Tungsten carbide

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Tungsten carbide
Identifiers
CAS number	[12070-12-1]
Properties
Molecular formula	WC
Molar mass	195.86 g·mol−1
Appearance	grey-black solid
Density	15.8 g·cm−3, solid
Melting point	2870 °C, 5198 °F (3143K)
Boiling point	6000°C, 10832 °F (6273K)
Solubility in water	Insoluble
Structure
Crystal structure	Hexagonal
Hazards
EU classification	not listed
Related compounds
Other anions	Tungsten boride Tungsten nitride
Other cations	Molybdenum carbide Titanium carbide Silicon carbide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Tungsten carbide, WC, or tungsten semicarbide, W₂C, is a chemical compound containing tungsten and carbon, similar to titanium carbide. Colloquially, tungsten carbide is often simply called carbide.

Contents 1 Chemical properties 2 Physical Properties 3 Structure 4 Applications 4.1 Machine tools 4.2 Military 4.3 Sports 4.4 Domestic 5 References 6 External links

[edit] Chemical properties

There are two well characterized compounds of tungsten and carbon, WC and W₂C. Both compounds may be present in coatings and the proportions can depend on the coating method, see e.g.^[1]

WC can be prepared by reaction of tungsten metal and carbon at 1400-2000°C.^[2] Other methods include a patented fluid bed process that reacts either tungsten metal or blue WO₃ with CO/CO₂ mixture and H₂ between 900 and 1200°C.^[3] Chemical vapor deposition methods that have been investigated include^[2]:

• tungsten hexachloride with hydrogen, as reducing agent and methane as the source of carbon at 670°C

WCl₆ + H₂ + CH₄ → WC + 6HCl

• reacting tungsten hexafluoride with hydrogen as reducing agent and methanol as source of carbon at 350°C

WCl₆ + H₂ + CH₃OH → WC + 6HF + H₂O

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.^[4]
Oxidation of WC starts at 500-600°C.^[2] It is resistant to acids and is only attacked by hydrofluoric acid/nitric acid (HF/HNO₃) mixtures above room temperature.^[2] It reacts with fluorine gas at room temperature and chlorine above 400°C and is unreactive to dry H₂ up to its melting point.^[2]
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.^[5] It has been proposed as a replacement for the iridium catalyst in hydrazine powered satellite thrusters.^[6]

[edit] Physical Properties

Tungsten carbide is a high melting, 2870°C, extremely hard 8.5 - 9.0 Mohs scale^{[citation needed]} and 22 GPa Vickers hardness with high electrical conductivity (Resistivity 1.7-2.2 10^-7ohm.m) comparable with metals (e.g vanadium 1.99 10^-7ohm.m).^[2][7]
WC is readily wetted by both molten nickel and cobalt.^[8] Investigation of the phase diagram of the W-C-Co system shows that WC and Co form a pseudo binary eutectic. The phase diagram also shows that there are so-called η-carbides with composition (W,Co)₆C that can be formed and the fact that these phases are brittle is the reason why control of the C content in WC-Co hardmetals is important.^[8]

[edit] Structure

There are two forms of WC, a hexagonal form, α-WC,^[9] and a cubic high temperature form, β-WC, which has the rock salt structure.^[10] 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.^[9] From the unit cell dimensions^[11] 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(CH₃)₆ (218pm) in which there is strongly distorted trigonal prismatic coordination of tungsten.^[12]
Molecular WC has been investigated and this gas phase species has a bond length of 171 pm for ¹⁸⁴W¹²C.^[13]

[edit] Applications

[edit] 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 endmills.

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 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 CVD and, for certain applications, with the mechanical PVD method. However if the deposition is performed at too high temperature, an eta phase of a Co₆W₆C tertiary carbide forms at the interface between the carbide and the cobalt phase, facilitating adhesion failure of the coating.

[edit] 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 W₂C 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 WWII. Owing to the limited German reserves of tungsten, W₂C 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 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.

[edit] Sports

Hard carbides, especially tungsten carbide, are used by athletes, generally on poles which impact hard surfaces. Trekking poles, used by many 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.^{[citation needed]}

While ski 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 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.^{[citation needed]}

Some tire manufacturers, such as 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.

[edit] Domestic

Tungsten carbide is used as the rotating ball in the tips of ballpoint pens to disperse ink during writing^[14].

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. ^[15]

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.^{[citation needed]}

[edit] References

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (December 2007)

[edit] External links

• International Chemical Safety Card 1320
• NIOSH Pocket Guide to Chemical Hazards