tungsten carbide

Tungsten carbide
Tungsten carbide milling bits
Identifiers
CAS number	12070-12-1 Y
Properties
Molecular formula	WC
Molar mass	195.86 g·mol−1
Appearance	Grey-black lustrous solid
Density	15.8 g·cm−3, solid
Melting point	2,870 °C, 5,198 °F (3,143 K)
Boiling point	6,000°C, 10,832 °F (6,273 K)
Solubility in water	Insoluble.
Structure
Crystal structure	Hexagonal, hP2, space group = P6m2, No. 187[1]
Hazards
EU classification	Not listed
Related compounds
Other anions	Tungsten boride Tungsten nitride
Other cations	Molybdenum carbide Titanium carbide Silicon carbide
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Tungsten carbide (WC) is an inorganic chemical compound (specifically, a carbide) containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery, tools, abrasives, as well as jewelry. Tungsten carbide is approximately three times stiffer than steel, with a Young's modulus of approximately 550 GPa,^[2] and is much denser than steel or titanium. It is comparable with corundum (α-Al₂O₃) or sapphire in hardness and can only be polished and finished with abrasives of superior hardness such as cubic boron nitride and diamond amongst others, in the form of powder, wheels, and compounds.

Chemical properties

There are two well characterized compounds of tungsten and carbon, WC and tungsten semicarbide, W₂C. Both compounds may be present in coatings and the proportions can depend on the coating method.^[3]

WC can be prepared by reaction of tungsten metal and carbon at 1400–2000 °C.^[4] 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.^[5] Chemical vapor deposition methods that have been investigated include:^[4] WC can also be produced by heating WO₃ with graphite in hydrogen at 670 °C following by carburization in Ar at 1000 °C or directly heating WO₃ with graphite at 900°C.^[6]

• tungsten hexachloride with hydrogen, as a reducing agent, and methane, as the source of carbon at 670 °C (1,238 °F)

WCl₆ + H₂ + CH₄ → WC + 6 HCl

• reacting tungsten hexafluoride with hydrogen, as reducing agent, and methanol, as source of carbon at 350 °C (662 °F)

WF₆ + 2 H₂ + CH₃OH → WC + 6 HF + 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.^[7]

• Oxidation of WC starts at 500–600 °C.^[4] It is resistant to acids and is only attacked by hydrofluoric acid/nitric acid (HF/HNO₃) mixtures above room temperature.^[4] It reacts with fluorine gas at room temperature and chlorine above 400 °C (752 °F) and is unreactive to dry H₂ up to its melting point.^[4]

WC has been investigated for its potential use as a catalyst and it has been found to resemble platinum in its catalysis of 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 isomerisation of 2,2-dimethylpropane to 2-methylbutane.^[8] It has been proposed as a replacement for the iridium catalyst in hydrazine powered satellite thrusters.^[9]

Physical properties

Tungsten carbide is high melting, 2,870 °C (5,200 °F), extremely hard (8.5–9.0 Mohs scale, Vickers hardness number = 2242) with low electrical resistivity (~2×10⁻⁷ Ohm·m), comparable with that of some metals (e.g. vanadium 2×10⁻⁷ Ohm·m).^[4][10]

WC is readily wetted by both molten nickel and cobalt.^[11] 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 carbon content in WC-Co hard metals is important.^[11]

Structure

α-WC structure, carbon atoms are gray.^[1]

There are two forms of WC, a hexagonal form, α-WC (hP2, space group P6m2, No. 187),^[1][12] and a cubic high-temperature form, β-WC, which has the rock salt structure.^[13] 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.^[12] From the unit cell dimensions^[14] 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 in W(CH₃)₆ (218 pm) in which there is strongly distorted trigonal prismatic coordination of tungsten.^[15]

Molecular WC has been investigated and this gas phase species has a bond length of 171 pm for ¹⁸⁴W¹²C.^[16]

Applications

Cutting tools for machining

Sintered tungsten carbide cutting tools were first introduced by Friedrich Krupp of Germany in 1927 under the name Widia (wie Diamant—like diamond).^[17] During World War II there was a tungsten shortage in Germany. It was found that tungsten in carbide cuts metal more efficiently than tungsten in high-speed steel so to economise on the use of tungsten, cemented carbides were used for metal cutting as much as possible.

Carbide tools are very abrasion resistant and can also withstand higher temperatures than standard high speed steel tools. Carbide cutting surfaces are often used for 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. Because carbide tools maintain a sharp cutting edge better than other tools, they generally produce a better finish on parts, and their temperature resistance allows faster machining. The material is usually called cemented carbide, hardmetal or tungsten-carbide cobalt: it is a metal matrix composite where tungsten carbide particles are the aggregate and metallic cobalt serves as the matrix. Manufacturers use tungsten carbide as the main material in some high-speed drill bits, as it can resist high temperatures and is extremely hard. ^[18][19]

Ammunition

Tungsten carbide is often used in armor-piercing ammunition, especially where depleted uranium is not available or is politically unacceptable. W₂C projectiles were first used by German Luftwaffe tank-hunter squadrons in World War II. Owing to the limited German reserves of tungsten, W₂C material was reserved for making machine tools and small numbers of projectiles. It is an effective penetrator due to its combination of great hardness and very high density.^[20][21]

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.^[22][23]

Nuclear

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 subcritical mass to go supercritical with the reflected neutrons.

Sports

A Nokian tyre with tungsten carbide spikes. The spikes are surrounded in aluminum.

Hard carbides, especially tungsten carbide, are used by athletes, generally on poles which strike 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); carbide tips last much longer than other types of tip.^[24]

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.

Sharpened carbide tipped spikes (known as studs) can be inserted into the drive tracks of 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.^[25]

Some tyre manufacturers offer bicycle tires with tungsten carbide studs for better traction on ice. These are generally preferred to steel studs because of their superior resistance to wear.^[26]

Tungsten carbide may be used in farriery, the shoeing of horses, to improve traction on slippery surfaces such as roads or ice. Carbide-tipped hoof nails may be used to attach the shoes,^[27] or alternatively borium, tungsten carbide in a matrix of softer metal, may be welded to small areas of the underside of the shoe before fitting.^[28]

Surgical instruments

It is also used for making surgical instruments meant for open surgery (scissors, forceps, hemostats, blade-handles, etc.) and laparoscopic 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.^[29]

Jewelry

Tungsten carbide, also called cemented carbide, has become a popular material in the bridal jewelry industry due to its extreme hardness and high resistance to scratching. While the material’s hardness contributes to its scratch resistance, this trait translates into low ductility (a type of plasticity) which results in a material that is brittle. This can be improved using a cobalt or nickel binder. Using nickel as the binder also makes the ceramic extremely resistant to corrosion from acids or sea water.^[30]

Other structural factors influence durability. When fashioned into a wedding band, any grooves, plastic, metal inlays, diamonds, precious or non-precious gemstones set within Tungsten Carbide create additional voids ^[31] beyond what is already a non-solid material. These voids, or lack of material, translate into a lack of support under compression or pressure. When Tungsten Carbide is dropped or repeatedly tapped on a hard surface (like a table top or stair railing), a tungsten carbide wedding ring may fracture, chip or break. This is due to fatigue, the combination of low ductility and lack of material support within any given void.

Design variations can make a tungsten carbide wedding band even more prone to fracture. Holes formed to hold gemstones leave a maximum of only two points of support,^[32] while both “grooved"^[33] and “channeled” designs, which are popular within the jewelry industry, further weaken the structural integrity of the ring. When coupled with low ductility, all three design motifs magnify the rings ability to break. Tungsten Carbide is not a solid metal – it is a ceramic. "Cemented carbide" is formed by sintering fine powders (powder metallurgy) of tungsten ( or sometimes tungsten oxide) and graphite which will form the carbide along with powdered nickel (and / or cobalt) based alloys as bonding agent, creating a metal matrix composite through high pressure. It is the gaps between bonding agent and aggregate that creates the brittleness that leads to structural failure. A SEM micrograph of cobalt and of tungsten carbide reveals the crystal-lattice differences.

Other

Tungsten carbide is sometimes used to make the rotating ball in the tips of ballpoint pens that disperse ink during writing.^[34]

Toxicity

The primary health risks associated with carbide relate to inhalation of dust, leading to fibrosis.^[35] Cobalt–Tungsten Carbide is also reasonably anticipated to be a human carcinogen by the National Toxicology Program.^[36]

References

External links

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