Tungsten Carbide Colloidals

Tungsten carbide (chemical formula: WC) is a chemical compound (specifically, a carbide) containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery, cutting tools, abrasives, armor-piercing rounds, other tools and instruments, and jewelry.

Tungsten carbide is approximately two times stiffer than steel, with a Young's modulus of approximately 550 GPa, and is much denser than steel or titanium. It is comparable with corundum (α-Al2O3), sapphire and ruby in hardness and can only be polished and finished with abrasives of superior hardness such as cubic boron nitride and diamond, in the form of powder, wheels, and compounds.

Tungsten carbide products typically have a high resistance to wear and can be used at high temperatures, allowing tungsten carbide's combined hardness and toughness to significantly outperform its steel product equivalents.

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 just tungsten.

Synthesis

WC can be 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 WO3 with CO/CO2 mixture and H2 between 900 and 1200 °C.

WC can also be produced by heating WO3 with graphite: directly at 900 °C or in hydrogen at 670 °C following by carburization in Ar 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, W2C. 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.

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 84.02 W·m−1·K−1, and a coefficient of thermal expansion of 5.8 µm·m−1·K−1.

Tungsten carbide is extremely hard, ranking ~9 on Mohs scale, and with a Vickers number of 1700–2400. It has a Young's modulus of approximately 550 GPa, a bulk modulus of 439 GPa, and a shear modulus of 270 GPa. It has an ultimate tensile strength of 344.8 MPa. It has a Poisson's ratio of 0.234

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

With a low electrical resistivity of (~2×10−7 Ohm·m), tungsten carbide's resistivity is comparable with that of some metals (e.g. vanadium 2×10−7 Ohm·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 fact that these phases are brittle is the reason why control of the carbon content in WC-Co hard metals is 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(CH3)6 (218 pm) in which there is strongly distorted trigonal prismatic coordination of tungsten.

 

Applications

Cutting tools for machining, Tungsten carbide drilling/milling bits, armor-piercing ammunition, an effective neutron reflector and as such was used during early investigations into nuclear chain reactions, particularly for weapons. A Nokian bicycle tire has tungsten carbide spikes. Tungsten carbide, are used by athletes, generally on poles that 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.

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.

Some tire 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.

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.

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.

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.

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

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

Tungsten Carbide Colloidals

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