Tungsten carbide 746462 226068470 2008-07-16T18:09:53Z Ndemarco 2004683 Added references (snowmobile). {{Chembox new | Name = Tungsten carbide | ImageFile = Tungsten carbide.jpg <!-- | ImageSize = thumb|center|250px --> | ImageName = Tungsten carbide milling bits | Section1 = {{Chembox Identifiers | CASNo = 12070-12-1 }} | Section2 = {{Chembox Properties | Formula = WC | MolarMass = 195.86 g·mol<sup>&minus;1</sup> | Appearance = grey-black solid | Density = 15.8 g·cm<sup>&minus;3</sup>, solid | Solubility = Insoluble. | MeltingPt = 2870 °C, 5198 °F (3143K) | BoilingPt = 6000°C, 10832 °F (6273K) }}<!-- | [[Thermal conductivity]] | 84.02 W·m<sup>&minus;1</sup>·K<sup>&minus;1</sup> |- | [[Tensile strength]] | 0.3448 GPa |- | [[Mohs hardness]] | 9 |- --> | Section3 = {{Chembox Structure | Coordination = | CrystalStruct = [[Hexagonal_crystal_system|Hexagonal]] }} | Section7 = {{Chembox Hazards | EUClass = not listed }} | Section8 = {{Chembox Related | OtherAnions = [[Tungsten boride]]<br />[[Tungsten nitride]] | OtherCations = [[Molybdenum carbide]]<br />[[Titanium carbide]]<br />[[Silicon carbide]] }} }} '''Tungsten carbide''', '''WC''', or '''tungsten semicarbide''', '''W<sub>2</sub>C''', is a chemical compound containing [[tungsten]] and [[carbon]], similar to [[titanium carbide]]. Colloquially, tungsten carbide is often simply called ''[[carbide]]''. ==Chemical properties== There are two well characterized compounds of tungsten and carbon, WC and W<sub>2</sub>C. Both compounds may be present in coatings and the proportions can depend on the coating method, see e.g.<ref>Comparative study of WC-cermet coatings sprayed via the HVOF and the HVAF Process, Jacobs L., Hyland M.M.,De Bonte M., Journal of Thermal Spray Technology 7, 2,(1998), 213-218, {{doi|10.1361/105996398770350954}}</ref> WC can be prepared by reaction of tungsten metal and carbon at 1400-2000°C.<ref name = "Pierson">Hugh O. Pierson (1992) ''Handbook of Chemical Vapor Deposition (CVD): Principles, Technology, and Applications'' William Andrew Inc. ISBN 0815513003</ref> Other methods include a patented fluid bed process that reacts either tungsten metal or blue [[tungsten(VI) oxide|WO<sub>3</sub>]] with CO/CO<sub>2</sub> mixture and H<sub>2</sub> between 900 and 1200°C.<ref>Lackner, A.,Filzwieser A., Gas carburizing of tungsten carbide (WC) powder, United States Patent 6447742, (2002),</ref> Chemical vapor deposition methods that have been investigated include<ref name = "Pierson"/>: *[[tungsten hexachloride]] with hydrogen, as reducing agent and [[methane]] as the source of carbon at 670°C :WCl<sub>6</sub> + H<sub>2</sub> + CH<sub>4</sub> → WC + 6HCl *reacting [[tungsten hexafluoride]] with hydrogen as reducing agent and [[methanol]] as source of carbon at 350°C :WF<sub>6</sub> + H<sub>2</sub> + CH<sub>3</sub>OH → WC + 6HF + H<sub>2</sub>O At high temperatures WC decomposes to tungsten and carbon and this can occur during high temperature thermal spray e.g high velocity oxygen fuel (HVOF) and high energy plasma (HEP) methods.<ref>Microstructural evaluation of tungsten carbide-cobalt coatings, Nerz, J.; Kushner, B.; Rotolico, A., Journal of Thermal Spray Technology,(1992), 1, 2, 147-152, {{doi|10.1007/BF02659015}}</ref><br /> Oxidation of WC starts at 500-600°C.<ref name = "Pierson"/> It is resistant to acids and is only attacked by [[hydrofluoric acid]]/[[nitric acid]] (HF/HNO<sub>3</sub>) mixtures above room temperature.<ref name = "Pierson"/> It reacts with fluorine gas at room temperature and chlorine above 400°C and is unreactive to dry H<sub>2</sub> up to its melting point.<ref name = "Pierson"/><br /> WC has been investigated for its potential use as a catalyst and it has been found to resemble [[platinum]] in its catalysis the production of water from hydrogen and oxygen at room temperature, the reduction of tungsten trioxide by hydrogen in the presence of water, and the isomerization of 2,2-dimethylpropane to 2-methylbutane.<ref>Platinum-Like Behavior of Tungsten Carbide in Surface Catalysis , R. B. Levy 1 and M. Boudart,Science (1973), 181, 4099, 547 - 549, doi|10.1126/science.181.4099.547}}</ref> It has been proposed as a replacement for the [[iridium]] catalyst in [[hydrazine]] powered satellite thrusters.<ref>Nitride and carbide of molybdenum and tungsten as substitutes of iridium for the catalysts used for space communication, Rodrigues J.A.J., Cruz G.M., Bugli G.,Boudart M., Djéga-Mariadassou G., Catalysis Letters, 45, 1-2 (1997), {{doi|10.1023/A:1019059410876}}</ref> ==Physical Properties== Tungsten carbide is a high melting, 2870°C, extremely hard 8.5 - 9.0 [[Mohs scale of mineral hardness|Mohs scale]]{{Fact|date=June 2008}} and 22 GPa [[Vickers hardness test|Vickers hardness]] with low [[resistivity|electrical resistivity]] (1.7-2.2 10<sup>-7</sup>ohm.m) comparable with metals (e.g [[vanadium]] 1.99 10<sup>-7</sup>ohm.m).<ref name = "Pierson"/><ref name = "Kittel">Charles Kittel, Introduction to Solid State Physics- 7th Edition (1995) Wiley-India ISBN 1081-265-1045-5</ref><br /> WC is readily wetted by both molten [[nickel]] and [[cobalt]].<ref name = "Ettmayer">Peter Ettmayer, Walter Lengauer, ''Carbides: transition metal solid state chemistry encyclopedia of inorganic chemistry (1994) John Wiley & Sons, ISBN 0471936200</ref> Investigation of the phase diagram of the W-C-Co system shows that WC and Co form a pseudo binary [[Eutectic point|eutectic]]. The [[phase diagram ]] also shows that there are so-called η-carbides with composition (W,Co)<sub>6</sub>C that can be formed and the fact that these phases are brittle is the reason why control of the carbon content in WC-Co hard metals is important.<ref name = "Ettmayer"/> ==Structure== There are two forms of WC, a hexagonal form, α-WC,<ref name = "Wells"> Wells A.F. (1984) ''Structural Inorganic Chemistry'' 5th edition Oxford Science Publications ISBN 0-19-855370-6 </ref> and a cubic high temperature form, β-WC, which has the rock salt structure.<ref>Phase Equilibria in the System Tungsten—Carbon, Sara R. V., Journal of the American Ceramic Society 48, (1965), 5, 251–257 {{doi|10.1111/j.1151-2916.1965.tb14731.x}}</ref> The hexagonal form can be visualized as made up of hexagonally close packed layers of metal atoms with layers lying directly over one another, with carbon atoms filling half the interstices giving both tungsten and carbon a regular trigonal prismatic, 6 coordination.<ref name = "Wells"/> From the unit cell dimensions<ref>Untersuchungen im System Tantal-Wolfram-Kohlenstoff, Rudy E., Rudy E., Benesovsky F., Monatshefte für chemie, (1962), 93, 3, 1176-1195, {{doi|10.1007/BF01189609}}</ref> the following bond lengths can be determined; the distance between the tungsten atoms in an hexagonally packed layer is 291pm, the shortest distance between tungsten atoms in adjoining layers is 284 pm, and the tungsten carbon bond length is 220 pm. The tungsten-carbon bond length is therefore comparable to the single bond in W(CH<sub>3</sub>)<sub>6</sub> (218pm) in which there is strongly distorted trigonal prismatic coordination of tungsten.<ref>Kleinhenz, S.; Pfennig, V.; Seppelt, K.;''Chem. Eur. J.''<b>1998<b>,''4'', 1687-91 {{doi|10.1002/(SICI)1521-3765(19980904)4:9<1687::AID-CHEM1687>3.0.CO;2-R}}</ref><br /> Molecular WC has been investigated and this gas phase species has a bond length of 171 pm for <sup>184</sup>W<sup>12</sup>C.<ref> Optical spectroscopy of tungsten carbide (WC), Sickafoose S.M., Smith A.W., and Morse M. D., J. Chem. Phys. 116, 993 (2002); {{doi|10.1063/1.1427068}}</ref> ==Toxicity== The primary health risks associated with carbide relate to inhalation of dust, leading to [[fibrosis]]. ==Applications== ===Machine tools=== Carbide cutting surfaces are often useful when machining through materials such as [[carbon steel]] or [[stainless steel]], as well as in situations where other tools would wear away, such as high-quantity production runs. Sometimes, carbide will leave a better finish on the part, and allow faster machining. Carbide tools can also withstand higher temperatures than standard [[high speed steel]] tools. The material is usually '''tungsten-carbide cobalt''', also called "cemented carbide", a [[metal matrix composite]] where tungsten carbide particles are the aggregate and metallic [[cobalt]] serves as the matrix. The process of combining tungsten carbide with [[cobalt]] is referred to as sintering or [[Hot Isostatic Pressing]] (HIP). During this process [[cobalt]] eventually will be entering the liquid stage and WC grains (>> higher melting point) remain in the solid stage. As a result of this process [[cobalt]] is embedding/cementing the WC grains and thereby creates the [[metal matrix composite]] with its distinct material properties. The naturally ductile cobalt metal serves to offset the characteristic brittle behavior of the tungsten carbide ceramic, thus raising its toughness and durability. Such parameters of tungsten carbide can be changed significantly within the carbide manufacturers sphere of influence, primarily determined by grain size, cobalt content, dotation (e.g. alloy carbides) and carbon content. Machining with carbide can be difficult, as carbide is more brittle than other tool materials, making it susceptible to chipping and breaking. To offset this, many manufacturers sell carbide inserts and matching insert holders. With this setup, the small carbide insert is held in place by a larger tool made of a less brittle material (usually steel). This gives the benefit of using carbide without the high cost of making the entire tool out of carbide. Most modern face mills use carbide inserts, as well as some lathe tools and [[endmill]]s. To increase the life of carbide tools, they are sometimes coated. Four such coatings are TiN ([[titanium nitride]]), TiC ([[titanium carbide]]), Ti(C)N (titanium carbide-nitride), and TiAlN ([[Titanium Aluminum Nitride]]). (Newer coatings, known as DLC (Diamond Like Coating) are beginning to surface, enabling the cutting power of diamond without the unwanted chemical reaction between real diamond and iron.) Most coatings generally increase a tool's hardness and/or lubricity. A coating allows the cutting edge of a tool to cleanly pass through the material without having the material [[galling|gall]] (stick) to it. The coating also helps to decrease the temperature associated with the cutting process and increase the life of the tool. The coating is usually deposited via thermal [[chemical vapor deposition|CVD]] and, for certain applications, with the mechanical [[Physical vapor deposition|PVD]] method. However if the deposition is performed at too high temperature, an ''eta phase'' of a Co<sub>6</sub>W<sub>6</sub>C tertiary carbide forms at the interface between the carbide and the cobalt phase, facilitating adhesion failure of the coating. ===Military=== Tungsten carbide is often used in armor-piercing ammunition, especially where [[depleted uranium]] is not available or not politically acceptable. The first use of W<sub>2</sub>C projectiles occurred in Luftwaffe tank-hunter squadrons, which used 37 mm [[autocannon]] equipped Ju-87G [[Stuka]] attack planes to destroy Soviet T-34 tanks in [[World War II|WWII]]. Owing to the limited German reserves of tungsten, W<sub>2</sub>C material was reserved for making machine tools and small numbers of projectiles for the most elite combat pilots, like [[Hans Rudel]]. It is an effective penetrator due to its high hardness value combined with a very high density. Tungsten carbide ammunition can be of the [[sabot|sabot type]] (a large arrow surrounded by a discarding push cylinder) or a subcaliber ammunition, where copper or other relatively soft material is used to encase the hard penetrating core, the two parts being separated only on impact. The latter is more common in small-caliber arms, while sabots are usually reserved for artillery use. Tungsten carbide is also an effective [[neutron reflector]] and as such was used during early investigations into nuclear chain reactions, particularly for weapons. A [[criticality accident]] occurred at [[Los Alamos National Laboratory]] on [[21 August]] [[1945]] when [[Harry K. Daghlian, Jr.]] accidentally dropped a tungsten carbide brick onto a [[plutonium]] sphere causing the sub-critical mass to go critical with the reflected [[neutrons]]. ===Sports=== Hard carbides, especially tungsten carbide, are used by athletes, generally on poles which impact hard surfaces. [[Trekking poles]], used by many [[hiking|hikers]] for balance and to reduce pressure on leg joints, generally use carbide tips in order to gain traction when placed on hard surfaces (like rock); such carbide tips last much longer than other types of tips. Rocks along many popular hiking trails, such as the [[Appalachian Trail]] and [[Pacific Crest Trail]], are scratched and pockmarked from hundreds or thousands of impacts from pole tips. {{Fact|date=February 2007}} While [[ski]] [[Ski pole|pole]] tips are generally not made of carbide, since they do not need to be especially hard even to break through layers of ice, [[rollerskiing|rollerski]] tips usually are. Roller skiing emulates [[cross country skiing]] and is used by many skiers to train during warm weather months. Because skiers require traction on [[bitumen]] ([[asphalt]]) carbide tips are used in the sport. {{Fact|date=February 2007}} Sharpened carbide tipped spikes (known as studs) can be inserted into the drive tracks of [[snowmobile|snowmobiles]]. These studs enhance traction on icy surfaces. Longer v-shaped segments fit into grooved rods called wear rods under each snowmobile ski. The relatively sharp carbide edges enhance steering on harder icy surfaces. The carbide tips and segments reduce wear encountered when the snowmobile must cross roads and other abrasive surfaces. Some tire manufacturers, such as [[Nokian Tyres|Nokian]] and [[Schwalbe]], offer bicycle tires with tungsten carbide studs for better traction on ice. These are generally preferred over steel studs because of their wear resistance. ===Domestic=== Tungsten carbide is used as the rotating ball in the tips of [[ballpoint pen]]s to disperse ink during writing<ref>{{cite web | title = How does a ballpoint pen work? | work = Engineering | publisher = HowStuffWorks | date = 1998-2007 | url = http://science.howstuffworks.com/question683.htm | accessdate = 2007-11-16 }}</ref>. Tungsten carbide can now be found in the inventory of some jewelers, most notably as the primary material in men's wedding bands. When used in this application the bands appear with a lustrous dark hue often buffed to a mirror finish. The finish is highly resistant to scratches and scuffs, holding its mirror-like shine for years. <ref>[http://www.forevermetals.com/jewelry-tungsten-carbide-ring/ Tungsten Carbide Manufacturing]</ref> A common misconception held concerning tungsten carbide rings is they cannot be removed in the course of emergency medical treatment, requiring the finger to be removed instead. Emergency rooms are usually equipped with jewelers' saws that can easily cut through gold and silver rings without injuring the patient when the ring cannot be slipped off easily. However, these saws are incapable of cutting through tungsten carbide. Although standard ring cutting tools cannot be used due to the hardness of this material, there are specialty cutters available that are just as effective on tungsten carbide as they are on gold and platinum. Tungsten carbide rings may be removed in an emergency situation by cracking them into pieces with standard vice grip–style locking pliers. Many manufacturers of this emerging jewelry material state that the use of a cobalt binder may cause unwanted reactions between the cobalt and the natural oils on human skin. Skin oils cause the cobalt to leach from the material. This is said to cause possible irritation of the skin and permanent staining of the jewelry itself. Many manufacturers now advertise that their jewelry is "cobalt free". This is achieved by replacing the cobalt with nickel as a binder.{{Fact|date=December 2007}} ==References== {{Refimprove|date=December 2007}} {{reflist}} ==External links== *[http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc13/icsc1320.htm International Chemical Safety Card 1320] *[http://www.cdc.gov/niosh/npg/npgd0647.html NIOSH Pocket Guide to Chemical Hazards] [[Category:Tungsten compounds]] [[Category:Carbides]] [[Category:Superhard materials]] [[zh-min-nan:O͘-kǹg]] [[de:Wolframkarbid]] [[es:Carburo de tungsteno]] [[fr:Carbure de tungstène]] [[it:Carburo di tungsteno]] [[no:Wolframkarbid]] [[pl:Węglik wolframu]] [[fi:Kovametalli]]