaces were investigated in two categories. The categories were called adhesive and abrasive wear processes, respectively. In the adhesive wear process, the adhesion, friction and wear of silicon carbide were markedly dependent on crystallographic orientation. The force to reestablish the shearing fracture of adhesive bond at the interface between silicon carbide and metal was the lowest in the preferred orientation of silicon carbide slip system. The fracturing of silicon carbide occurred near the adhesive bond to metal and it was due to primary cleavages of both prismatic (10(-1)0) and basal (0001) planes.

Miyoshi, K.↗
NASA NTRS1982

Surface chemistry and wear behavior of single-crystal silicon carbide sliding against iron at temperatures to 1500 C in vacuum

X-ray photoelectron and Auger electron spectroscopy analyses and morphological studies of wear and metal transfer were conducted with a single-crystal silicon carbide 0001 surface in contact with iron at various temperatures to 1500 C in a vacuum of 10 to the minus 8th power pascal. The results indicate that below 800 C, carbide-carbon and silicon are primarily seen on the silicon carbide surface. Above 800 C the graphite increases rapidly with increase in temperature. The outermost surficial layer, which consists mostly of graphite and little silicon at temperatures above 1200 C is about 2 nm thick. A thicker layer, which consists of a mixture of graphite, carbide, and silicon is approximately 100 nm thick. The closer the surface sliding temperature is to 800 C, the more the metal transfer produced. Above 800 C, there was a transfer of rough, discontinuous, and thin iron debris instead of smooth, continuous and thin iron film which was observed to transfer below 800 C. Two kinds of fracture pits were observed on the silicon carbide surface: (1) a pit with a spherical asperity; and (2) multiangular shaped pits.

Miyoshi, K.↗
NASA NTRS1982

MC carbide structures in M(lc2)ar-M247

The morphologies and distribution of the MC carbides in Mar-M247 ingot stock and castings were investigated using metallographic, X-ray diffraction and energy-dispersive X-ray analysis techniques. The MC carbides were found to form script structures during solidification. The script structures were composed of three distinct parts. The central cores and elongated arms of the MC carbide script structures had compositions (Ti, Cr, Hf, Ta, W)C and lattice parameters of 4.39 A. The elongated script arms terminated in enlarged, angular "heads". The heads had compositions (Ti, Hf, Ta, W)C and lattice parameters of approximately 4.50 A. The heads had a higher Hf content than the cores and arms. The size of the script structures, as well as the relative amount of head-type to core and arm-type MC carbide, was found to be determined by solidification conditions. No carryover of the MC carbides from the ingot stock to the remelted and cast material was observed.

Wawro, S. W.↗
NASA NTRS1983

Epsilon carbide - A low-temperature component of interplanetary dust particles

Transmission electron microscope study of a chondritic interplanetary dust particle has revealed the presence of epsilon iron-nickel carbide, a low-temperature carbide previously encountered only in metallurgical studies. In these studies epsilon-carbide was synthesized by carburization of iron or nickel grains in a stream of carbon monoxide or carbon monoxide plus hydrogen. Similar carburization of an iron-nickel metal in situ may have produced epsilon-carbide during particle heating on atmospheric entry or in solar orbit. Alternatively, the epsilon-carbide may be a by-product of Fischer-Tropsch reactions in the solar nebula. Such reactions have been proposed as the mechanism of hydrocarbon formation in the early solar system.

Christoffersen, R.↗
NASA NTRS1984

Carbides in iron-rich Fe-Mn-Cr-Mo-Al-Si-C systems

The optimization of high carbon iron-base superalloy properties with duplex microstructure gamma + M7C3 carbide requires analysis in the context of a seven-component system. Data are first provided here for the Fe-Mn-Cr-Mo-C quinary system, at 30 at. pct carbon. A characterization of competing carbides, according to a pseudoternary phase diagram at 35 wt pct iron, is made from isothermal sections. It is noted that while M7C3 and M3C carbides' occurrences are respectively favored at the Cr and Mn corners, the M2C carbide and molybdenum cementite are predominant with increasing amounts of Mo. Lattice parameters are reported for the various carbides.

Lemkey, F. D.↗