Structure Of WC Research Articles

Influence of the Structure and Properties OF WC – Co Alloy Powders on the Structure and Wear-Resistance of Detonation Coatings

We have made comparative investigations of the structure, phase composition and abrasive wear resistance of WC – Co coatings sprayed by the detonation method using powders of identical chemical composition and different particle structure, because the powders are prepared by different technologies. The laws we have found that govern how the structure evolves from the initial powder to a coating allow us to predict the physicotechnical properties of the coating. The more porous the powders, the more they are affected by the gas atmosphere in the spraying process and thus, the more significant changes in the phase composition are observed in the powder – coating system. The use of powders with highly porous particles results in the formation of porous coatings with low wear resistance.

Structure and properties of WC–CrAlN superlattice films by cathodic arc ion plating process

New WC–CrAlN superlattice films have been deposited on Si(100) substrate using a cathodic arc ion plating system. A substrate holder was rotated with a range from 1 to 12 rev./min to control the thickness of layered structures, and on arc power density of Al cathodes was controlled to change the Al concentration in the film. The microstructure and mechanical properties of the film depend on the superlattice period (λ) and Al concentration. The chemical composition was evaluated by glow discharge optical emission spectroscopy (GDOES). Using X-ray diffraction analysis (XRD), it is observed that the preferred orientation of microstructure is changed according to the superlattice periods (λ) and Al concentration. Through transmission electron microscope (TEM) analysis, the microstructure and superlattice period (λ) of the WC–CrAlN superlattice film were confirmed. The hardness of the deposited film was evaluated by a nanoindentation test. As a result of the nanoindentation test, the hardness of WC–CrAlN superlattice film gained approximately 40 GPa at the superlattice period (λ) of 7 nm. The structure of WC–CrAlN superlattice film was transformed from dense columnar structure to a nanocomposite structure.

Effect of metal and carbon vacancies on the electronic structure of hexagonal WC and cubic TaC

The electronic states and energy characteristics of carbon and metal vacancies in hexagonal β-WC and cubic TaC were examined by the full-potential LMTO method.

Advanced and new grades of WC and binder powder – their properties and application

This paper deals with properties and applications of advanced WC and new (Fe/Ni/Co) powders. Special attention is focused on (1) fine-grained and highly sinteractive (Fe/Ni/Co) powder and (2) ultrafine WC powder with linear WC intercepts averaging 100–130 nm in structures of WC–10 wt% Co hardmetals. The quality, individual features and properties of the new powders are shown to be a key factor in achieving excellent and improved properties and performance of hardmetals and diamond tools. A number of correlations are established between powder properties, sintering behaviour and hardmetal structures and properties. In addition to other well-known parameters, the lattice microstructure, as well as the mixed-crystal phases of the new powders, are shown to be essential for optimized sintering, structure formation and improved properties of the tool materials.

Cemented Carbides Alloyed with Ruthenium, Osmium, and Rhenium

A study was made of the structure and mechanical properties of WC ― Co cemented carbides alloyed with Ru, Re, and Os. It was shown that the alloying elements form solid solutions with the binder phase, Co(Ru, W, C), Co(Re, W, C), Co(Os, W, C), which have higher microhardnesses and elastic moduli than the solid solution Co(W, C). Ruthenium, rhenium, and osmium reduce the stacking fault energy of the cobalt phase and promote the transformation of the cubic modification of cobalt to the hexagonal. This results in precipitation hardening of the binder phase. Additions of Ru, Re, and Os increase the hardness, and bend, compression and yield strengths, of WC ― Co alloys. Examples of the successful use of WC ― Co cemented carbide tools alloyed with noble metals for the machining of heat-resistant alloys and high-alloy steels are given.

Stacking Faults Created by Mechanical Milling in Nanostructured WC-Co Composite Powder

ABSTRACTNanostructured WC-Co powders obtained by mechanical milling were investigated by combination of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques. HRTEM image analysis shows that in the as-milled nanostructured powder, many WC grains contain stacking faults lying on the plane{10.0}. Analysis of phase images showed that these defects were nearly periodically ordered along the [10.0] direction. Based on these observations, a structural model is proposed for the WC grains with ordered stacking faults, which is in fact equivalent to a superstructure of WC with space group Amm2. When this model is introduced together with the normal WC structure (space group P6m2) into the Rietveld refinement, a much better agreement between the calculated and experimental XRD profiles is obtained. This study allowed obtaining the lattice parameters, grain size, microstrain and other structural information on the as-milled powders.

The electronic structure of MoC and WC by anion photoelectron spectroscopy

Photoelectron spectra of MoC− and WC− are reported at two detachment energies, 532 and 355 nm. The electron affinities of MoC and WC were measured to be 1.358 (0.010) and 1.022 (0.010) eV, respectively. Seven low-lying electronic states were observed for MoC within 2 eV above its ground state whereas six were observed for WC within 2.4 eV above its ground state. The bonding and spectroscopy of MoC and WC were found to be different and were attributed to their different atomic configurations due to the strong relativistic effects in W. The current study reports the first spectroscopic information on WC.

Structural studies of WC(0001) and the adsorption of benzene

We report on studies dealing with the structure of WC(0001) and the adsorption of benzene on this surface. An I(V)-low-energy electron diffraction structure analysis has been performed to elucidate the surface structure of WC(0001). These studies indicate that the surface consists of a tungsten layer covered by carbon randomly distributed on the hcp sites with a coverage of 30% that of a full carbon layer. The distance between this carbon layer and the tungsten layer beneath is enlarged by 5% with respect to the spacing between carbon and tungsten layers in the bulk. Only a small deviation from the bulk value was found for the distance between the first tungsten layer and the carbon layer below. No indications of surface reconstruction have been observed. Benzene adsorption was studied on clear oxygen covered and oxidized WC(0001). The benzene multilayer desorbs at T≤200 K. On stoichiometric WC(00001), molecular benzene of (sub)monolayer coverage is found up to temperatures of T≈230 K. After desorption of this species, small signals of fragments are visible in the photoelectron spectra up to T≈1000 K. Above this temperature, a graphite covered surface remains. On a surface covered by a thin closed oxide phase (WO) only multilayer adsorption is found; above T≈200 K no adsorption takes place under UHV conditions. Weakly oxidized WC(0001) interacts more strongly with benzene in that strong photoemission signals of a (sub)monolayer species are visible up to a temperature of T≈340 K.

Features of the electronic strucutre of different modifications of tantalum mononitrides

The electronic structure of tantalum mononitride having a lattice of the CoSn, WC, and NaCl type has been studied by X-ray emission, absorption and photoelectron spectroscopy. For all compounds the X-ray emission and absorption spectra were combined in a common energy scale. The charge transfer from tantalum atoms to nitrogen atoms was revealed in the nitrides studied. Strong hybridization of the Ta5d and N2p electron states is characteristic of all examined compounds. Significant narrowing of the NKα-band is observed in the sequence TaN (CoSn structure type) ↠TaN (WC structure type) ↠ TaN (NaCl structure type).

Chemical synthesis and magnetic properties of nanocrystalline FeMoN 2

We have synthesized a new ternary nitride, FeMoN 2, phase crystallizing in hexagonal WC structure with lattice parameters, a=0.2843 nm and c=0.2735 nm. Ultrafine FeMoN 2 nitride is Pauli-paramagnetic in the temperature range 77–300 K having a magnetic susceptibility value of 230×10 −6 emu/g at room temperature. FeMoN 2 exhibits metallic behavior. The reduction of intrinsic magnetic moments of Fe occurs because of the spin-pairing effect and d-orbital coupling of Fe and Mo. Mossbauer study of ultrafine FeMoN 2 particles appropriately corroborates the observed magnetic properties.

Electronic Structure of Substoichiometric Tantalum Nitrides Studied by the XES Method

The X-ray emission spectroscopy (XES) method has been used to study the electronic structure of substoichiometric tantalum mononitrides. Both the Ta Lβ5 and N Kα X-ray emission spectra of cubic nitrides TaNx (NaCl structure, 0.81 ≤ x ≤ 1.172), hexagonal nitrides TaN0.99 (CoSn structure) and TaN0.90 (WC structure) have been derived and compared on a common energy scale. In the sequence TaN (CoSn structure) → TaN (WC structure) → TaN (NaCl structure) a decrease of the N Kα band halfwidth was observed. In most substoichiometric cubic tantalum mononitrides with vacancies on the metal sublattice (1.134 ≤ x ≤ 1.172) a subband of non-bonding N 2p-like states appeared in the energy region between the Ta 5d–N 2p- and Ta 5d-like bands. In all tantalum nitrides studied the Ta 5d-like and N 2p-like states are strongly hybridized, and a charge transfer occurs from Ta to N atoms.

High-Resolution Transmission Electron Microscope Analysis of Tungsten Carbide Thin Films

ABSTRACTElectron diffraction patterns and high-resolution transmission electron microscope (HREM) images show that the dominant phase in tungsten carbide thin films grown by plasma enhanced chemical vapor deposition is WC(1−x). The f.c.c crystal structure and the unit cell size of WC(1−x) have been determined via electron powder diffraction. The two largest and most dominant spacings in HREM images are the {111} and {002} spacings of WC(1−x). Cross lattice fringes along the two most densely populated zones of WC(1−x) are seen. The sizes and aspect ratios of nano-crystals have been measured from HREM images. Stereo analysis of individual nano-crystals has been done. Confirmation of the 3-D structure of WC(1−x) via spacings larger than 0.15 nm will require a tilt larger than 35° between images.

Structure and properties of bulk nano-structured WC–CO alloy by mechanical alloying

Mixtures of elemental powders of nominal composition WC–6wt%Co and WC–6wt%Co–1wt%VC were prepared using 99.5% purity tungsten, graphite, cobalt and vanadium powders with particle sizes smaller than 75 μm. Mechanical alloying (MA) was performed in a QM-1 planetary ball mill. The structural evolution and the crystallite size changes of the powders during MA were monitored by X-ray diffraction. The results show that cemented carbides of WC–Co powder with crystalline sizes of about 10 nm were directly synthesized from elemental powders by mechanical alloying. Cold compacting was carried out at a pressure of 800 MPa using a manual uniaxial press with carbide insert dies. The hardness and sintered density of sintered samples were measured. The effects of small VC additions on the grain size, density and hardness of sintered samples were also investigated.

Characterisation of red and white thrombus by intravascular ultrasound using radiofrequency and videodensitometric data-based texture analysis

Visual assessment of intravascular ultrasound (IVUS) video images cannot reliably identify thrombus. We examined if texture analysis of radiofrequency (r.f.) data or videodensitometric data (VD) could distinguish thrombi of different ages and cell compositions. Whole human blood (red clot = RC), platelet-rich plasma (white clot = WC) and plasma ( n = 6/group) were imaged at 4 and 24 h with 30 MHz IVUS transducers. At 4 h, VD- and r.f.-based analyses revealed significant differences between RC and WC with variance (VD red 26.4 ± 2.5, white 33.9 ± 7.8; r.f. red 1.4 ± 0.5, white 4.9 ± 1.3), kurtosis (VD red 0.29 ± 0.9, white 0.23 ± 0.3) and skewness (VD red 0.23 ± 0.13, white 0.35 ± 0.52; r.f. red 0.06 ± 0.01, white −0.06 ± 0.05). Also mean grey-level from both data sets was higher in RC (VD 134.8 ± 18.0; r.f. −13.3 ± 1.2) than in WC (VD 105.3 ± 17.4, r.f. 16.5 ± 2.2) ( p < 0.01). With increasing time, variance increased in WC (5.5 ± 1.5 at 24 h) and decreased in RC (0.9 ± 0.3.3 at 24 h). The more heterogeneous structure of WC may be distinguished from that of RC using texture analysis of either VD or r.f.-signals.

Cohesion in tungsten carbide

A recent method for analysing the cohesion of transition metal carbides is applied to the carbides of tungsten. In the most interesting of these, WC, the W atoms are not close packed but are arranged in a simple hexagonal sub lattice of lower coordination number. It is proposed that this occurs so that the electrons of the W–W bonds can avoid the negatively charged C atoms more effectively than when in a carbide such as the NaCI type of the early transition metals. The ensuing stability of this WC structure exists, however, only so long as virtually all the C sites are full. When the C content drops appreciably below stoichiometric, separation into a two phase field occurs, consisting of stoichiometric WC and a lower carbide with a close packed metal sublattice.MST/3092

Cohesion in tungsten carbide

AbstractA recent method for analysing the cohesion of transition metal carbides is applied to the carbides of tungsten. In the most interesting of these, WC, the W atoms are not close packed but are arranged in a simple hexagonal sub lattice of lower coordination number. It is proposed that this occurs so that the electrons of the W–W bonds can avoid the negatively charged C atoms more effectively than when in a carbide such as the NaCI type of the early transition metals. The ensuing stability of this WC structure exists, however, only so long as virtually all the C sites are full. When the C content drops appreciably below stoichiometric, separation into a two phase field occurs, consisting of stoichiometric WC and a lower carbide with a close packed metal sublattice.MST/3092

Degradation of refractory metal carbides by hot hydrogen

The degradation of materials (TaC, WC and NbC) for advanced energy systems induced by hydrogen at high temperatures was investigated. Experiments were conducted in vacuum (control experiments) and hydrogen for temperatures ranging from 1000 to 1600 K. Weight loss, microstructure and phase analyses were performed to correlate the degradation processes in these materials. No hybride formation was observed in any of the specimens when conducted in the presence or absence of hot hydrogen. Oxides were formed in the hydrogen and vacuum runs for TaC. However, oxidation of TaC ceased above 1250 K in the presence of hydrogen. Hydrogen had virtually no effect on the crystal structure of WC or NbC. Weight loss in NbC was enhanced by the presence of hot hydrogen whereas the contrary was observed in TaC. The degradation processes were found to depend strongly on materials and experimental conditions.

Superconductivity of WC in the NaCl-Type Structure under Pressure

Here we report the theoretical calculations of band structure and superconductivity of WC in the NaCl-type structure. The effect of pressure on the band structure is obtained using the linear muffin-tin orbital method. The superconducting transition temperature is calculated using McMillan's formula. The value of T c at ambient conditions is 5.98 K which increases on compression. The increase in T c values with pressure may be due to continuous transfer of s electrons from the carbon site to d states of the tungsten site. The calculated value of T c at ambient pressure and the positive pressure coefficient of T c are in agreement with other transition metal carbides and nitrides.

Temperature stability range of the binary MoC phase

The MoC phase with the WC structure was prepared by a novel synthetic route that involved the stoichiometric disproportionation of a high-temperature phase, Mo 3C2, to MoC and M02C at 1000 °C. MoC was found to be thermodynamically stable at temperatures below 1080 °C. From consideration of equilibrium and calorimetric data, the thermodynamic properties of MoC can be established as S298 0/R = 4.41(0.3) andΔH 298 0/R = -3430(800)K,

Structure of Ta3N5 at 16 K by time-of-flight neutron diffraction

Polycrystalline tritantalum pentanitride (Ta3Ns) has been prepared from the reaction of TaC15 with ammonia, and its structure has been refined from time-of-flight neutron diffraction data. M,. = 612-88, orthorhombic, Cmcm, a = 3.8862 (1), b = 10.2118 (2), c = 10.2624 (3) A, V = 407.26 (3) A, 3, Z = 4, D,. = 9.99 g cm-3, neutron time-of-flight, Rietveld refinement, wRp, Rp, reduced X 2= 0.061, 0.043, 4.21 for data collected at 16 K. Ta3N5 crystal- lizes with the pseudobrookite (Fe2TiOs) structure. Irregular TaN6 octahedra share edges and corners. Introduction. Several authors have mapped out the phase diagram of the Ta-N system (Brauer & Zapp, 1954; Sch6nberg, 1954b). y-Ta2N crystallizes with close-packed metal atoms; the N atoms fill the octa- hedral sites, e-TaN has the B35 structure type which consists of very densely packed metal atoms; N atoms fill deformed octahedral holes (Brauer & Zapp, 1953; Christensen & Lebech, 1978). Ta4N5 and TasN6 can be prepared as thin films and have been analyzed by electron diffraction (Terao, 1971). At elevated temperatures, 6'-TaNx forms with the rock- salt structure (Gatterer, Dufek, Ettmayer & Kieffer, 1975). Thin films of 6-TaNo.8_09 are reported with the hexagonal WC structure (Brauer & Mohr- Rosenbaum, 1971). Some of these interstitial nitrides