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Control of eta carbide formation in tungsten carbide powders sputter-coated with (Fe/Ni/Cr)

Fernandes, C. M.; Senos, A. M. R.; Vieira, M. T.
Fonte: Universidade de Coimbra Publicador: Universidade de Coimbra
Tipo: Artigo de Revista Científica Formato: aplication/PDF
ENG
Relevância na Pesquisa
66.97%
Composite powders of tungsten carbide with variable coating contents of Fe/Ni/Cr based alloys (1-15 wt.%) were prepared by sputtering. The coated powders have revealed a good performance to shape-forming and sintering. However, the presence of nanocrystalline coatings could enhance the formation of a brittle phase during sintering, named [eta]-phase, (M, W)6C. To investigate this effect, the powders were heat-treated at variable temperatures, up to 1400 °C. For comparison, conventionally prepared powders were studied, too, and the content of [eta]-phase was found to be higher for the sputter-coated ones. This result is attributed to the content of binder elements with affinity to carbon, like chromium, enhanced by the difference on binder characteristics, such as nanocrystallinity and morphology. It was concluded that an effective control of the reaction can be achieved by increasing in the binder composition the ratio between the elements without affinity and those with affinity to carbon and/or enlarging the carbon content.; http://www.sciencedirect.com/science/article/B6TY1-4MBJY6R-1/1/f383fedbf0fa61a789c1d40bd9608baf

On-line mass spectrometry of the electro-oxidation of methanol in acidic media on tungsten carbide

ANGELUCCI, Camilo A.; DEINER, L. Jay; NART, Francisco C.
Fonte: SPRINGER Publicador: SPRINGER
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
67.09%
The electro-oxidation of methanol at supported tungsten carbide (WC) nanoparticles in sulfuric acid solution was studied using cyclic voltammetry, potentiostatic measurements, and differential electrochemical mass spectroscopy (DEMS). The catalyst was prepared by a sonochemical method and characterized by X-ray diffraction. Over the WC catalyst, the oxidation of methanol (1 M in a sulfuric acid electrolyte) begins at a potential below 0.5 V/RHE during the anodic sweep. During potentiostatic measurements, a maximum current of 0.8 mA mg(-1) was obtained at 0.4 V. Measurements of DEMS showed that the methanol oxidation reaction over tungsten carbide produces CO2 (m/z=44); no methylformate (m/z=60) was detected. These results are discussed in the context of the continued search for alternative materials for the anode catalyst of direct methanol fuel cells.; Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP)[04/03051-6]; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Conselho Nacional de Desenvolvimento Cientfico e Tecnologico (CNPq); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Usinabilidade do carbeto de tungstênio no torneamento com ferramenta de diamante; Single point diamond turning of the tungsten carbide

Gonçalves, André da Motta
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 06/11/2009 PT
Relevância na Pesquisa
67.21%
Este trabalho apresenta o estudo da usinabilidade do carbeto de tungstênio utilizando ferramenta de ponta única de diamante em máquina-ferramenta de ultraprecisão, em função de suas características de dureza e potencial uso para a fabricação de micromoldes. O carbeto de tungstênio foi submetido a testes de usinagem para a determinação dos parâmetros e condições de corte para a obtenção do regime dúctil. Com base nos resultados experimentais, concluiu-se que, para os avanços da ordem de 1 micrômetro/revolução, a profundidade de corte (AP) não influencia significativamente no resultado de rugosidade, e para avanços da ordem de 3 micrômetros, observou-se a formação de trincas na superfície usinada. Além disso, para profundidades de corte maiores que 2 micrômetros as ferramentas começaram a apresentar lascamento. Para avanços da ordem de 3 micrômetro/revolução a profundidade de corte influencia com grande significância no resultado de rugosidade. O torneamento do carbeto de tungstênio usando ferramenta de diamante mostrou-se uma opção viável à produção de superfícies em termos de qualidade óptica, porém, devido à alta dureza deste material (aproximadamente 4000HV) o torneamento mostrou ser um processo com condições limitadas para a produção em série de componentes em função da baixa taxa de remoção de material permitida. É possível que a retificação possa apresentar taxas de remoção maiores...

Conversão catalítica de celulose utilizando catalisadores de carbeto de tungstênio suportado em carvão ativo e promovido por paládio; Catalytic conversion of cellulose using catalysts of tungsten carbide supported on activated carbon and promoted by palladium

Leal, Glauco Ferro
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 08/08/2014 PT
Relevância na Pesquisa
56.97%
A celulose é o biopolímero mais abundante na natureza e apresenta grande potencial para ser processada e produzir de maneira sustentável biocombustíveis e produtos químicos. A conversão catalítica é um dos meios mais promissores para transformação da celulose. A separação entre produtos e catalisadores é uma etapa importante para indústria, o que coloca a catálise heterogênea em posição privilegiada como via de conversão, devido à facilidade de separação entre produto e catalisador. A hidrogenólise é uma via de transformação que promove a quebra de ligações C-C e a retirada de átomos de oxigênio, levando a uma gama de combustíveis e produtos químicos. Os carbetos de metais de transição suportados em carvão ativo são efetivos na quebra de ligações C-C, enquanto o paládio atua tanto na quebra de ligações C-C como em etapas de hidrogenação. Assim, este trabalho estudou as propriedades estruturais e catalíticas de catalisadores de carbeto de tungstênio suportados em carvão ativado e promovidos com paládio. Foram preparados e caracterizados catalisadores de WXC sem promotor e com 1 e 2% de Pd. As medidas de fisissorção de N2 revelou que os catalisadores são formados por uma mistura de micro e mesoporos. A análise de difração de raios X revelou predominância da fase W2C nos catalisadores promovidos por Pd...

Effects of surface treatments on the fatigue strength of AISI 4340 aeronautical steel

Nascimento, M. P.; Souza, R. C.; Pigatin, W. L.; Voorwald, HJC
Fonte: Elsevier B.V. Publicador: Elsevier B.V.
Tipo: Artigo de Revista Científica Formato: 607-618
ENG
Relevância na Pesquisa
47.04%
Internal residual stresses significantly influence the fatigue strength of coated materials. It is well known that chromium plating is the most used electrodeposited coating for important industrial applications. However, pressure to identify alternatives or to improve the chromium electroplating process have increased in recent years, related to the reduction in fatigue strength of the base material and to environmental requirements. The high efficiency and fluoride free hard chromium electroplating there called accelerated) is an improvement to the conventional process. One environmentally safer and cleaner alternative to hard chromium plating is tungsten carbide thermal spray coating applied by the High Velocity Oxy-Fuel (HVOF) process. To increase the fatigue strength of chromium plated materials, coating thickness and microcracks density are important parameters to be controlled. Techniques as compressive residual stresses induced by shot peening and multilayers, are also used. The aim of this study was to analyse the effects on AISI 4340 steel, in the rotating bending fatigue behaviour, of the: tungsten carbide thermal spray coating applied by HP/HVOF process; chemical nickel underplate, and shot peening process applied before coating deposition...

Tungsten permanent chemical modifier with co-injection of Pd(NO3)(2)+Mg(NO3)(2) for direct determination of Pb in vinegar by graphite furnace atomic absorption spectrometry

Oliveira, Silvana Ruella; Oliveira, Adriana Paiva; Gomes Neto, Jose Anchieta
Fonte: Elsevier B.V. Publicador: Elsevier B.V.
Tipo: Artigo de Revista Científica Formato: 236-241
ENG
Relevância na Pesquisa
56.77%
A tungsten carbide coating on the integrated platform of a transversely heated graphite atomizer (THGA((R))) used together with Pd(NO3)(2) + Mg(NO3)(2) as modifier is proposed for the direct determination of lead in vinegar by graphite furnace atomic absorption spectrometry. The optimized heating program (temperature, ramp time, hold time) of atomizer involved drying stage (110 degrees C, 5 s, 30 s; 130 degrees C, 5 s, 30 s), pyrolysis stage (1000 degrees C, 15 s, 30 s), atomization stage (1800 degrees C, 0 s, 5 s) and clean-out stage (2450 degrees C, I s, 3 s). For 10 mu L of vinegar delivered into the atomizer and calibration using working standard solutions (2.5-20.0 mu g L-1 Pb) in 0.2% (v/v) HNO3, analytical curve with good linear correlation (r = 0.9992) was established. The characteristic mass was 40 pg Pb and the lifetime of the tube was around 730 firings. The limit of detection (LOD) was 0.4 mu g L-1 and the relative standard deviations (n = 12) were typically <8% for a sample containing 25 pg L-1 Pb. Accuracy of the proposed method was checked after direct analysis of 23 vinegar samples. A paired t-test showed that results were in agreement at 95% confidence level with those obtained for acid-digested vinegar samples. The Pb levels varied from 2.8 to 32.4 pg L-1. Accuracy was also checked by means of addition/recovery tests and recovered values varied from 90% to 110%. Additionally...

Coating residual stress effects on fatigue performance of 7050-T7451 aluminum alloy

Marin de Camargo, Jose Andre; Cornelis, Herman Jacobus; Odila Hilario Cioffi, Voorwald Maria; Pitanga Costa, Midori Yoshikawa
Fonte: Elsevier B.V. Publicador: Elsevier B.V.
Tipo: Artigo de Revista Científica Formato: 9448-9455
ENG
Relevância na Pesquisa
46.89%
The tendency of the aircraft industry is to enhance customer value by improving performance and reducing environmental impact. In view of availability, aluminum alloys have a historically tendency to faster insertion due to their lower manufacturing and operated production infrastructure. In landing gear components, wear and corrosion control of many components is accomplished by surface treatments of chrome electroplating on steel or anodizing of aluminum. One of the most interesting environmentally safer and cleaner alternatives for the replacement of hard chrome plating or anodizing is tungsten carbide thermal spray coating, applied by the high velocity oxy fuel (HVOF) process. However, it was observed that residual stresses originating from these coatings reduce the fatigue strength of a component.An effective process as shot peening treatment, considered to improve the fatigue strength, pushes the crack sources beneath the surface in most of medium and high cycle cases, due to the compressive residual stress field induced. The objective of this research is to evaluate a tungsten carbide cobalt (WC-Co) coating applied by the high velocity oxy fuel (HVOF) process, used to replace anodizing. Anodic films were grown on 7050-T7451 aluminum alloy by sulfuric acid anodizing...

Non-isothermal kinetic of oxidation of tungsten carbide

Ribeiro, C. A.; de Souza, W. R.; Crespi, Marisa Spirandeli; Neto, J. A. Gomes; Fertonani, F. L.
Fonte: Springer Publicador: Springer
Tipo: Artigo de Revista Científica Formato: 801-805
ENG
Relevância na Pesquisa
66.89%
Tungsten carbide, WC, has shown dissimilar thermal behavior when it is heated on changeable heating rate and flow of oxidant atmosphere. The oxidation of WC to WO3 tends to be in a single and slow kinetic step on slow heating rate and/or low flux of air. Kinetic parameters, on non-isothermal condition, could be evaluated to the oxidation of WC to heating rate below 15 degrees C min(-1) or low flow of air (10 mL min(-1)). The reaction is governed by nucleation and growth at 5 to 10 degrees C min(-1) then the tendency is to be autocatalytic, JMA and SB, respectively.

Influência de diferentes métodos de limpeza sobre a eficiência de corte e as características morfológicas de fresas Carbide

Fais, Laiza Maria Grassi; Scardueli, Cassio Rocha; Bordignon, Daniela Cristina; Silva, Regina Helena Barbosa Tavares da; Guaglianoni, Dalton Geraldo; Pinelli, Ligia Antunes Pereira
Fonte: Universidade Estadual Paulista Publicador: Universidade Estadual Paulista
Tipo: Artigo de Revista Científica Formato: 166-173
POR
Relevância na Pesquisa
56.97%
Introduction: Currently, there are many questions regarding the cleaning methods seeking greater efficiency and less loss of burs. Aim: the aim of this study was to evaluate the influence of cleaning methods on the cutting efficiency and morphological characteristics of stainless steel burs tungsten carbide (carbide). Materials and method: Thirty burs were divided into five groups (n = 5) according with the cleaning method: L1 - steel brush, L2 - nylon brush, L3 - ultrasound  +  distilled water, L4 - ultrasound + descaling solution and L5 - no cleaning method (control). The burs were used for the cutting of bovine enamel during six periods of 12 minutes each. After each period, the burs were cleaned (except L5 ) following the protocol established for each group. The cutting efficiency was determined by mass loss and morphological characteristics. Result: The average amount of wear after 72 minutes of use were L1  = 0.3558 g; L2  = 0.4275 g; L3  = 0.4652 g; L4  = 0.4396 g e L5  = 0.4854 g; significant differences in the time of use (p  <  0.001) and cleaning method (p  <  0.001). The L1 group showed the worst performance. Regardless of the experimental group, morphological analysis revealed alterations in the cutting blades soon after the first 12 minutes...

Properties of amorphous SiC coatings deposited on WC-Co substrates

Costa,A.K.; Camargo Jr,S.S.
Fonte: ABM, ABC, ABPol Publicador: ABM, ABC, ABPol
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2003 EN
Relevância na Pesquisa
46.85%
In this work, silicon carbide films were deposited onto tungsten carbide from a sintered SiC target on a r.f. magnetron sputtering system. Based on previous results about the influence of r.f. power and argon pressure upon the properties of films deposited on silicon substrates, suitable conditions were chosen to produce high quality films on WC-Co pieces. Deposition parameters were chosen in order to obtain high deposition rates (about 30 nm/min at 400 W rf power) and acceptable residual stresses (1.5 GPa). Argon pressure affects the energy of particles so that films with higher hardness (30 GPa) were obtained at low pressures (0.05 Pa). Wear rates of the coated pieces against a chromium steel ball in a diamond suspension medium were found to be about half of the uncoated ones. Hardness and wear resistance measurements were done also in thermally annealed (200-800 °C) samples revealing the effectiveness of SiC coatings to protect tool material against severe mechanical degradation resulting of high temperature (above 500 °C) oxidation.

Microstructure and mechanical properties of WC-Co reinforced with NbC

Acchar,Wilson; Zollfrank,Cordt; Greil,Peter
Fonte: ABM, ABC, ABPol Publicador: ABM, ABC, ABPol
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/09/2004 EN
Relevância na Pesquisa
46.85%
Cemented carbides such as tungsten carbide-cobalt WC-Co composites have been widely used as cutting tool materials. Several reports have shown the influence of different factors such as grain size, type and amount of binder phase and the addition of hard particles on the properties of WC-Co. The purpose of this work was to investigate the effect of niobium carbide on the microstructure and mechanical properties of WC-Co. Specimens of WC-Co reinforced with NbC were mixed and subsequently hot-pressed in a inert atmosphere. The WC-Co-NbC composite material exhibited high hardness values (18.9 GPa), flexural strength (2100 MPa) and fracture toughness (11.2 MPa.m½ ). TEM analysis has shown a bimodal grain size distribution of WC.

Aspects on the optimization of die-sinking EDM of tungsten carbide-cobalt

Amorim,Fred Lacerda; Weingaertner,Walter Lindolfo; Bassani,Irionson Antonio
Fonte: Associação Brasileira de Engenharia e Ciências Mecânicas - ABCM Publicador: Associação Brasileira de Engenharia e Ciências Mecânicas - ABCM
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2010 EN
Relevância na Pesquisa
67.18%
At present, due to their properties, the tungsten carbide-cobalt (WC-Co) composite materials are in huge demand by industry to manufacture special tools, dies/molds and components under erosion. The powder metallurgy is the usual process applied to obtain WC-Co products, but in some cases this process is unable to produce tools of very complex shapes and highly intricate details. Thus, additional conventional and non-conventional machining processes are required. In this context, the electrical discharge machining (EDM) is an efficient alternative process. However, the EDM parameters have to be properly set for any different tungsten carbide-cobalt composition and electrode material to achieve an appropriate level of machining performance. In this work, a special grade of tungsten carbide-cobalt was used as workpiece and a copper-tungsten alloy as electrode. Experiments on important EDM electrical and non-electrical parameter settings with reference to material removal rate, electrode wear ratio and surface roughness were carried out under typical rough and finish machining. This paper contributes with an attempt to provide insightful guidelines to optimize electrical discharge machining of WC-Co composite materials using CuW alloy electrodes.

Maintenance of stellite and tungsten carbide saw tips: respiratory health and exposure-response evaluations.

Kennedy, S M; Chan-Yeung, M; Marion, S; Lea, J; Teschke, K
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /03/1995 EN
Relevância na Pesquisa
47.21%
OBJECTIVE--To study exposure to cobalt and chromium in saw maintenance rooms and test respiratory health among saw filers at lumber mills. Hard-metal lung disease is associated with cobalt in the manufacture of tungsten carbide tools; recently it has also been reported among tool maintenance workers. Lumber mills often use saws tipped with tungsten carbide or with a newer alloy, stellite (containing more cobalt, as well as chromium). METHODS--A cross sectional study of 118 saw filers at eight lumber mills was carried out that included a standardised questionnaire, spirometry, personal air sampling, and examination of tasks every 10 minutes (by observation). Comparison data were from a study of bus mechanics tested with similar methods. RESULTS AND CONCLUSION--Cobalt exposure was associated with tungsten carbide grinding but not with stellite grinding. Chromium exposure was associated mainly with stellite welding. Saw filers had a twofold increase in phlegm and wheeze (P < 0.01) and a threefold increase in cough, phlegm, and wheeze related to work (P < 0.001), but no increase in breathlessness. Stellite welding was associated with a significant increase in nasal symptoms and cough related to work and a small decrease in airflow (forced expiratory volume in one second/forced vital capacity (FEV1/FVC%)...

Vector drop-on-demand production of tungsten carbide-cobalt tooling inserts by three dimensional printing

Guo, David, 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 120 p.; 10279604 bytes; 10294208 bytes; application/pdf; application/pdf
EN_US
Relevância na Pesquisa
47.04%
Three Dimensional Printing (3DP) is a solid freeform fabrication process used to generate solid parts directly from three-dimensional computer models. A part geometry is created by selectively depositing binder into sequentially spread layers of powder. In slurry-based 3DP, a suspension of powder in a solvent is used to form the powderbed layer. This slurry-based powderbed yields higher green density and part resolution than dry powder-based 3DP because of smaller particle size. Vector printing requires that the printhead trace and define the external geometries of a part before raster filling the interior, a new approach in comparison to conventional, raster-only printing. Drop-on-demand (DOD) printheads allow binder droplets to be ejected when needed rather than relying upon charge-and-deflect mechanisms used in continuous jet printheads. Integrating these concepts for vector, DOD printing has the potential to enhance the 3DP process by providing greater part resolution and surface finish. The 3DP slurry-based process and vector, drop-on-demand printing are examined as potential methods to produce Tungsten Carbide-Cobalt (WC-Co) tooling inserts. The research focuses on three fundamental process steps: (1) development of a stable slurry...

Tungsten carbide-cobalt by Three Dimensional Printing

Kelley, Andrew, III
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 93 p.; 5549025 bytes; 5553728 bytes; application/pdf; application/pdf
ENG
Relevância na Pesquisa
47.14%
Three Dimensional Printing is an additive manufacturing process for rapid prototyping ceramic and metallic parts [Sachs, et al, 1990]. Green (not sintered) tungsten carbide-cobalt parts must have a density greater than 50% of the theoretical density, 14.9 g/cc, for proper sintering and post-processing. Two approaches were assessed for feasibility and robustness: printing slurry into tungsten carbide-cobalt spray dried powder and printing a solvent in spray dried tungsten carbide powder that readily dissolves. For slurry administered to a powder bed of solid, spherical particles, it has been found that the resulting packing primitive packing fraction increases almost linearly with the volume loading of the slurry over a range of powder size. The increase in density is approximately half what would be calculated by assuming that the slurry fills all the porosity in the powder bed. The maximum green density achieved by printing slurry into a spray dried tungsten carbide-cobalt bed was 41%, midway between the lower bound calculated by assuming the vehicle in the slurry infiltrates only the large pores between the spray dried power and the upper bound calculated by assuming that the vehicle of the slurry also infiltrates the find pores within a spray dried granule. A re-dispersible spray dried powder (38-53 micron size range) was fabricated using only the Duramax 3007 dispersant as the binder. This powder redisperses in water. Administering a drop of water to this powder resulted in primitives with 47% packing density...

STUDIES OF THE COMPOSITION AND REACTIVITY OF CARBON SPECIES GENERATED THROUGH LASER VAPORIZATION OF GRAPHITE, TANTALUM-CARBIDE AND TUNGSTEN-CARBIDE

ORTMAN, BRYAN JOHN
Fonte: Universidade Rice Publicador: Universidade Rice
ENG
Relevância na Pesquisa
46.85%
The vapor compositions above graphite, tantalum carbide and tungsten carbide together with the chemical reactions of C$sb1$, C$sb2$, and C$sb3$ have been studied by the techniques of CW laser vaporization, matrix isolation, and FTIR spectroscopy. The carbon species were scavenged by cocondensing reactive gases (N$sb2$, H$sb2$, O$sb2$, CO and H$sb2$O) with the vapors of graphitic samples in argon or nitrogen matrices. Products were identified from their infrared spectra. Photolysis and annealing studies were also conducted to detect reaction intermediates, formation of carbon clusters, and metastable C$sb1$, C$sb2$ and C$sb3$ adducts with the added reactants. From these experiments CO was shown to be potentially the most useful scavenger molecule, since it reacted with both C$sb1$ and C$sb2$ to form C$sb2$O and C$sb3$O, respectively. CO also reacted with C$sb3$ to form an adduct, which rearranges upon photolysis to form C$sb4$O, a previously unknown molecule. Its identity was confirmed by isotopic studies and comparison of its vibrations to those of related molecules, C$sb3$O, C$sb2$O and CO. In H$sb2$O/carbon studies, only C$sb3$ reacted with H$sb2$O upon cocondensation to form an adduct. In O$sb2$/carbon studies, the formation of CO$sb2$ and CO has been explained in terms of reactions of C$sb1$ and C$sb2$...

Growth of WC-Cr-N and WC-Al-N coatings in a RF-magnetron sputtering process

STANISHEVSKY, Andrei; WALOCK, Michael; ZOU, Yujiao; IMHOFF, Luc; ZAIRI, Amel; NOUVEAU, Corinne
Fonte: Elsevier Publicador: Elsevier
EN_US
Relevância na Pesquisa
47.17%
Tungsten carbide-based coatings have been used in a wide variety of industrial applications such as high speed cutting tools, extrusion dies, drills, aerospace industries, and more. A few reports on ternary and quaternary coatings of WC with other elements indicate good prospects for these material systems. The present study focuses on the formation of quaternary WCeCreN and WCeAleN coatings during the simultaneous reactive RF-magnetron sputtering of tungsten carbide and Al or Cr targets in an argon/nitrogen gas mixture. The resulting coatings, with thicknesses of 3.5 mme8.2 mm, were characterized by using several analytical techniques including X-ray diffraction, SEM/EDS, AFM, and X-ray photoelectron spectroscopy. WCeCreN and WCeAleN coatings with high levels of tungsten (i.e. more than 50 at.% of the total metal content) demonstrated dense microstructure. Coatings with lower tungsten content formed columnar grain microstructure, with different surface morphologies depending on the process parameters. It was proposed that crystalline tungsten carbide (with partial N-substitution of C atoms) and chromium (or aluminum) nitride phases coexist in the coatings when the amount of tungsten was greater than 50 at.% of the total metal content; while at lower tungsten content...

First principle study of intrinsic defects in hexagonal tungsten carbide

Kong, Xiang-Shan; You, Yu-Wei; Xia, J. H.; Liu, C. S.; Fang, Q. F.; Luo, G. -N.; Huang, Qun-Ying
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 16/08/2010
Relevância na Pesquisa
46.85%
The characteristics of intrinsic defects are important for the understanding of self-diffusion processes, mechanical strength, brittleness, and plasticity of tungsten carbide, which present in the divertor of fusion reactors. Here, we use first-principles calculations to investigate the stability of point defects and their complexes in WC. Our calculation results confirm that the formation energies of carbon defects are much lower than that of tungsten defects. The outward relaxations around vacancy are found. Both interstitial carbon and interstitial tungsten atom prefer to occupy the carbon basal plane projection of octahedral interstitial site. The results of isolated carbon defect diffusion show that the carbon vacancy stay for a wide range of temperature because of extremely high diffusion barriers, while carbon interstitial migration is activated at lower temperatures for its considerable lower activation energy. These results provide evidence for the presumption that the 800K stage is attributed by the annealing out of carbon vacancies by long-range migration.; Comment: Submitted to Journal of Nuclear Materials

Novel Route to Submicrometer Tungsten Carbide

Welham, N
Fonte: American Institute of Chemical Engineers Publicador: American Institute of Chemical Engineers
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
66.89%
A mixture of graphite and scheelite (CaWO4) have been mechanically milled together for 40 h. The resultant powder and an unmilled powder were examined by thermal analysis, isothermal annealing, and X-ray diffraction to determine the effect of milling on the reduction of scheelite to tungsten carbide. The milled powder underwent a rapid reduction reaction at 800°C- 300°C lower than the unmilled powder. The reduction sequence to WC was shown to differ for the two powders with W2C intermediate in the milled powder and W in the unmilled powder. The milled powder showed complete reduction to WC in 1 h at 1,000°C, whereas the unmilled powder was incompletely reduced, even after 1 h at 1,200°C. Separation of the final phases was readily achieved by a simple acid leach, leaving WC grains around 200 nm in size.

Room Temperature Reduction of Scheelite (CaWO4)

Welham, N
Fonte: Materials Research Society Publicador: Materials Research Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
47.02%
A mixture of scheelite and magnesium has been mechanically milled together for 100 h, either with graphite or in a nitrogen atmosphere, with the intention of forming tungsten carbide or nitride. The resultant powders were examined by thermal analysis, isothermal annealing, and x-ray diffraction to determine the effect of milling on the reduction of scheelite. With graphite, nanocrystallite W2C was the exclusive tungsten product; WC was not detected even after annealing at 1000°C. No nitride formed in the system milled with nitrogen; however, 10 nm crystallites of elemental tungsten were formed. The unwanted phases, MgO and CaO, were readily removed by leaching in acid, leaving a fine powder composed of impact welded aggregates of either carbide or 99% pure tungsten metal.