Tungsten carbide

Naming

Historically referred to as Wolfram, Wolf Rahm, wolframite ore discovered by Peter Woulfe was then later carburized and cemented with a binder creating a composite now called "cemented tungsten carbide". Tungsten is Swedish for "heavy stone".

Colloquially among workers in various industries (such as machining and carpentry), tungsten carbide is often simply called carbide, despite the inaccuracy of the usage. Among the lay public, the growing popularity of tungsten carbide rings has also led to consumers calling the material tungsten.

Synthesis

Tungsten carbide is prepared by reaction of tungsten metal and carbon at 1400–2000 °C. Other methods include a patented lower temperature fluid bed process that reacts either tungsten metal or blue WO
3 with CO/CO₂ mixture and H
2 between 900 and 1200 °C.

WC can also be produced by heating WO
3 with graphite: directly at 900 °C or in hydrogen at 670 °C following by carburization in argon at 1000 °C. Chemical vapor deposition methods that have been investigated include:

Chemical properties

There are two well-characterized compounds of tungsten and carbon, WC and tungsten semicarbide, W
2C. Both compounds may be present in coatings and the proportions can depend on the coating method.

At high temperatures WC decomposes to tungsten and carbon and this can occur during high-temperature thermal spray, e.g., in high velocity oxygen fuel (HVOF) and high energy plasma (HEP) methods.

Oxidation of WC starts at 500–600 °C (932–1,112 °F). It is resistant to acids and is only attacked by hydrofluoric acid/nitric acid (HF/HNO
3) mixtures above room temperature. It reacts with fluorine gas at room temperature and chlorine above 400 °C (752 °F) and is unreactive to dry H
2 up to its melting point. Finely powdered WC oxidizes readily in hydrogen peroxide aqueous solutions. At high temperatures and pressures it reacts with aqueous sodium carbonate forming sodium tungstate, a procedure used for recovery of scrap cemented carbide.

Physical properties

Tungsten carbide has a high melting point at 2,870 °C (5,200 °F), a boiling point of 6,000 °C (10,830 °F) when under a pressure equivalent to 1 standard atmosphere (100 kPa), a thermal conductivity of 110 W·m⁻¹·K⁻¹, and a coefficient of thermal expansion of 5.5 µm·m⁻¹·K⁻¹.

Tungsten carbide is extremely hard, ranking about 9 on Mohs scale, and with a Vickers number of around 2600. It has a Young's modulus of approximately 530–700 GPa, a bulk modulus of 630–655 GPa, and a shear modulus of 274 GPa. It has an ultimate tensile strength of 344 MPa, an ultimate compression strength of about 2.7 GPa and a Poisson's ratio of 0.31.

The speed of a longitudinal wave (the speed of sound) through a thin rod of tungsten carbide is 6220 m/s.

Tungsten carbide's low electrical resistivity of about 0.2 µΩ·m is comparable with that of some metals (e.g. vanadium 0.2 µΩ·m).

WC is readily wetted by both molten nickel and cobalt. 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 brittleness of these phases makes control of the carbon content in WC-Co hard metals important.

Structure

There are two forms of WC, a hexagonal form, α-WC (hP2, space group P6m2, No. 187), and a cubic high-temperature form, β-WC, which has the rock salt structure. 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. From the unit cell dimensions 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(CH
3)
6 (218 pm) in which there is strongly distorted trigonal prismatic coordination of tungsten.

Molecular WC has been investigated and this gas phase species has a bond length of 171 pm for 184
W12
C.

Cutting tools for machining

Sintered tungsten carbide cutting 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, and 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.

Ammunition

Tungsten carbide is often used in armor-piercing ammunition, especially where depleted uranium is not available or is politically unacceptable. However, it is also common to use powder metallurgic tungsten alloys (in which metallic tungsten powder has been cemented with a metallic binder). W
2C projectiles were first used by German Luftwaffe tank-hunter squadrons in World War II. Owing to the limited German reserves of tungsten, W
2C material was reserved for making machine tools and small numbers of projectiles. Tungsten carbide-cored HVAP (high-velocity armor-piercing) ammunition allowed the U.S. Army's 3-inch, 76mm, and 90mm anti-tank guns to knock out heavily armored German tanks, which they could not do with more conventional ammunition. It is an effective penetrator due to its combination of great hardness and very high density.

Tungsten carbide in its monolithic sintered form, or much more often in tungsten carbide cobalt composite (in which fine ceramic tungsten carbide particles are embedded in metallic cobalt binder forming a metal matrix composite or MMC) can be of the sabot type. SLAP, or saboted 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 bullets. Both of the designs are, however, common in designated light armor-piercing small arms ammunition.

Discarding sabots like used with M1A1 Abrams main gun are more commonplace in precision high-velocity gun munition.

Mining

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, and tunnel boring machines. 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.

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 Daghlian accidentally dropped a tungsten carbide brick onto a plutonium sphere, known as the demon core, causing the subcritical mass to go supercritical with the reflected neutrons.

Sports

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.

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.

Car, motorcycle and bicycle tires with tungsten carbide studs provide better traction on ice. The tungsten carbide insert protruding from inside of a zinc or aluminium seating is commonly called a "soul" in the tire manufacturing business. These are generally preferred to steel studs because of their superior resistance to wear.

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, or alternatively Borium, a trademark for tungsten carbide in a matrix of softer metal, may be welded to small areas of the underside of the shoe before fitting.

Surgical instruments

Tungsten carbide 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.

Jewelry

Tungsten carbide, typically in the form of a cemented carbide (carbide particles brazed together by metal), has become a popular material in the bridal jewelry industry due to its extreme hardness and high resistance to scratching. Even with high-impact resistance, this extreme hardness also means that it can occasionally be shattered under certain circumstances. 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. Special tools may be required if such a ring must be removed quickly (e. g. due to medical emergency following a hand injury accompanied by swelling).

Other

Tungsten carbide is widely used to make the rotating ball in the tips of ballpoint pens that disperse ink during writing.

Tungsten carbide is a common material used in the manufacture of gauge blocks, used as a system for producing precision lengths in dimensional metrology.

English guitarist Martin Simpson is known to use a custom-made tungsten carbide guitar slide. The hardness, weight, and density of the slide give it superior sustain and volume compared to standard glass, steel, ceramic, or brass slides.

Tungsten carbide 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. It has been proposed as a replacement for the iridium catalyst in hydrazine-powered satellite thrusters.

Toxicity

The primary health risks associated with carbide relate to inhalation of dust, leading to fibrosis. Cobalt–cemented tungsten carbide is also reasonably anticipated to be a human carcinogen by the National Toxicology Program.