100% Silver Products

Cermet's unique processing technologies allow the precipitation of monosized, highly spherical powders which are particularly useful as multilayer capacitor electrodes, especially the new generation of ULF dielectrics being introduced into the marketplace. The smooth, dense laydowns which can be obtained make these products ideal for hybrid applications as well.

Besides our spherical materials, Cermet also manufactures a line of silver powders, flakes and powder/flake combinations. These materials are well suited to the manufacture of terminations for numerous types of passive components.

100% Palladium Powders

Cermet has the capability of producing palladium powders in a wide variety of surface areas. These powders are produced using Cermet's proprietary chemical precipitation techniques and yield spherical, monosized, highly dispersed powders. By carefully controlling our processes, Cermet can produce these powders as:

• Crystalline Powder

• Oxidation Resistant (B series)

• Controlled Sintering

• Controlled Wetting Characteristics

Cermet is committed to the environment, safety, and quality. Through our ISO 9001:2008 Quality Management System, Cermet meets all the requirements of this important standard.

Cermet provides:

1. High quality products

2. On-time and in-full delivery

3. Quick responses to our customers’ requests.

Our ISO 14001:2004 Environmental Management System provides a healthy and safe workplace through proper treatment of our employees and the environment.

Cermet:

1) Adheres to all environmental regulations

2) Recycles all forms of usable waste

3) Trains employees in safe practices and environmental awareness

4) Continually strives to improve our environmental stewardship and safety record.

UNIT 5

CARBIDES

Hardmetal is the term used to signify a group of sintered, hard, wear-resisting materials based on the carbides of one or more of the elements tungsten, tantalum, titanium, molybdenum, niobium and vanadium, bonded with a metal of lower melting point usually cobalt. Tungsten carbide is the most widely used.

In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and “intermediate” transition metal carbides. Examples include calcium carbide, silicon carbide, tungsten carbide (often called simply carbide), and cementite, each used in key industrial applications.

Salt-like carbides. These carbides are composed of highly electropositive elements such as the alkali metals, alkaline earths, and group 3 metals including scandium, yttrium and lanthanum. Aluminium from group 13 forms carbides, but gallium, indium and thallium do not. These materials feature isolated carbon centers, often described as "C⁴⁻", in the methanides or methides; two atom units, "C₂²⁻" in the acetylides; and three atom units "C₃⁴⁻" in the sesquicarbides. The naming of ionic carbides is not consistent and can be quite confusing.

Methanides. Carbides of this class decompose in water producing methane. Two such examples are aluminium carbide Al₄C₃ and beryllium carbide Be₂C.

Methanides in general chemical context refers to any compound that hydrolyzes to methane, which might include also salts with hydrogenated anions such as CH³⁻, CH2−2, and CH−3. However, according to IUPAC systematic naming conventions, only the last is properly called "methanide". In theory one can describe compounds that contain the methyl group, with relatively large bond polarity between the carbon and non-hydrogen atom, as salts of this anion; however in truth most such compounds, if not all, are, in fact covalent.

Acetylides. Several carbides are assumed to be salts of the acetylide anion C₂^2– (also called percarbide), which has a triple bond between the two carbon atoms. Alkali metals, alkaline earth metals, and lanthanoid metals form acetylides, e.g., sodium carbide Na₂C₂, calcium carbide CaC₂, and LaC₂. Lanthanoids also form acetylides with formula M₂C₃. Metals from group 11 too tend to form acetylides, such as copper(I) acetylide and silver acetylide. Carbides of the actinide elements, which have stoichiometry MC₂ and M₂C₃, are also described as salt-like derivatives of C₂^2–. The C-C triple bond length ranges from 109.2 pm in CaC₂ (similar to ethyne), to 130.3 pm in LaC₂ and 134 pm in UC₂. The bonding in LaC₂ has been described in terms of La^III with the extra electron delocalised into the antibonding orbital on C₂²⁻, explaining the metallic conduction.^[1]

Sesquicarbides. The polyatomic ion C₃^4–, sometimes called sesquicarbide, is found in Li₄C₃, Mg₂C₃. The ion is linear and is isoelectronic with CO₂. The C-C distance in Mg₂C₃ is 133.2 pm. Mg₂C₃ yields methylacetylene, CH₃CCH, on hydrolysis which was the first indication that it may contain C₃^4–.

Covalent carbides. The carbides of silicon and boron are described as "covalent carbides", although virtually all compounds of carbon exhibit some covalent character. Silicon carbide has two similar crystalline forms, which are both related to the diamond structure. Boron carbide, B₄C, on the other hand has an unusual structure which includes icosahedral boron units linked by carbon atoms. In this respect boron carbide is similar to the boron rich borides. Both silicon carbide, SiC, (carborundum) and boron carbide, B₄C are very hard materials and refractory. Both materials are important industrially. Boron also forms other covalent carbides, e.g. B₂₅C.

Interstitial carbides. The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as interstitial compounds. These carbides have metallic properties and are refractory. Some exhibit a range of stoichiometries, e.g. titanium carbide, TiC. Titanium carbide and tungsten carbide are important industrially and are used to coat metals in cutting tools. The longheld view is that the carbon atoms fit into octahedral interstices in a close packed metal lattice when the metal atom radius is greater than approximately 135 pm:

When the metal atoms are cubic close packed, (ccp), then filling all of the octahedral interstices with carbon achieves 1:1 stoichiometry with the rock salt structure, (note that in rock salt, NaCl, it is the chloride anions that are cubic close packed).

When the metal atoms are hexagonal close packed, (hcp), as the octahedral interstices lie directly opposite each other on either side of the layer of metal atoms, filling only one of these with carbon achieves 2:1 stoichiometry with the CdI₂ structure.

The following table shows actual structures of the metals and their carbides. (N.B. the body centred cubic structure adopted by vanadium, niobium, tantalum, chromium, molybdenum and tungsten is not a close packed lattice.) The notation "h/2" refers to the M₂C type structure described above, which is only an approximate description of the actual structures. The simple view that the lattice of the pure metal "absorbs" carbon atoms can be seen to be untrue as the packing of the metal atom lattice in the carbides is different from the packing in the pure metal, although it is technically correct that the carbon atoms fit into the octahedral interstices of a close-packed metal lattice.

Metal	Structure of pure metal	Metallic radius (pm)	MC metal atom packing	MC structure	M2C metal atom packing	M2C structure	Other carbides
titanium	hcp	147	ccp	rock salt
zirconium	hcp	160	ccp	rock salt
hafnium	hcp	159	ccp	rock salt
vanadium	cubic body centered	134	ccp	rock salt	hcp	h/2	V4C3
niobium	cubic body centered	146	ccp	rock salt	hcp	h/2	Nb4C3
tantalum	cubic body centered	146	ccp	rock salt	hcp	h/2	Ta4C3
chromium	cubic body centered	128					Cr23C6, Cr3C, Cr7C3, Cr3C2
molybdenum	cubic body centered	139		hexagonal	hcp	h/2	Mo3C2
tungsten	cubic body centered	139		hexagonal	hcp	h/2

For a long time the non-stoichiometric phases were believed to be disordered with a random filling of the interstices, however short and longer range ordering has been detected.

Intermediate transition metal carbides. In these carbides, the transition metal ion is smaller than the critical 135 pm, and the structures are not interstitial but are more complex. Multiple stoichiometries are common, for example iron forms a number of carbides, Fe₃C, Fe₇C₃ and Fe₂C. The best known is cementite, Fe₃C, which is present in steels. These carbides are more reactive than the interstitial carbides, for example the carbides of Cr, Mn, Fe, Co and Ni all are hydrolysed by dilute acids and sometimes by water, to give a mixture of hydrogen and hydrocarbons. These compounds share features with both the inert interstitials and the more reactive salt-like carbides.^[1]

Molecular carbides. The complex [Au₆C(PPh₃)₆]²⁺, containing a carbon-gold core. Metal complexes containing C_n fragments are well known. Most common are carbon-centered clusters, such as [Au₆C(PPh₃)₆]²⁺. Similar species are known for the metal carbonyls and the early metal halides. Even terminal carbides have been crystallized, e.g., CRuCl₂(P(C₆H₁₁)₃)₂.

Impossible carbides. Some metals, such as lead and tin, are believed not to form carbides under any circumstances.^[5] There exists however a mixed titanium-tin carbide, which is a two-dimensional conductor.^[6] (In 2007, there were two reports of a lead carbide PbC₂, apparently of the acetylide type; but these claims have yet to be published in reviewed journals.)

Related materials

In addition to the carbides, other groups of related carbon compounds exist:

• graphite intercalation compounds

• alkali metal fullerides

• endohedral fullerenes, where the metal atom is encapsulated inside a fullerene molecule

• metallacarbohedrenes(met-cars) which are cluster compounds containing C₂ units

• tunable nanoporous carbon, where gas chlorination of metallic carbides removes metal molecules to form a highly porous, near-pure carbon material capable of high-density energy storage

• transition metal carbene complexes.

UNIT 6

TUNGSTEN CARBIDE

Carbide is any one of a group of compounds that contain carbon and one other element that is either a metal, boron, or silicon. Generally, a carbide is prepared by heating a metal, metal oxide, or metal hydride with carbon or a carbon compound. Calcium carbide, CaC₂, can be made by heating calcium oxide and coke in an electric furnace; it reacts with water to yield acetylene and is an important source of the gas. Barium carbide reacts similarly. Aluminum carbide reacts with water to yield methane. Some carbides are unaffected by water, e.g., chromium carbide and silicon carbide . Silicon carbide, almost as hard as diamond, is used as an abrasive. Tungsten carbide, also very hard, is used for cutting edges of machine tools. Iron carbides are present in steel, cast iron, and some other iron alloys.

Tungsten carbide is the most widely used.

Tungsten carbide (WC) is an inorganic chemical compound 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, 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.

WC can be prepared by reaction of tungsten metal and carbon at 1400–2000 °C. 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. Chemical vapor deposition methods that have been investigated include: 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. 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. 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. 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 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.

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).

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.^[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.

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

Applications

Machine 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. Carbide generally produces a better finish on the part, and allows faster machining. Carbide tools can also withstand higher temperatures than standard high speed steel tools. 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.

Military

Tungsten carbide is often used in armor-piercing ammunition, especially where depleted uranium is not available or is politically unacceptable. The first use of W₂C projectiles occurred in German Luftwaffe tank-hunter squadrons, which used 37 mm autocannon equipped Junkers Ju 87G dive bomber aircraft to destroy Soviet T-34 tanks 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 for elite combat pilots, like Hans-Ulrich Rudel. It is an effective penetrator due to its combination of great hardness and 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 subcritical mass to go supercritical with the reflected neutrons.

S ports

A Nokian tire 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.

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