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| Names | |
|---|---|
| IUPAC name Tungsten carbide | |
| Other names Tungsten(IV) carbide | |
| Identifiers | |
3D model (JSmol) |
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| ChemSpider | |
| ECHA InfoCard | 100.031.918 |
| EC Number |
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| RTECS number |
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| UNII | |
| UN number | 3178 |
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| Properties | |
| WC | |
| Molar mass | 195.85 g·mol−1 |
| Appearance | Grey-black lustrous solid |
| Density | 15.6 g/cm3[1] |
| Melting point | 2,785–2,830 °C (5,045–5,126 °F; 3,058–3,103 K)[3][2] |
| Boiling point | 6,000 °C (10,830 °F; 6,270 K) at 760 mmHg[2] |
| Insoluble | |
| Solubility | Soluble inHNO 3,HF[3] |
| 1·10−5 cm3/mol[3] | |
| Thermal conductivity | 110 W/(m·K)[4] |
| Structure | |
| Hexagonal,hP2[5] | |
| P6m2, No. 187[5] | |
| 6m2[5] | |
a = 2.906 Å,c = 2.837 Å[5] α = 90°, β = 90°, γ = 120° | |
| Trigonal prismatic (center at C)[6] | |
| Thermochemistry | |
| 39.8 J/(mol·K)[4] | |
Std molar entropy(S⦵298) | 32.1 J/(mol·K) |
| Related compounds | |
Otheranions | |
Othercations | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Tungsten carbide (chemical formula:WC) is achemical compound (specifically, acarbide) containing equal parts oftungsten andcarbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into shapes throughsintering[7] for use inindustrial machinery, engineering facilities,[8]molding blocks,[9]cutting tools,chisels,abrasives,armor-piercing bullets andjewelry.
Tungsten carbide is approximately three times as stiff assteel, with aYoung's modulus of approximately 530–700 GPa,[4][10][11][12] and is twice as dense assteel. It is comparable withcorundum (α-Al2O3) inhardness, approaching that of a diamond,[7] and can be polished and finished only with abrasives of superior hardness such ascubic boron nitride and diamond powder, wheels and compounds. Tungsten carbide tools can be operated at cutting speeds much higher thanhigh-speed steel (a special steel blend for cutting tools).[7]
Tungsten carbide powder was first synthesized by H. Moissan in 1893, and the industrial production of the cemented form started 20 to 25 years later (between 1913 and 1918).[10]
Colloquially among workers in various industries (such asmachining), tungsten carbide is often simply calledcarbide.
Tungsten carbide powder is prepared by reaction of tungsten metal (or powder) and carbon at 1,400–2,000 °C.[13] Other methods include a lower temperature fluid bed process that reacts either tungsten metal (or powder) or blueWO3 withCO/CO2 gas mixture andH2 gas between 900 and 1,200 °C.[14]
WC can also be produced by heating WO3 withgraphite, either directly at 900 °C or in hydrogen at 670 °C, followed bycarburization inargon at 1,000 °C.[15]Chemical vapor deposition methods that have been investigated include:[13]
Solid tungsten carbide is prepared using techniques frompowder metallurgy developed in the 1920s.[7] Powdered tungsten carbide is mixed with another powdered metal, usuallycobalt (alternatives includenickel,iron andparaffin wax[10]) which acts as abinder.[7] The mixture is pressed, thensintered by heating it to temperatures of 1,400 °C (2,550 °F) to 1,600 °C (2,910 °F); the binder melts, wets, and partially dissolves the tungsten grains, binding them together.[7] The cobalt-tungsten composites specifically are known by a number of trade names, including Widia and Carboloy.[7]
There are two well-characterized compounds of tungsten and carbon: tungsten carbide, WC, andtungsten semicarbide,W2C. Both compounds may be present in coatings and the proportions can depend on the coating method.[16]
Another meta-stable compound of tungsten and carbon can be created by heating the WC phase to high temperatures using plasma, then quenching in inert gas (plasma spheroidization).[17] This process causes macrocrystalline WC particles to spheroidize and results in the non-stoichiometric high temperature phaseW1−x existing in a meta-stable form at room temperature. The fine microstructure of this phase provides high hardness (2800–3500 HV) combined with good toughness when compared with other tungsten carbide compounds. The meta-stable nature of this compound results in reduced high temperature stability.[citation needed]
At high temperatures WC decomposes to tungsten and carbon and this can occur during high-temperaturethermal spray, e.g., in high velocity oxygen fuel (HVOF) and high energy plasma (HEP) methods.[18]
Oxidation of WC starts at 500–600 °C (773–873 K).[13] It is resistant toacids and is only attacked byhydrofluoric acid/nitric acid (HF/HNO3) mixtures above room temperature.[13] It reacts withfluorine gas at room temperature andchlorine above 400 °C (673 K) and is unreactive to dryH2 up to its melting point.[13] Finely powdered WC oxidizes readily inhydrogen peroxide aqueous solutions.[19] At high temperatures and pressures it reacts with aqueoussodium carbonate formingsodium tungstate, a procedure used for recovery of scrap cemented carbide due to its selectivity.[citation needed]
Tungsten carbide has a high melting point at 2,870 °C (3,140 K), a boiling point of 6,000 °C (6,270 K) when under a pressure equivalent to 1 standard atmosphere (101.325 kilopascals),[2] a thermal conductivity of 110 W/(m·K),[4] and acoefficient of thermal expansion of 5.5 μm/m·K.[10]
Tungsten carbide is extremely hard, ranking about 9.0–9.5 on theMohs scale, and with aVickers number of around 2600.[11] It has aYoung's modulus of approximately 530–700 GPa,[4][10][11][12] abulk modulus of 379–381 GPa,[20] and ashear modulus of 274 GPa.[21] It has an ultimate tensile strength of 344 MPa,[12] an ultimate compression strength of about 2.7 GPa and a Poisson's ratio of 0.31 .[21]
The speed of a longitudinal pressure wave (thespeed of sound) through a thin rod of tungsten carbide is 6220 m/s.[22]
Tungsten carbide's lowelectrical resistivity of about 0.2 μΩ·m is comparable with that of some metals (e.g.vanadium 0.2 μΩ·m).[13][23]
WC is readilywetted by both moltennickel andcobalt.[24] Investigation of the phase diagram of the W-C-Co system shows that WC and Co form a pseudo binaryeutectic. Thephase diagram also shows that there are so-called η-carbides with composition(W,Co)6C that can be formed and the brittleness of these phases makes control of the carbon content in WC-Co cemented carbides important.[24] In the presence of a molten phase such as cobalt,abnormal grain growth is known to occur in the sintering of tungsten carbide, with this having significant effects on the performance of the product material.[citation needed]
There are two forms of WC, ahexagonal form, α-WC (hP2,space group P6m2, No. 187),[5][6] and acubic high-temperature form, β-WC, which has therock salt structure.[25] The hexagonal form can be visualized as made up of a simple hexagonal lattice of metal atoms of layers lying directly over one another (i.e. not close packed), with carbon atoms filling half the interstices giving both tungsten and carbon a regular trigonal prismatic, 6 coordination.[6] From the unit cell dimensions[26] the following bond lengths can be determined: The distance between the tungsten atoms in a hexagonally packed layer is 291 pm, 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 inW(CH3)6 (218 pm) in which there is strongly distorted trigonal prismatic coordination of tungsten.[27]
Molecular WC has been investigated and this gas phase species has a bond length of 171 pm for184
W12
C.[28]
Sintered tungsten carbide–cobaltcutting tools are very abrasion resistant and can also withstand higher temperatures than standardhigh-speed steel (HSS) tools. Carbide cutting surfaces are often used formachining tough materials such ascarbon steel orstainless steel, and in applications where steel tools would wear quickly, such as high-quantity and high-precision production. Because carbide tools maintain a sharp cutting edge better than steel tools, they generally produce a better finish on parts, and their temperature resistance allows faster machining. The material is usually calledcemented carbide, solid carbide, hardmetal or tungsten-carbide cobalt. It is ametal matrix composite, where tungsten carbide particles are the aggregate, and metalliccobalt serves as the matrix.[29][30] It has been found wear and oxidation properties of cemented carbide can be improved by replacing cobalt with iron aluminide.[31][32][33] Using iron also reduces cost, as cobalt is particularly expensive, but the mixing is best done with resonant acoustic mixing.[34] Tungsten carbide cutting tools can be further enhanced with coatings such astitanium aluminium nitride ortitanium chromium nitride to increase their thermal stability, and prolong tool life.[citation needed]
Tungsten carbide, in its monolithic sintered form, or much more often in cemented tungsten carbide cobalt composite (see above), is often used inarmor-piercing ammunition, especially wheredepleted uranium is not available or is politically unacceptable.W2C projectiles were first used by GermanLuftwaffetank-hunter squadrons inWorld War II. However, owing to the limited German reserves of tungsten,W2C material was reserved for making machine tools and small numbers ofprojectiles. It is an effective penetrator due to its combination of great hardness and very high density.[35][36]
Tungsten carbide ammunition is now generally of thesabot type. SLAP, orsaboted light armour penetrator, where a plastic sabot discards at the barrel muzzle, is one of the primary types of saboted small arms ammunition. Non-discarding jackets, regardless of the jacket material, are not perceived as sabots but as bullets. Both of the designs are, however, common in designated light armor-piercing small arms ammunition. Discarding sabots such as are used with M1A1 Abrams main gun are more commonplace in precision high-velocity gun ammunition.[37][38]
Tungsten carbide is used extensively in mining in top hammer rock drill bits,downhole hammers,roller-cutters,long wall plough chisels,long wall shearer picks,raiseboring reamers, andtunnel boring machines. In these applications it is also used for wear and corrosion resistant components in inlet control for well screens, sub-assemblies, seal rings and bushings common in oil and gas drilling.[39] It is generally utilised as a button insert, mounted in a surrounding matrix of steel that forms the substance of the bit. As the tungsten carbide button is worn away the softer steel matrix containing it is also worn away, exposing yet more button insert.[citation needed]
Tungsten carbide is also an effectiveneutron reflector and as such was used during early investigations into nuclear chain reactions, particularly for weapons. Acriticality accident occurred atLos Alamos National Laboratory on 21 August 1945 whenHarry Daghlian accidentally dropped a tungsten carbide brick onto aplutonium sphere, known as thedemon core, causing thesubcritical mass to go supercritical with the reflectedneutrons. He fell into a coma and died 25 days after the accident.[40][41][42]
Trekking poles, used by manyhikers 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); carbide tips last much longer than other types of tip.[43]
Whileski pole tips are generally not made of carbide, since they do not need to be especially hard even to break through layers of ice, rollerski tips usually are.Roller skiing emulatescross country skiing and is used by many skiers to train during warm weather months.[citation needed]
Sharpened carbide tipped spikes (known as studs) can be inserted into the drive tracks ofsnowmobiles. 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.[44]
Car, motorcycle and bicycletires with tungsten carbide studs provide better traction on ice. They are generally preferred to steel studs because of their superior resistance to wear.[45]
Tungsten carbide may be used infarriery, the shoeing ofhorses, to improve traction on slippery surfaces such as roads or ice. Carbide-tipped hoof nails may be used to attach theshoes;[46] in the United States, borium – chips of tungsten carbide in a matrix of softer metal such asbronze or mild steel – may be welded to small areas of the underside of the shoe before fitting.[47]: 73
Tungsten carbide is also used for making surgical instruments meant for open surgery (scissors, forceps, hemostats, blade-handles, etc.) andlaparoscopic surgery (graspers, scissors/cutter, needle holder, cautery, etc.). They are much costlier than their stainless-steel counterparts and require delicate handling, but give better performance.[48]
Tungsten carbide, typically in the form of acemented carbide (carbide particlesbrazed together by metal), has become a popular material in the bridal jewelry industry due to its extreme hardness and high resistance to scratching.[49][50] Even with high-impact resistance, this extreme hardness also means that it can occasionally be shattered.[51] Some consider this useful, since an impact would shatter a tungsten ring, quickly removing it, where precious metals would bend flat and require cutting. Tungsten carbide is roughly 10 times harder than 18k gold. In addition to its design and high polish, part of its attraction to consumers is its technical nature.[49] Special tools, such as locking pliers, may be required if such a ring must be removed quickly (e.g. due to medical emergency following a hand injury accompanied by swelling).[52]
Tungsten carbide is widely used to make the rotating ball in the tips ofballpoint pens that disperse ink during writing.[53]
English guitaristMartin Simpson uses a custom-made tungsten carbideguitar slide, which gives it superiorsustain and volume.[54]
Tungsten carbide has been investigated for its potential use as acatalyst and it has been found to resembleplatinum in its catalysis of the production of water from hydrogen and oxygen at room temperature, thereduction oftungsten trioxide by hydrogen in the presence of water, and theisomerisation of 2,2-dimethylpropane to 2-methylbutane.[55] It has been proposed as a replacement for theiridium catalyst inhydrazine-poweredsatellite thrusters.[56]
A tungsten carbide coating has been utilized on brake discs in high performance automotive applications to improve performance, increase service intervals and reduce brake dust.[57]
The primary health risks associated with tungsten carbide relate to inhalation of dust, leading tosilicosis-like pulmonaryfibrosis.[58] Cobalt-cemented tungsten carbide is also anticipated to be a humancarcinogen by the AmericanNational Toxicology Program.[59]
