Monotungsten carbide, WC, or Ditungsten Carbide, W2C, is a chemical
compound containing tungsten and carbon, similar to titanium carbide.
Its extreme hardness makes it useful in the manufacture of cutting
tools, abrasives and bearings, as a cheaper and more heat-resistant
alternative to diamond. Tungsten carbide is also used as a scratch-resistant material for jewelry including watch bands and wedding rings.
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 tungstencarbide particles are the aggregate and metallic cobalt serves as the matrix. The process of combining tungstencarbide with cobalt is referred to as sintering or HIP (Hot Isostatic Pressing). 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 tungstencarbide ceramic, thus raising its toughness and durability. Such parameters of tungstencarbide can be changed significantly within the carbide
manufacturers sphere of influence, primarily determined by grain size,
cobalt content, dotation (e.g. aloy 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(CN) (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. 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.
Tungstencarbide 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.
Tungstencarbide 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.
Tungstencarbide 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 tungstencarbide brick onto a plutonium sphere causing the sub-critical mass to go critical with the reflected neutrons.
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