Showing papers on "Carbide published in 1993"

Single crystal metals encapsulated in carbon nanoparticles.

Abstract: Single-domain microcrystals of LaC2 encapsulated within nanoscale polyhedral carbon particles have been synthesized in a carbon arc. Typical particle sizes are on the order of 20 to 40 nanometers. The stoichiometry and phase of the La-containing crystals have been assigned from characteristic lattice spacings observed by high-resolution transmission electron microscopy and energy dispersive spectroscopy (EDS). EDS spectra show that La and C are the only elements present. Characteristic interatomic distances of 3.39 and 2.78 angstroms identify the compound inside the nanoparticle cavities as α-LaC2, the phase of LaC2 that is stable at room temperature. Bulk α-LaC2 is metallic and hydrolytic. Observation of crystals of pure encapsulated α-LaC2 that were exposed to air for several days before analysis indicates that the LaC2 is protected from degradation bythe carbon polyhedral shells of the nanoparticles. A high percentage of the carbon nanoparticles have encapsulated LaC2 single crystals. These carbon-coated metal crystals form a new class of materials that can be protected in their pure or carbide forms and may have interesting and useful properties.

LaC2 Encapsulated in Graphite Nano-Particle

Abstract: Arc discharging carbon rods containing 88 wt% La2O3 produces a carbonaceous deposit containing nano-particles (nm-size particles) consisting of graphite shells around core nano-crystals averaging 10-30 nm in diameter but sometimes 3-4 nm Transmission electron microscopy, nm-size electron diffraction and electron energy-loss spectroscopy have revealed that the nano-crystals are La-carbide, LaC2, that the interfaces between the carbide and the graphite are flat, and that no intermediate layers are formed

Ion implantation effects in silicon carbide

Abstract: Results from a program, which has existed for some years, on ion implantation effects in α and β silicon carbide will be summarized. Silicon carbide is easily amorphized by ion implantation at room temperature. Amorphization as determined by Rutherford backscattering spectrometry (RBS) occurs for damage energies of about 20 eV/atom, corresponding to 0.2 to 0.3 displacements per atom (dpa), at room temperature. Implantation at higher temperatures (≈ 500°C or above) does not produce an amorphous region for damage levels as high as 17 dpa. Recovery of damage at the subamorphous damage level is fairly complete by 1000°C. Epitaxial regrowth after amorphization occurs over a very narrow temperature range at ≈ 15000°C in an almost “explosive” fashion. Damage and amorphization are accompanied by swelling of up to 15%. The hardness and elastic modulus values of amorphous SiC are 40 and 70%, respectively, of the unimplanted single crystalline values, but before amorphization, the hardness first increases during the early damage phase and then decreases upon amorphization. The oxidation and chemical rates of the amorphous state are higher than for crystalline material. Amorphization kinetics, annealing kinetics and properly changes are broadly compatible with the idea of a critical accumulation model for amorphization.

Matrix cracking in unidirectional ceramic matrix composites under quasi-static and cyclic loading

Abstract: Experimental data associated with the development of matrix cracks in unidirectional continuous silicon carbide fibre/calcium alumino-silicate matrix laminates under quasi-static loading are presented, including crack density, residual strain and hysteresis behaviour as functions of applied stress. Simple models are developed, based on an assumption of purely frictional load transfer between the fibre and matrix, which describe reasonably well the quasi-static stress-strain behaviour in the presence of cracks. Under tension-tension mechanical fatigue cycling it is found that the crack density stabilises at a relatively early stage in the test. Based on the quasi-static model, the changes in fatigue hysteresis loops on fatigue cycling are interpreted in terms of a reduction in the effective frictional interfacial shear stress.

In-situ x-ray absorption spectroscopy studies of hydride and carbide formation in supported palladium catalysts

Abstract: Extended X-ray absorption fine structure (EXAFS) spectroscopy as used to characterize hydride and carbide phases in supported Pd catalysts. Transmission EXAFS measurements were made at room temperature on 5% Pd/C and 5% Pd/[gamma]-Al[sub 2]O[sub 3] catalysts. Combined EXAFS and transmission electron microscopy (TEM) results indicate that the average Pd particle diameter is 26 [+-] 8 Angstroms (or Pd dispersion is approximately 45%) in both catalysts. Supported Pd particles in air-exposed catalysts were found to be approximately 98% converted to a disordered Pd oxide; the remaining Pd is in metallic cores (average diameter is approximately 6 Angstroms) inside the oxidized Pd particles. Catalysts were reduced in situ and cooled to 25[degrees]C in H[sub 26] (partial pressure of 26 Torr), yielding a hydride phase with a lattice expansion of 2.2 [+-] 0.2%. The stoichiometry of the hydride phase, PdH[sub x] (x to [approximately] 0. 44), is consistent with previous reports of decreased H[sub 2] sorption capacity, relative to bulk Pd, in supported Pd catalysts. Decomposition of the hydride phase yielded metallic Pd particles with a first-shell Pd-Pd coordination number of 9 [+-] 1. Reaction of the reduced 5% Pd/C catalyst with 1% C[sub 2]H[sub 4]/ Ar at 150[degrees]C for 20 min generatesmore » a Pd carbide phase with an average Pd-Pd distance 1.2% larger than that in Pd metal. The PdC[sub x] phase has a maximum carbon content, x to approximately 0.06, about half that of bulk PdC[sub x[prime]] x to approximately 0.13. 44 refs., 10 figs., 6 tabs.« less

Gas sensors for high temperature operation based on metal oxide silicon carbide (MOSiC) devices

Abstract: Catalytic metal gate-silicon dioxide-silicon carbide (MOSiC) capacitors operating to about 800-degrees-C are used as high temperature gas sensor devices. Hydrogen or hydrogen containing molecules, ...

Point defects in silicon carbide

Abstract: A review is given on recent progress made in a microscopic understanding of point defects in silicon carbide (SiC). Defect structures to be discussed include shallow nitrogen donors, group-III acceptors, the transition metal impurities vanadium and titanium and radiation induced defects, like the silicon vacancy in SiC.

Microstructure and hydroabrasive wear behaviour of high velocity oxy-fuel thermally sprayed WCCo(Cr) coatings

Abstract: Sand erosion tests were performed on WC-Co and WC-CoCr coatings deposited by the high velocity oxy-fuel spraying method. Several analytical techniques, including X-ray diffraction, Auger electron spectroscopy and energy-dispersive spectroscopy in a transmission electron microscope were used to characterize the microstructures formed during powder processing and spraying. It was found that a substantial fraction of WC decomposed into W2C or reacted with the cobalt matrix to form ternary carbides such as Co3W3C and other mixed compounds. In both cases the binder phase had a nanocrystalline structure of size 4-8 nm containing tungsten, cobalt, carbon and chromium elements. The addition of chromium inhibits to a large extent the decomposition of WC and avoids the formation of metallic tungsten. In addition, chromium improved the erosion resistance by several times compared with the WC-Co coating. Scanning electron microscopy showed that the CoCr matrix binds carbides better than the cobalt matrix, thereby inhibiting carbide loss at the spray particle boundaries. The hydroabrasive wear behaviour of coatings and the mechanisms for material removal are discussed with respect to the microstructures formed during spraying.

Metal Dusting of High-Temperature Alloys

Abstract: Metal dusting, i.e. disintegration into fine metal particles and carbon, was induced on a selection of chromia forming high temperature alloys in a flowing CO-H2-H2O atmosphere in exposures at 650°C, 600°C, 500°, and 450°C. The materials were pretreated by annealing in H2 at 1000°C and electropolishing, this leads to large grain size and low surface deformation, both is disadvantageous for formation of a Cr2O3 scale. The resistance to metal dusting is only dependent on the ability to form a protective Cr2O3 scale, thus the high Cr ferritic steels proved to be very resistant, the ferritic steels with 12–13% Cr were less resistant. Due to the lower Cr diffusivity in the austenitic steels, these were very susceptible, especially two alloys with about 30% Ni (Alloy 800, AC 66). The appearance of metal dusting was somewhat different for Ni-base materials but they were also attacked under pitting. The metal dusting is preceded in all cases by internal carburization whereby the chromium is tied up, afterwards the remaining Fe or Fe-Ni matrix can react to the instable intermediate carbide M3C which decomposes to metal particles and carbon, in case of Ni-base materials a supersaturated solid solution of carbon is the intermediate.

Acoustic emission from particulate-reinforced metal matrix composites

Abstract: A systematic study of the effect of microstructural parameters on the fracture behaviour of silicon carbide particle reinforced aluminium matrix composites has been carried out. Acoustic emissions have been monitored during tensile testing, giving the size and number of emmissions as a function of strain. This has been shown to be simply related to the rate of void nucleation at the reinforcing phase. Both particle fracture and particle/matrix decohesion mechanisms can be detected. Void nucleation was observed from the onset of plastic deformation and a linear relationship between damage initiation rate and strain was found. The rate of emission increased with reiforcing particle size and volume fraction but was independent of matrix alloy composition and heat treatment. These results show that the failure strain of particulate metal matrix composites is not controlled solely by the onset of void nucleation at the reinforcing phase. Local failure processes in the matrix are shown to promote void coalescence and dominate the ductility. However, suppression of void nucleation at the particles increases the ductility. It is suggested that a critical number of fractured particles is required before failure.

Oxidation of Hafnium Carbide in the Temperature Range 1400° to 2060°C

Abstract: After hafnium carbide has been oxidized at temperatures in the range of 1400° to 2060°C, three distinct layers are present in the film cross section: (a) a residual carbide layer with dissolved oxygen in the lattice, (b) a dense-appearing oxide interlayer containing carbon, and (c) a porous outer layer of hafnium oxide. Experimental measurements of layer thicknesses and oxygen concentrations are combined with an extended formulation of moving-boundary diffusion theory to obtain the diffusion constants of oxygen in each of the three layers. The results indicate that the oxide interlayer is a better diffusion barrier for oxygen than either of the other layers. Based on X-ray microanalysis, X-ray diffraction, and resistance measurements, the interlayer is an oxygen-deficient oxide of hafnium with a carbon impurity. The interlayer hardness equals that of the residual carbide layer.

Tribological properties of SiCp-reinforced Al-4.5% Cu-1.5% Mg alloy composites

Abstract: SiC p -reinforced Al-4.5% Cu-1.5% Mg composite specimens were processed through vigorous stirring of the carbide in a semi-solid alloy slurry, remelting and casting. The specimens were examined microstructurally and their tribological properties were evaluated both in the as-cast and heat-treated conditions by pin-on-disk tribometry and automatic scratch testing. The specific wear rate and the friction coefficient between a diamond stylus and the polished specimen surface decreased, and the composite microhardness increased with increasing volume fraction carbide, and decreasing carbide particle size and spacing. A solution and ageing treatment led to an increase in wear resistance and composite microhardness, and a reduction in friction coefficient.

Microstructure Development of Oxycarbide Composites during Active‐Filler‐Controlled Polymer Pyrolysis

Abstract: The microstructure development of a ceramic composite material fabricated by active-filler-controlled polymer pyrolysis (AFCOP) was investigated. During heating of a polysiloxane precursor mixed with titanium powder in argon atmosphere up to 1400°C, thermally induced decomposition of the polymer phase is combined with simultaneous carburization of the transition metal filler. Precipitation of nanocrystalline titanium carbide at the filler particle surface starts above 400°C, and larger, faceted carbide particles have growth above 800°C. A skeleton of turbostratic carbon is formed above 800°C in the polymer-derived silicon oxycarbide matrix from which b-silicon carbide and cristobalite crystallize above 1000°C. A pronounced reduction in linear shrinkage involved in polymer–ceramic conversion is observed. The shrinkage reduction ranges from more than 25% for the filler-free precursor to less than 10% in the presence of 30 vol% of the titanium filler. Thus, active-filler-controlled pyrolysis offers the possibility of controlling shrinkage and porosity formation during polymer–ceramic conversion in order to fabricate bulk components from organometallic polymer precursor systems.

Selective encapsulation of the carbides of yttrium and titanium into carbon nanoclusters

Abstract: Characterization of the arc‐discharge deposits at the cathode from anodes containing yttrium oxide and titanium by transmission electron microscopy and x‐ray diffraction shows different results with respect to an encapsulation of the metal carbides into carbon clusters. Yttrium carbide is encapsulated into carbon nanoclusters in a crystalline phase. The formation of titanium carbide, on the other hand, preempts the formation of the carbon—carbon bonds necessary to form the carbon cages, so that only titanium carbide clusters are observed. Thermodynamic data support the interpretation of the results.

Si3N4/SiC-Platelet Composite without Sintering Aids: A Candidate for Gas Turbine Engines

Abstract: A dense and isotropic Si3N4 composite body containing 25 vol% of α-SiC platelets, with average particle size of 24μm and aspect ratio of 8 to 10, was fabricated by hot isostatic pressing without any addition of sintering aids. In this composite, desirable properties for structural ceramics to be used in long-term high-temperature applications are conveniently combined: a fracture resistance comparable with that of Si3N4 sintered with conspicuous amounts of additives, as well as a superior time-dependent strength and deformation behavior up to 1400°C, was found. The high-temperature reliability in the present composite was improved further than that of the additive-free Si3N4, mainly due to mechanisms operating in the wake of the crack. The key to the attainment of a valid synergism between nitride and carbide phase resides both in the presence of pure SiO2 glassy phase at the grain boundary and in the morphology of the reinforcement.

Binder-phase enrichment by dissolution of cubic carbides

Abstract: Sintering of a cemented carbide body containing nitrogen in a vacuum or in a nitrogen-free atmosphere results in the formation of a cubic-carbide-free gradient zone adjacent to the insert surface. The mechanism for the formation of the gradient zone is, despite its having been thoroughly investigated by others, not fully understood. In the present paper, a kinetic model based on thermodynamic calculations and some kinetic considerations will be presented. According to this model, gradient formation is the result of nitrogen diffusion out of the insert and the coupled diffusion of cubic-carbide-forming elements in the opposite direction, from the surface region into the interior of the insert. Based on the model, a comprehensible explanation of the influence of sharp edges on the gradient growth is given. The gradient is usually thinner close to sharp edges. The present model has been successfully used to rationalize experimental data and to explain predict the influence of variations in composition, sintering time, temperature, and atmosphere.

Solidification and solid-state transformation mechanisms in Si alloyed high-chromium white cast irons

Abstract: Chromium white cast irons are widely used in environments where severe abrasion resistance is a dominant requirement. To improve the wear resistance of these commercially important irons, the United States Bureau of Mines and CSIRO Australia are studying their solidification and solid-state transformation kinetics. A ternary Fe-Cr-C iron with 17.8 wt pct (pct) Cr and 3.0 pct C was compared with commercially available irons of similar Cr and C contents with Si contents between 1.6 and 2.2 pct. The irons were solidified and cooled at rates of 0.03 and 0.17 K · s-1 to 873 K. Differential thermal analysis (DTA) showed that Si depresses the eutectic reaction temperature and suggests that is has no effect upon the volume of eutectic carbides formed during solidification. Microprobe analysis revealed that austenite dendrites within the Si alloyed irons cooled at 0.03 and 0.17 K·s-1 had C and Cr contents that were lower than those of dendrites within the ternary alloy cooled at the same cooling rate and a Si alloyed iron that was water quenched from the eutectic temperature. These lower values were shown by image analysis to be the result of both solid-state growth (coarsening) of the eutectic carbides and some secondary carbide formation. Hardness measurements in the as-cast condition and after soaking in liquid nitrogen suggest an increase in the martensite start temperature as the Si content was increased. It is concluded that Si’s effect on increasing the size and volume fraction of eutectic carbides and increasing the matrix hardness should lead to improved wear resistance over regular high-chromium white cast irons.

High resistivity silicon carbide substrates for high power microwave devices

Abstract: A substrate for use in semiconductor devices, fabricated of silicon carbide and having a resistivity of greater than 1500 Ohm-cm. The substrate being characterized as having deep level impurities incorporated therein, wherein the deep level elemental impurity comprises one of a selected heavy metal, hydrogen, chlorine and fluorine. The selected heavy metal being a metal found in periodic groups IIIB, IVB, VB, VIB, VIIB, VIIIB, IB and IIB.

Mechanism and kinetics of the chemical interaction between liquid aluminium and silicon-carbide single crystals

Abstract: Previous investigations of phase equilibria in the ternary system Al-C-Si have shown that silicon carbide is attacked by pure aluminium at temperatures higher or equal to 923±3 K and up to about 1600 K, according to the chemical reaction: 4Al+3SiC ↔ Al4C3+3Si In the present work, a study has been carried out to obtain more detailed information on the mechanism and kinetics of this reaction. For that purpose, 6H silicon carbide platelets with broad Si (0 0 0 1) and C (0 0 0 ¯1) faces were isothermally heated at 1000 K in a large excess of liquid aluminium. Characterization of the resulting samples by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) revealed that the reaction proceeds in both faces via a dissolution-precipitation mechanism. However, the polarity of the substrate surface strikingly influences the rate at which silicon carbide decomposes: dissolution starts much more rapidly on the Si face than on the C face, but, while a barrier layer of aluminium carbide is formed on the Si face protecting it against further attack, the major part of the C face remains directly exposed to liquid aluminium and thus may continue to dissolve at a low but constant rate up to complete decomposition of the α-SiC crystal.

The wettability of silicon carbide by liquid aluminium : the effect of free silicon in the carbide and of magnesium, silicon and copper alloy additions to the aluminium

Abstract: Results from the sessile-drop method are reported for the effects on wetting angle, θ, of free silicon in the silicon carbide substrate and of alloy additions of silicon, copper or magnesium to the aluminium drop for the temperature range 700–960 or 1040 °C in a titanium-gettered vacuum (10−4/10−5 torr; 1 torr=133.322 Pa). Wetting angle, θ, was reduced by a factor as large as 2.8 for pure aluminium on reaction-bonded, compared with sintered silicon carbide, attributable to partial dissolution by the aluminium of the 18 wt% free silicon present in the reaction-bonded material. For wetting of reaction-bonded silicon carbide, the addition of 5 wt% silicon, copper or magnesium to the aluminium gave contact angles that decreased in the sequence Si→Cu→Mg, with the magnesium addition being the only one to result in wetting (i.e. θ<90 °) for all conditions studied. These results may have implications for design of conditions for joining or promotion of infiltration of silicon carbide parts, preforms or arrays with aluminium alloy melts.

Cemented carbide with binder phase enriched surface zone

Abstract: The present invention relates to a cemented carbide insert with improved toughness and resistance against plastic deformation containing WC and cubic phases of carbide and/or carbonitride in a binder phase based on Co and/or Ni with a binder phase enriched surface zone. The binder phase content in the insert is 3.5-12 weight-%. In a zone below the binder phase enriched surface zone the binder phase content is 0.85-1 of the content in the inner portion of the insert and the content of cubic phases essentially constant and equal to the content in the inner portion of the insert.

Thermal conductivities of thin, sputtered optical films

Abstract: The normal component of thin-film thermal conductivity has been measured for the first time, to the best of our knowledge, for several advanced sputtered optical materials. Included are data for single layers of boron nitride, silicon aluminum nitride, silicon aluminum oxynitride, silicon carbide, and for dielectric-enhanced metal reflectors of the form Al(SiO2/Si3N4)n and Al(Al2O3/AlN)n. Sputtered films of more conventional materials such as SiO2, Al2O3, Ta2O5, Ti, and Si have also been measured. The data show that thin-film thermal conductivities are typically 10 to 100 times lower than conductivities for the same materials in bulk form. Structural disorder in the amorphous or fine-grained films appears to account for most of the conductivity difference. Conclusive evidence for a film–substrate interface contribution is presented.

Phase Reactions and Chemical Stability of Ceramic Carbide and Solid Lubricant Particulate Additions within Sintered High Speed Steel Matrix

Abstract: Efforts to improve the wear properties of sintered high speed steels have made use of the simultaneous addition of hard ceramic particles (TiC or NbC) alongside particles which might act as a solid lubricant (MnS or CaF 2 ). Preliminary investigations carried out to study interactions between such particles and a sintered high speed steel matrix indicated that the steel matrix carbides were modified by a chemical reaction which occurred between the added ceramic particles and the high speed steel matrix the result of which was to give a substantial improvement in hardness. The solid lubricant remained chemically unaltered by the sintering process, and tended to reduce hardness. Both types of particulate addition raised the sintering temperature needed to achieve full density due to their effect on solidus and liquidus temperatures

Magnetic metal or metal carbide nanoparticles and a process for forming same

Abstract: A magnetic metal or metal carbide nanoparticle is provided having a carbon coating. The nanoparticle has a diameter in the range of approximately 5 to 60 nm, and may be crystalline or amorphous. The magnetic metal or metal carbide nanoparticle is formed by preparing graphite rods which are packed with a magnetic metal oxide. The packed graphite rods are subjected to a carbon arc discharge to produce soot containing magnetic metal or metal carbide nanoparticles and non-magnetic species. The soot is subsequently subjected to a magnetic field gradient to separate the magnetic metal or metal carbide nanoparticles from the non-magnetic species. Ferromagnetic or paramagnetic compounds are made by starting with graphite rods packed with the oxides of iron, cobalt, nickel and manganese bismuth, or a rare earth element excluding lanthanum, lutetium and promethium, or a paramagnetic transition metal.