Showing papers on "Carbide published in 1998"

State of the art in hard coatings for carbide cutting tools

Abstract: The majority of carbide cutting tools in use today employ hard coatings because coatings offer proven benefits in terms of tool life and machining performance. Continuing development of the chemical vapor deposition (CVD) coating process, the most widely used technique, has produced complex multilayer coatings tailored for specific applications and workpiece materials. These coatings include alumina layers of different crystal structures, and TiCN layers applied by high- or moderate-temperature (MT-CVD) processes. Over the last decade, coatings applied by physical vapor deposition (PVD) have gained acceptance in applications requiring sharp edges or those featuring interrupted cuts. Originally limited to TiN coatings, the PVD offering now includes TiCN and TiA1N coatings which provide better high-speed performance and increased abrasive wear resistance. In the area of superhard coatings, improvements in deposition processes and coating adhesion have resulted in diamond-coated carbide tools that have begun to play an important role in machining non-ferrous and non-metallic materials. This paper presents the state of the art in hard coatings for carbide cutting tools including discussion of coating characteristics and applications.

Effects of alloying elements on microstructure and fracture properties of cast high speed steel rolls. Part I : Microstructural analysis

Abstract: A study was made of the effects of alloying elements on microstructural factors of six high speed steel (HSS) rolls manufactured by centrifugal casting method. Particular emphasis was placed on the role of hard carbides located along solidification cell boundary and the type of the martensite matrix. Microstructural observation, X-ray diffraction analysis, and hardness measurement were conducted on the rolls to identify carbides. Various types of carbides were formed depending on the contents of strong carbide forming elements. In the rolls containing the high Cr content, MC carbides inside cells and M 7 C 3 carbides along cell boundaries were primarily formed, while in the rolls containing the high W and Mo contents, MC carbides inside the cells and fibrous M 2 C carbides in the intercellular regions were dominantly formed. The most important microstructural factor affecting overall roll hardness was the intercellular carbides and their distribution. The effects of alloying elements were analyzed on the basis of the liquidus surface diagram, suggesting that the proper contents of carbon, tungsten, molybdenum, chromium, and vanadium were 1.9–2.0, 3–4, 3–4, 5–7, and 5–6%, respectively.

Atomic Force Microscopy Study of an Ideally Hard Contact: The Diamond(111)/Tungsten Carbide Interface

Abstract: A comprehensive nanotribological study of a hydrogen-terminated diamond(111)/tungsten carbide interface has been performed using ultrahigh vacuum atomic force microscopy. Both contact conductance, which is proportional to contact area, and friction have been measured as a function of applied load. We demonstrate for the first time that the load dependence of the contact area in UHV for this extremely hard single asperity contact is described by the Derjaguin-M{umlt u}ller-Toporov continuum mechanics model. Furthermore, the frictional force is found to be directly proportional to the contact area. {copyright} {ital 1998} {ital The American Physical Society}

Synthesis of nanocrystalline silicon carbide powder by carbothermal reduction

Abstract: A solution of sugar in silica sol was used as parent material for the manufacture of nanocrystalline SiC powders. After mixing and freeze drying of the components the conversion was performed in two steps. First the sugar was converted to carbon particles and secondly the silica carbon mixture was heated up to the approved reaction temperatures between 1550 and 1800 °C under an argon pressure of 180 kPa. When the synthesis temperature was reached the pressure was reduced to 0.02 kPa what leads to a vigorous reaction resulting in extremely fine particles. The obtained powders were characterized with regard to the particle and crystallite size, the BET surface and their chemical composition. The oxygen content was < 0.5 wt% and the particles were < 0.5 μm with crystallites < 100 nm and specific surface areas in the range of 20–30 m2g−1. Conclusions concerning the synthesis parameters to the resulting powder properties are given in this paper.

A Tough, Thermally Conductive Silicon Carbide Composite with High Strength up to 1600°C in Air

Abstract: A sintered silicon carbide fiber-bonded ceramic, which consists of a highly ordered, close-packed structure of very fine hexagonal columnar fibers with a thin interfacial carbon layer between fibers, was synthesized by hot-pressing plied sheets of an amorphous silicon-aluminum-carbon-oxygen fiber prepared from an organosilicon polymer. The interior of the fiber element was composed of sintered beta-silicon carbide crystal without an obvious second phase at the grain boundary and triple points. This material showed high strength (over 600 megapascals in longitudinal direction), fibrous fracture behavior, excellent high-temperature properties (up to 1600 degreesC in air), and high thermal conductivity (even at temperatures over 1000 degreesC).

Photoemission and x-ray-absorption study of boron carbide and its surface thermal stability

Abstract: We present a photoemission and x-ray-absorption study on boron carbide which identifies the spectroscopic features of this material, discussing them in connection with current structural models. A study on the surface thermal stability is also performed, revealing that despite the high melting point of boron carbide $(\ensuremath{\sim}2600\mathrm{K}),$ significant surface modifications occur in the material at about 1800 K. At this temperature the surface becomes covered with a graphite layer formed from bulk segregation of carbon, leaving boron-rich boron carbide buried beneath the graphitic surface.

Infrared and x-ray photoelectron spectroscopy studies of as-prepared and furnace-annealed radio-frequency sputtered amorphous silicon carbide films

Abstract: The effects of annealing on the structural properties of radio-frequency sputtered amorphous silicon carbide films prepared under different hydrogen partial pressures (PH) were investigated. Infrared (IR) results of the as-prepared films suggest that as PH increases, more hydrogen is incorporated into the film to form the Si–H and C–H bonds and less silicon and carbon atoms are available to form the Si–C bonds. X-ray photoelectron spectroscopy (XPS) results of the as-prepared films agree with the IR results in that the percent of Si–C decreases and the percent of Si–H and C–H increases as PH increases. IR and XPS results of the annealed films suggest that as the annealing temperature increases, the dangling Si and C bonds will combine to form the Si–C bonds for the unhydrogenated samples. The increase in Si–C bonds for the hydrogenated samples is more likely to be due to the formation of Si–C bonds from the breaking up of the Si–H and C–H bonds.

Diffusion kinetics of the carburization and silicidation of Ti3SiC2

Abstract: The ternary carbide Ti3SiC2 possesses a unique set of properties that could render it a material of considerable technological impact The motivation for this work was to enhance the hardness and oxidation resistance of Ti3SiC2 by altering its surface chemistry Reaction of Ti3SiC2 with single crystal Si wafers in the 1200–1350 °C temperature range resulted in the formation of a dense surface layer composed of a two phase mixture of TiSi2 and SiC This layer grows in two distinct morphologies; an outer layer with fine (1–5 μm) SiC particles and an inner coarser (10–15 μm) one The overall growth rates of the layers were parabolic Comparison with previously published results supports the conclusion that diffusion of Si through TiSi2 is rate limiting In the 1400–1600 °C temperature range, reaction of Ti3SiC2 with graphite foils resulted in the formation of a 15 vol % porous surface layer of TiCx (where x>08) It is shown that the carburization kinetics are rate limited by the diffusion of C through TiCx Both carburization and silicidation increased surface hardness, the latter also enhanced the oxidation resistance by about three orders of magnitude

Micromechanisms of metal dusting on Fe‐base and Ni‐base alloys

Abstract: Phenomena of metal dusting on iron and nickel (and their alloys) were studied by characterizing both microstructure and nanochemistry of the reaction zones using TEM and AEM techniques. While in case of iron a metastable carbide (cementite) is formed nickel directly disintegrates. In the chromium-rich steel HK40 and the chromium-rich Ni-base alloy Inconel 600 some microstructural chromium dependent features were observed with protective effects against metal dusting. Independent of these differences, in all groups of materials a fundamental common starting mechanism on the atomic scale could be deduced, which first of all comprises the arrangement of basal graphite lattice planes perpendicularly oriented to the carbide (or metal) surface acting as active sites in the disintegration process.

Carbon nanotubes–Fe–alumina nanocomposites. Part I: influence of the Fe content on the synthesis of powders

Abstract: Oxides based on a-alumina and containing various amounts of Fe (2, 5, 10, 15 and 20 cat.%) were prepared by decomposition and calcination of the corresponding mixed-oxalates. Selective reduction of the oxides in a H2-CH4 atmosphere produces nanometric Fe particles which are active for the in-situ nucleation and growth of carbon nanotubes. These form bundles smaller than 100 nm in diameter and several tens of micrometers long. However, the carbon nanotubes-Fe-Al2O3 nanocomposite powders may also contain Fe carbide nanoparticles as well as undesirable thick, short carbon tubes and thick graphene layers covering the Fe/Fe carbide nanoparticles. The influence of the Fe content and the reduction temperature on the composition and micro/nanostructure of the nanocomposite powders have been investigated with the aim of improving both the quantity of nanotubes and the quality of carbon, i. e. a smaller average tube diameter and/or more carbon in tubular form. A higher quantity of carbon nanotubes is obtained using a-Al1.8Fe0.2O3 as starting compound, i. e. the maximum Fe concentration (10 cat.%) allowing to retain the monophase solid solution. A further increase in Fe content provokes a phase partitioning and the formation of a Fe2O3-rich phase which upon reduction produces too large Fe particles. The best carbon quality is obtained with only 5 cat.% Fe (a-Al1.9Fe0.1O3), probably because the surface Fe nanoparticles formed upon reduction are a bit smaller than those formed from a-Al1.8Fe0.2O3, thereby allowing the formation of carbon nanotubes of a smaller diameter. For a given Fe content (≤ 10 cat.%), increasing the reduction temperature favours the quantity of nanotubes because of a higher CH4 sursaturation level in the gas atmosphere, but also provokes a decrease in carbon quality.

The shape of WC crystals in cemented carbides

Abstract: A study has been made of the shape development of WC crystals in cemented carbides during liquid phase sintering. The relative influence of shape relaxation and carbide crystal growth processes on the shape is discussed. An effect of titanium additions on the shape of WC crystals in WC–Ni cemented carbides was observed. A shape parameter referred to as shape equiaxiality has been defined and calculated theoretically for various triangular prisms. The observed change of shape equiaxiality with titanium concentration in the binder phase of WC–Ni cemented carbides is discussed in terms of titanium segregation at the interphase boundary.

The thermal conductivity of metallic ceramics

Abstract: Transition metal carbides, nitrides, and borides can be called metallic ceramics because they are electronically conductive and extremely hard. Their various applications include cutting and grinding tools, thermal-barrier coatings, diffusion-resistant thin films, interconnects, and superconductivity devices. In each case, the ability of the material to resist or permit heat flow is important. Because of the high concentration of nonmetal atom vacancies in the carbides and nitrides, the carriers of heat—conduction electrons and phonons (the quanta of lattice waves)—are severely scattered, and the thermal conductivity, K, is strongly affected, although differently in high- and low-temperature regions. Measurements of both the electrical and thermal conductivity of single-crystal metallic ceramics at low temperatures and the application of the Callaway formalism help explain the puzzling temperature dependence of K. The finding of a large peak in K of NbC just below its superconducting transition temperature confirms phonon-electron scattering and could lead to a thermal switch. The single-crystal thermal conductivity behavior of TiC and WC is used to interpret the measured K values for cemented carbides TiC/Ni-Mo and WC/Co through a broad temperature range.

Flash memory with microcrystalline silicon carbide film floating gate

Abstract: A memory is described which has memory cells that store data using hot electron injection. The data is erased through electron tunneling. The memory cells are described as floating gate transistors wherein the floating gate is fabricated using a conductive layer of microcrystalline silicon carbide particles. The microcrystalline silicon carbide particles are in contact such that a charge stored on the floating gate is shared between the particles. The floating gate has a reduced electron affinity to allow for data erase operations using lower voltages.

MC carbide formation in directionally solidified MAR-M247 LC superalloy

Abstract: MC-type carbide formation in MAR-M247 LC superalloy was systematically investigated using directional solidification and quenching methods in sample growth rates between 0.8×10−6 and 15×10−6 m s−1. The results indicate that carbide growth rate, carbide forming element enrichment and surrounding solid geometry determine carbide morphology and carbide composition. The carbide forming element enrichment and interface energy control carbide nucleation. Heterogeneous carbide nucleation can occur above the alloy liquidus temperature. Carbide forming element enrichment and trapping behavior of the solid–liquid interface control carbide growth, which occurs at inter-secondary γ-dendrite arm positions and the mushy zone bottom which are rich in carbide forming elements. At fast sample growth rates, the solid–liquid interface can trap carbide nuclei in front of it. This trapping tends to occur at the inter-secondary γ-dendrite arm region.

Black mask, color filter and liquid crystal display

Abstract: A black mask comprises: a first and a second antireflection films each comprising oxide, nitride, carbide, oxide nitride, oxide carbide, nitride carbide, or oxide nitride carbide of at least one kind of metal selected from the group consisting of chromium, molybdenum, tungsten, nickel, and germanium, and said films each having a different composition; and a screening film comprising at least one kind of metal selected from the group consisting of chromium, molybdenum, tungsten, nickel, and germanium; wherein said three films are successively formed on a transparent substrate, and at least one but not all of said three films contains chromium. Having improved etching characteristics at the manufacturing process, low reflectivity in the whole visible wavelength range, and a small wavelength dependence of reflectivity, the black mask can be preferably used for a color filter and a liquid crystal display which can show clear pictures.

Physiso‐Chemical and Thermophysical Properties of Cubic Binary Carbides

Abstract: A summary of selected thermophysical properties of refractory metallic carbides is reported with a critical evaluation of the data. A target of this work was to collect a wide and selected set of data spanning from physical and physico-chemical parameters to more specialistic physico-mechanical items of informations, all useful for technological applications. A second target was to obtain a comprehensive view of the matter concerning cubic carbides. On the collected data the authors have carried out a number of elaborations, correlation analyses, search for trends and graphic presentations in order to assess the validity of numerical figures inside a more general context and to extract some physical and/or physico-chemical meaning also through the aid of proposed equations.

Solidification microstructure and M2C carbide decomposition in a spray-formed high-speed steel

Abstract: The solidified carbide morphology, the decomposition behavior of the M2C carbide, and the carbide distribution after forging of an Fe-1.28C-6.4W-5.0Mo-3.1V-4.1Cr-7.9Co (wt pct) high-speed steel prepared by spray forming have been investigated. The spray-formed microstructure has been characterized as a discontinuous network of plate-shaped M2C carbides and a uniform distribution of fine, spherical MC carbides. The metastable M2C carbides formed during solidification have been fully decomposed into MC and M6C carbides after sufficient annealing at high temperatures. Initially, the M6C carbides nucleate at M2C/austenite interfaces and proceed to grow. In the second stage, the MC carbides form either inside the M6C carbides or at the interfaces between M6C carbides. With this increasing degree of decomposition of the M2C carbide, the carbides become more uniformly distributed through hot forging, which produces a significant increase in ultimate bend strength. The decomposition treatment of M2C carbide has been found to be most important for obtaining a fine homogeneous carbide distribution after hot forging.

Factors affecting chromium carbide precipitate dissolution during alloy oxidation

Abstract: Ferrous alloys containing significant volumefractions of chromium carbides were formulated so as tocontain an overall chromium level of 15% (by weight) buta nominal metal matrix chromium concentration of only 11%. Their oxidation at 850°C inpure oxygen led to either protectiveCr2O3 scale formation accompaniedby subsurface carbide dissolution or rapid growth ofiron-rich oxide scales associated with rapid alloy surface recession, which engulfedthe carbides before they could dissolve. Carbide sizewas important in austenitic alloys: an as-castFe-15Cr-0.5C alloy contained relatively coarse carbides and failed to form aCr2O3 scale, whereas the samealloy when hot-forged to produce very fine carbidesoxidized protectively. In ferritic alloys, however, evencoarse carbides dissolved sufficiently rapidly to provide the chromium flux necessary to formand maintain the growth of a Cr2O3scale, a result attributed to the high diffusivity ofthe ferrite phase. Small additions of silicon to theas-cast Fe-15Cr-0.5C alloy rendered it ferritic and led toprotective Cr2O3 growth. However,when the silicon-containing alloy was made austenitic(by the addition of nickel), it still formed aprotective Cr2O3 scale, showing that the principal function of silicon was inmodifying the scale-alloy interface.

FeAl-TiC and FeAl-WC composites-melt infiltration processing, microstructure and mechanical properties

Abstract: TiC-based and WC-based cermets were processed with iron aluminide, an intermetallic, as a binder by pressureless melt infiltration to near full density (>97% TD). Phase equilibria calculations in the quaternary Fe–Al–Ti–C and Fe–Al–W–C systems at 1450°C were performed to determine the solubility of the carbide phases in liquid iron aluminide. This was done by using Thermocalc™ and the results show that molten Fe–40 at.% Al in equilibrium with Ti 0.512 C 0.488 and graphite, dissolves 4.9 at.% carbon and 64 at. ppm titanium. In the Fe–Al–W–C system, liquid Fe–40 at.% Al in equilibrium with graphite dissolves ≈5 at.% carbon and 1 at.% tungsten. Due to the low values for the solubility of the carbide phases in liquid iron aluminide, liquid phase sintering of mixed powders does not yield a dense, homogenous microstructure for carbide volume fractions greater than 0.70. Melt infiltration of molten FeAl into TiC and WC preforms serves as a successful approach to process cermets with carbide contents ranging from 70 to 90 vol.%, to greater than 97% TD. Also, the microstructures of cermets prepared by melt infiltration were very homogenous. Typical properties such as hardness, bend strength and fracture toughness are reported. SEM observations of fracture surfaces suggest the improved fracture toughness to result from the ductility of the intermetallic phase. Preliminary experiments for the evaluation of the oxidation resistance of iron aluminide bonded cermets indicate that they are more resistant than WC–Co cermets.

Control of M23C6 carbides in 0.45C–13Cr martensitic stainless steel by means of three representative heat treatment parameters

Abstract: Heat treatment parameters closely control the amount of carbides in steels. In this study, the influence of three parameters (heating temperature, heating rate and cooling rate) on the area percentage of M23C6-type carbides in the quenching microstructures of 0.45C–13Cr martensitic stainless steel has been studied. The experimental results obtained in this study demonstrate that the amount of carbides in the quenching microstructures of this steel can be severely modified through the heating and cooling conditions of the heat treatment.

Microanalysis of two creep resistant 9–12% chromium steels

Abstract: The ferritic chromium steels P122 and P92 have been investigated using APFIM and TEM. The two steels are similar in composition with the exception of an addition of copper to P122, allowing a higher chromium content. The investigated materials were tempered at 770°C and isothermally aged at 600°C for times ranging from 0 to 10 000 h. The matrix and precipitates of type M 23 C 6 , MX and Laves phase have been analysed with APFIM. Steel P122 contains 0.9% copper and it was found that the matrix concentration of copper drops during ageing from 0.4% to an equilibrium level at 0.1%, which is in good agreement with previous thermodynamical calculations. No copper was found in M 23 C 6 , MX or Laves phase. Copper instead forms a separate phase. This phase was identified by TEM. During ageing the amount of tungsten in the matrix drops due to formation of Laves phase. This process is faster in steel P122 compared with steel P92, indicating an accelerating effect of copper on the nucleation of Laves phase. In both steels, enhanced concentrations of boron were found inside M 23 C 6 carbides. Phosphorous was found to segregate to a very narrow region at the carbide/matrix interface.

Characterization of β‐Silicon Carbide Powders Synthesized by the Carbothermal Reduction of Silicon Carbide Precursors

Abstract: Boron-doped and nondoped ultrafine β-silicon carbide (β-SiC) powders were synthesized via the carbothermal reduction of SiC precursors at temperatures of 1773–1973 K. Although the reaction rate of carbothermal reduction was generally higher when a boron-doped precursor was used, the reaction rate for the boron-doped precursor was reduced considerably at 1873 K. For boron-doped and nondoped precursors, the reaction rates were almost the same. Powder characterization via transmission electron microscopy indicated that the suppression of the reaction rate for boron-doped precursor at 1873 K was due to the formation of a special coexistent system with two types of particle agglomerates. As expected, boron doping inhibited the particle growth in the synthesis of SiC powder.