Showing papers on "Carbide published in 1991"

Vapor-phase fabrication and properties of continuous-filament ceramic composites.

Abstract: The continuous-filament ceramic composite is becoming recognized as necessary for new, high-temperature structural applications. Yet because of the susceptibility of the filaments to damage from traditional methods for the preparation of ceramics, vapor-phase infiltration has become the fabrication method of choice. The chemical vapor infiltration methods for producing these composites are now being studied in earnest, with the complexity of filament weaves and deposition chemistry being merged with standard heat and mass-transport relationships. Two of the most influential effects on the mechanical properties of these materials are the adhesion and frictional force between the fibers and the matrix, which can be controlled by a tailored interface coating. A variety of materials are available for producing these composites including carbide, nitride, boride, and oxide filaments and matrices. Silicon carbide-based materials are by far the most advanced and are already being used in aerospace applications.

A Model of Silicon Carbide Chemical Vapor Deposition

Abstract: This paper presents a model describing the interacting gas phase and surface chemistry present during the steady-state chemical vapor deposition (CVD) of silicon carbide (SiC). In this work, the authors treat the case of steady-state deposition of SiC from silane (SiH{sub 4}) and propane (C{sub 3}H{sub 8}) mixtures in hydrogen carrier gas at one atmosphere pressure. Epitaxial deposition is assumed to occur on a pre-existing epitaxial silicon carbide crystal. Pyrolysis of SiH{sub 4} and C{sub 3}H{sub 8} is modeled by 83 elementary gas-phase reactions. A set of 36 reactions of gas- phase species with the surface is used to simulate the deposition process. Rates for the gas/surface reactions were obtained from experimental measurements of sticking coefficients in the literature and theoretical estimates. The authors' results represent the first simulation of a silicon carbide deposition process that includes detailed description of both the gas phase and surface reactions. The chemical reaction mechanism is also combined with a model of a rotating disk reactor (RDR), which is a convenient way to study the interaction of chemical reactions with fluid mechanics. Transport of species from the gas to the surface is accounted for using multicomponent transport properties. Predictions of deposition rates as amore » function of susceptor temperature, disk rotation rate, and reactant partial pressure are presented. In addition, velocity, temperature, and concentration profiles normal to the heated disk for 41 gas-phase species are determined using reactor conditions typical of epitaxial silicon carbide deposition on silicon substrates.« less

Microstructural evolution of modified 9Cr-1Mo steel

Abstract: The tempering and subsequent annealing of modified 9Cr-lMo steel have been investigated to determine the influence of trace amounts of V and Nb on the sequence of precipitation processes and to identify the basis for the enhanced high-temperature strength compared to the standard 9Cr-lMo composition. Air cooling (normalizing) from 1045 °C results in the precipitation of fine (Fe, Cr)3C particles within the martensite laths. Additional carbide precipitation and changes in the dislocation structure occur during the tempering of martensite at 700 °C and 760 °C after normalizing. The precipitation of M23C6 carbides occurs preferentially at lath interfaces and dislocations. The formation of Cr2C was detected during the first hour of tempering over the range of 650 °C to 760 °C but was replaced by V4C3 within 1 hour at 760 °C. During prolonged annealing at 550 °C to 650 °C, following tempering, the lath morphology remains relatively stable; partitioning of the laths into subgrains and some carbide coarsening are evident after 400 hours of annealing at 650 °C, but the lath morphology persists. The enhanced martensite lath stability is attributed primarily to the V4C3 precipitates distributed along the lath interfaces and is suggested as the basis for the improved performance of the modified 9Cr-lMo alloy under elevated temperature tensile and creep conditions.

Fischer-Tropsch synthesis over iron catalysts

Abstract: Because of the decreased profitability of making synthetic fuels, Sasol intends expanding its production of the higher valued chemicals, in particular waxes and olefins. The advantages and disadvantages of using Fe, Co and Ru catalysts are discussed from the point of view of costs, availability, product selectivity, activity and sensitivity to poisons. The loss of activity and selectivity of iron based catalysts in both fixed and fluidized bed reactors is discussed. The main contributing factors are sulfur poisoning, oxidation and coke fouling. In fixed bed reactors sulfur poisoning and “coke” laydown deactivates the front end of the bed while hydrothermal sintering/oxidation deactivates the back end. In fluidized beds the deposition of large amounts of Boudouard carbon doesnot markedly lower the activity. The smaller catalyst particles end up consisting of small iron carbide entities embedded in a matrix of carbon. The larger catalyst particles consist of cores of inert magnetite surrounded by the carbide/carbon matrix. FT reactor development at Sasol is briefly reviewed.

Room-Temperature Mechanosynthesis of Carbides by Grinding of Elemental Powders

Abstract: The direct room-temperature synthesis (mechanochemical synthesis or mechanosynthesis—MS) of most metal carbides by milling metal—carbon powders (size some tens of micrometers) mixtures is described. The particle size of the carbides obtained is of the order of 20 nm. Stable, metastable, mixed, and new carbides can be formed. Moreover, metal—carbon alloys can be obtained, such as in the Fe—C system. The synthesized compounds have been characterized by X-ray diffraction and Mossbauer spectroscopy for iron-containing systems. Carbides of the following elements were obtained: Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Hf, Ta, W, Re, Al and Si.

Reaction-formed silicon carbide

Abstract: The reaction bonding of silicon carbide (SiC) typifies liquid-solid reaction processes for the synthesis of refractory ceramic composites. These processes have particular advantages over conventional sintering and hot-pressing techniques in their lower processing temperatures, shorter times and near-net shape fabrication capabilities. Two particular modifications that we have employed in order to improve the mechanical properties and the ultimate use temperature of reaction-bonded SiC are the use of microporous carbon pre-forms derived from polyfurfural alcohol for refinement of microstructure, and the use of alloyed melts in order to replace detrimental residual silicon with a refractory silicide. The control of reaction rate is always a key issue in reaction processing. We have studied the kinetics and mechanisms of the liquid SiC reaction. Experiments on carbon fibers and plates show that the principle mechanism is one of solution-reprecipitation. There is an increased solubility of carbon at very fine SiC particles formed by the spallation of the misfitting carbide from the carbon interface, leading to reprecipitation of SiC at defective seed crystals. Molybdenum and boron at low concentrations (3.2 mol.%) have little effect on reaction kinetics, whereas aluminum is able to impede the reaction through the formation of an interfacial carbide layer.

Metastable phases in the Al3X (X = Ti, Zr, and Hf) intermetallic system

Abstract: The paper concentrates on mechanical alloying (MA), a high-energy ball-milling technique, used for preparing powders of Al3Ti, Al3Zr, and Al3Hf. The synthesis of these alloys by MA is reported, and the formation of metastable phases, the thermal stability of these phases, and their lattice parameters are discussed. It is noted that the closeness of the lattice and structure parameters between Al and the Al3X intermetallic phases offers the possibility of coherent precipitates in the Al matrix, while the use of hexane during MA leads to the formation of fine uniformly dispersed carbide precipitates in the matrix. It is pointed out that this, in combination with the fine grain size of the matrix and intermetallic precipitates should lead to cumulative improvements in the properties of the alloys. 23 refs.

Low temperature aging behavior of type 308 stainless steel weld metal

Abstract: The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M23C6 carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness.

Ceramic bonding method

Abstract: A method of coating and bonding a substrate with particles of a ceramic selected from the group consisting of diamond, carbon, graphite, and graphite or carbon-carbon composite, comprising: providing the substrate and at least one of the ceramic particles; selecting at least a carbide-forming substance consisting principally of an element which is other than Ni, Cr, and Co and is capable of forming a carbide to provide a coating material; applying said coating material onto at least one component of the substrate and the at least one ceramic particle; placing the at least one ceramic particle on the substrate; and heating the product of step (D) at a temperature sufficient to form a liquid-diffusion formed, carbide coating on the at least one ceramic particle. The ceramic particles are then coated with strong, adherent, substantially defect-free, and thermomechanically shock resistant metallized layers which are capable of practical uses over 630° C.

Effects of surface pretreatments on nucleation and growth of diamond films on a variety of substrates

Abstract: The effects of surface pretreatments on nucleation and growth of chemical vapor deposited diamond films were studied. The pretreatments included scratching with diamond grit, coating with a low vapor pressure, high thermal stability hydrocarbon oil, and coating with a 100–200‐A‐thick layer of evaporated carbon. The effects of the treatments on carbide and noncarbide forming materials are described.

Energy dependence of electron damage and displacement threshold energy in 6H silicon carbide

Abstract: The frequency response of silicon carbide (SiC) light-emitting diodes has been used to measure the energy dependence of displacement damage produced in 6H SiC by energetic electrons. The minimum electron energy required to produce displacement damage was determined to be 108+or-7 keV, corresponding to an atomic displacement of silicon atoms. For electrons of energies greater than 0.5 MeV, the damage constant for lifetime degradation in SiC is lower than that for GaAs by more than three orders of magnitude, indicating a greatly superior resistance of SiC to displacement damage in most radiation environments. >

Permanent magnets by mechanical alloying (invited)

Abstract: Mechanical alloying is applied to prepare Nd‐Fe‐B, Sm‐Fe‐TM type (TM: V, Ti, Zr), and interstitial nitride and carbide permanent magnets. Starting from elemental powders, the hard magnetic phases are formed by milling in a planetary ball mill and a following solid‐state reaction at relatively low temperatures. For Nd‐Fe‐B, the magnetically isotropic particles are microcrystalline, show a high coercivity (up to 16 kA/cm for ternary alloys and above for Dy‐substituted samples), and can be either used for making bonded magnets or compacted to dense isotropic magnets by hot uniaxial pressing. Magnetically anisotropic samples with a remanence up to 1.31 T and an energy product up to 326 kJ/m3 are formed by die upsetting. The mechanical alloying process has also been applied to prepare magnetic material of three new Sm‐Fe‐TM phases: Sm‐Fe‐V with the ThMn12 structure, Sm‐Fe‐Zr with the PuNi3 structure, and Sm‐Fe‐Ti with the A2 structure. They all show high or ultrahigh coercivities (up to 51.6 kA/cm for Sm‐Fe‐Ti...

High‐Temperature Active Oxidation of Chemically Vapor‐Deposited Silicon Carbide in an Ar─O2 Atmosphere

Abstract: Active oxidation behavior of chemically vapor-deposited silicon carbide in an Ar─O2 atmosphere at 0.1 MPa was examined in the temperature range between 1840 and 1923 K. The transition from active oxidation (mass loss) to passive oxidation (mass gain) was observed at certain distinct oxygen partial pressures (PO2t). The values of PO2t increased with increasing temperature and with decreasing total gas flow rates. This behavior was well explained by Wagner's model and thermodynamic calculations. Active oxidation rates (ka) increased with increasing O2 partial pressures and total gas flow rates. The rate-controlling step of the active oxidation was concluded to be O2 diffusion through the gaseous boundary layer.

Composite PTCR thermistors, utilizing conducting borides, silicides, and carbide powders

Abstract: Ceramic-polymer composite thermistors using conducting boride, silicide, and carbide powders that include TiB2, ZrB2, NbB2, NbSi2, WSi2, MoSi2, and TiC have been fabricated. Percolation and subsequent PTCR effects were observed for all the powders in both semi-crystalline (polyethylene) and amorphous (epoxy) polymer matrix materials, however, as found for carbon black and metal fillers, both niobate powders did not exhibit a PTCR effect in the amorphous polymer. Results indicate that percolation and PTCR behaviour are related to the powder characteristics (size/distribution), composite microstructure and ceramic-polymer interface. Composite thermistors with room temperature resistivities as low as 1 Ω cm and a nine-order of magnitude change (Δϱ) at 1 kHz (12 Δϱ at d.c.) were achieved.

Physically vapor deposited coatings on tools: performance and wear phenomena

Abstract: Coatings produced by physical vapor deposition (PVD) enhance the performance of tools for a broad variety of production processes. In addition to TiN, nowadays (Ti, Al)N and Ti(C,N) coated tools are available. This gives the opportunity to compare the performance of different coatings under identical machining conditions and to evaluate causes and phenomena of wear. TiN, (Ti,Al)N and Ti(C,N) coatings on high speed steel (HSS) show different performances in milling and turning of heat treated steel. The thermal and frictional properties of the coating materials affect the structure, the thickness and the flow of the chips, the contact area on the rake face and the tool life. Model tests show the influence of internal cooling and the thermal conductivity of coated HSS inserts. TiN and (Ti,Zr)N PVD coatings on cemented carbides were examined in interrupted turning and in milling of heat treated steel. Experimental results show a significant influence of typical time-temperature cycles of PVD and chemical vapor deposition (CVD) coating processes on the physical data and on the performance of the substrates. PVD coatings increase tool life, especially towards lower cutting speeds into ranges which cannot be applied with CVD coatings. The reason for this is the superior toughness of the PVD coated carbide. The combination of tough, micrograin carbide and PVD coating even enables broaching of case hardened sliding gears at a cutting speed of 66 m min −1 .

Carbide phases on iron-based Fischer-Tropsch synthesis catalysts part I: Characterization studies

Abstract: The characterization of iron carbide phases formed on iron-based Fischer-Tropsch catalysts is reviewed in this part with particular emphasis on Mossbauer and XPS studies of selected iron and supported iron catalysts, in combination with some specific results in activity and selectivity. Mossbauer studies have shown clearly the evolution of bulk carbide species on iron catalysts under synthesis reaction conditions; the relationship of these to the nature of near-surface species and their temporal evolution under synthesis conditions is indicated by XPS results. Comparisons among reduced, unreduced and carbided iron catalysts are given; XPS and reaction results suggest that Fe3O4 is active for synthesis even in the absence of carbide phases.

High sensitivity ultraviolet radiation detector

Abstract: A high sensitivity radiation detecting photodiode formed in silicon carbide comprises a monocrystalline silicon carbide substrate; a first monocrystalline portion of silicon carbide upon the substrate and having a first conductivity type; a second monocrystalline portion of silicon carbide adjacent the first portion and having the opposite conductivity type from the first portion; and a p-n junction between the adjacent first and second portions. The photodiode provides a dark current density of no more than about 1×10-9 amps/cm2 at a reverse bias of -1.0 volts and at temperatures of 170° C. or less.

Cemented carbide body for rock drilling, mineral cutting and highway engineering

Abstract: The invention relates to a coated cemented carbide body for rock drilling having a substrate containing at least one metal carbide and a binder metal and an at least partly covering coating comprising at least one diamond- or cBN-layer applied by CVD- or PVD-technique The cemented carbide body has a core of cemented carbide containing eta-phase surrounded by a surface zone of cemented carbide free of eta-phase

Oxidation of hafnium carbide and hafnium carbide with additions of tantalum and praseodymium

Abstract: The oxidation behavior of HfC, HfC-25 wt. % TaC, and HfC-7 wt.% PrC2 has been studied between 1200–2200° C. Parabolic growth of the oxide layer has been observed for both HfC and HfC-TaC over the entire temperature range. A break in the temperature dependence of the oxidation kinetics occurs around 1600°C. At lower temperatures, the kinetics are limited by gaseous diffusion via pores in the oxide. Above 1800°C, gaseous diffusion through pores becomes less important as scale-growth kinetics are dominated by bulk (ambipolar) diffusion of oxygen and electrons through the oxide.

An experimental and theoretical study of cementite dissolution in an Fe-Cr-C alloy

Abstract: The dissolution of cementite at 910 °C in an Fe-2.06Cr-3.91C (at. pct) alloy is investigated experimentally. The Cr concentration profiles in austenite and cementite are measured by means of the scanning transmission electron microscopy/energy dispersive spectrometry (STEM/EDS) technique at different dissolution times. The measurements show the Cr enrichment in the cementite during the dissolution process. The measurements suggest that the main part of the reaction for this alloy is controlled by Cr diffusion in the cementite or in the austenite matrix. This observation is in agreement with predictions of the local equilibrium hypothesis. The carbide fraction and average particle diameter are evaluated as functions of dissolution time. The Cr enrichment of the cementite results in a supersaturation and a possible decomposition of the cementite. Microstructural evidence for such a decomposition is found by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A new program package called DICTRA,[11] which is suitable for the simulation of diffusional reactions in multicomponent alloys, has been applied to the present case. The simulation is compared with the experimental data, and a good agreement between the two is found.

Junction field-effect transistor formed in silicon carbide

Abstract: A junction field-effect transistor is disclosed that comprises a bulk single crystal silicon carbide substrate having respective first and second surfaces opposite one another, the substrate having a single polytype and having a concentration of suitable dopant atoms so as to make the substrate a first conductivity type. A first epitaxial layer of silicon carbide is formed on the first surface of the substrate, and having a concentration of suitable dopant atoms that give the first epitaxial layer the first conductivity type. A second epitaxial layer of silicon carbide is formed on the first epitaxial layer, the second epitaxial layer having a concentration of suitable dopant atoms to give the second epitaxial layer a second conductivity type opposite from the first conductivity type. A higher conductivity region of silicon carbide is formed on the second epitaxial layer, A trench is formed in the second epitaxial layer and higher conductivity region extending entirely through the higher conductivity region and partially into the second epitaxial layer toward the first surface of the substrate for defining a gate region in the second epitaxial layer between the trench and the first epitaxial layer. The trench divides the second epitaxial layer and higher conductivity region into respective first and second regions with the trench therebetween.