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TUNGSTEN CARBIDE
From Wikipedia, the free encyclopedia
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.
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.
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:
tungsten hexachloride with hydrogen, as reducing agent
and methane as the source of carbon at 670 °C (1,238 °F)
WCl6 + H2 + CH4 → WC + 6HCl
reacting tungsten hexafluoride with hydrogen as reducing
agent and methanol as source of carbon at 350 °C (662
°F)
WF6 + 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.
Oxidation of WC starts at 500–600 °C. 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 (752 °F) and is
unreactive to dry H2 up to its melting point.
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. It has been proposed as a replacement
for the iridium catalyst in hydrazine powered satellite
thrusters.
Physical properties
Tungsten carbide is high melting, 2,870 °C (5,200 °F),
extremely hard 8.5–9.0 Mohs scale[citation needed] at 22
GPa Vickers hardness with low electrical resistivity
(1.7–2.2x10-7 ohm-m), comparable with metals (e.g
vanadium 1.99x10-7 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, and a
cubic high temperature form, β-WC, which has the rock
salt structure. 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. From the unit cell dimensions the
following bond lengths can be determined; the distance
between the tungsten atoms in an 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 (218pm) 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 184W12C.
Toxicity
The primary health risks associated with carbide relate
to inhalation of dust, leading to fibrosis.
Applications
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. Most
of the time, 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.
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.
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.
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, 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.
Domestic
Tungsten carbide is used as the rotating ball in the
tips of ballpoint pens to disperse ink during writing.
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.
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.
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