CHEMISTRY OF CARBIDES

Carbide

carbide,  any of a class of chemical compounds in which carbon is combined with a metallic or semimetallic element. Calcium carbide is important chiefly as a source of acetylene and other chemicals, whereas the carbides of silicon, tungsten, and several other elements are valued for their physical hardness, strength, and resistance to chemical attack even at very high temperatures. Iron carbide (cementite) is an important constituent of steel and cast iron.

Preparation of carbides :

Carbides: Carbides are binary compounds of carbon with elements of lower or about equal electronegativity.

Preparation : Carbides are generally prepared by heating the elements orits oxide with carbon or hydrocarbon at very high temperatures.

 Ca + 2C ——→ BaC₂; 2Li + 2C ——→ Li₂C₂

CaO + 3C ——→ CaC2 + CO

4Li + C₂H₂ ——→ Li₂C₂ + LiH

Carbides are classified into three types on the basis of chemical bonding.

Carbide

Classification of carbides :

Classification of carbides based on structural type is rather difficult, but three broad classifications arise from general trends in their properties. The most electropositive metals form ionic or saltlike carbides, the transition metals in the middle of the periodic table tend to form what are called interstitial carbides, and the nonmetals of electronegativity similar to that of carbon form covalent or molecular carbides.

Ionic carbides

Ionic carbides have discrete carbon anions of the forms C⁴⁻, sometimes called methanides since they can be viewed as being derived from methane, (CH₄); C₂²⁻, called acetylides and derived from acetylene (C₂H₂); and C₃⁴⁻, derived from allene (C₃H₄). The best-characterized methanides are probably beryllium carbide (Be₂C) and aluminum carbide (Al₄C₃). Beryllium oxide (BeO) and carbon react at 2,000 °C (3,600 °F) to produce the brick-red beryllium carbide, whereas pale yellow aluminum carbide is prepared from aluminum and carbon in a furnace. Aluminum carbide reacts as a typical methanide with water to produce methane.Al₄C₃ + 12H₂O → 4Al(OH)₃ + 3CH₄

There are many acetylides that are well known and well characterized. In addition to those of the alkali metals and the alkaline-earth metals mentioned above, lanthanum (La) forms two different acetylides, and copper (Cu), silver (Ag), and gold (Au) form explosive acetylides. Zinc (Zn), cadmium (Cd), and mercury (Hg) also form acetylides, although they are not as well characterized. The most important of these compounds is calcium carbide, CaC₂. The primary use for calcium carbide is as a source of acetylene for use in the chemical industry. Calcium carbide is synthesized industrially from calcium oxide (lime), CaO, and carbon in the form of coke at about 2,200 °C (4,000 °F). Pure calcium carbide has a high melting point (2,300 °C [4,200 °F]) and is a colourless solid. The reaction of CaC₂ with water yields C₂H₂ and a significant amount of heat, so the reaction is carried out under carefully controlled conditions.CaO + 3C → CaC₂ + CO CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂ Calcium carbide also reacts with nitrogen gas at elevated temperatures (1,000–1,200 °C [1,800–2,200 °F]) to form calcium cyanamide, CaCN₂.CaC₂ + N₂ → CaCN₂ + C This is an important industrial reaction because CaCN₂ finds extensive use as a fertilizer owing to its reaction with water to produce cyanamide, H₂NCN. Most MC₂ acetylides have the CaC₂ structure, which is derived from the cubic sodium chloride (NaCl) structure. The C₂ units lie parallel along the cell axes, causing a distortion of the cell from cubic to tetragonal.

Salt-like (saline) carbides :

Salt-like carbides are composed of highly electropositive elements such as the alkali metals, alkaline earths, and group 3 metals including scandium, yttrium and lanthanum. Aluminum 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 graphite intercalation compound KC₈, prepared from vapour of potassium and graphite, and the alkali metal derivatives of C₆₀ are not usually classified as carbides.

Methanides

Tungsten carbide drill bits.

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

The reaction of transition metal carbides with water is very slow and is usually neglected. For example, depending on surface porosity, 5–30 atomic layers of titanium carbide are hydrolyzed within 5 minutes at ambient conditions, following by saturation of the reaction.

Acetylides

Sesquicarbides :

The polyatomic ion C₃^4–, sometimes called sesquicarbide, is found in Li₄C₃ and Mg₂C₃. The ion is linear and is isoelectronic with CO₂. The C-C distance in Mg₂C₃ is 133.2 pm. Mg₂C₃ yields methyl acetylene, CH₃CCH, and propadiene , CH₂CCH₂, 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.
• 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.

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

Molecular carbides :

The complex [Au₆C(PPh₃)₆]²⁺, containing a carbon-gold core.

Some metals, such as lead and tin, are believed not to form carbides under any circumstances. There exists however a mixed titanium-tin carbide, which is a two-dimensional conductor. (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.)

Calcium carbide :

Calcium carbide is a chemical compound with the chemical formula of CaC₂. Its main use industrially is in the production of acetylene and calcium cyanamide.

The pure material is colorless, however pieces of technical-grade calcium carbide are grey or brown and consist of only 80-85% of CaC₂ (the rest is CaO, Ca₃P₂, CaS, Ca₃N₂, SiC, etc.). Because of presence of PH₃, NH₃, and H₂S, technical-grade calcium carbide has a distinctive smell which some find unpleasant.

Production

Calcium carbide is produced industrially in an electric arc furnace from a mixture of lime and coke at approximately 2000 °C. This method has not changed since its invention in 1888:

CaO + 3 C → CaC₂ + CO

The high temperature required for this reaction is not practically achievable by traditional combustion, so the reaction is performed in an electric arc furnace with graphite electrodes. The carbide product produced generally contains around 80% calcium carbide by weight. The carbide is crushed to produce small lumps that can range from a few mm up to 50 mm. The impurities are concentrated in the finer fractions. The CaC₂ content of the product is assayed by measuring the amount of acetylene produced on hydrolysis. As an example, the British and German standards for the content of the coarser fractions are 295 L/kg and 300 L/kg respectively. Impurities present in the carbide include phosphide, which produces phosphine when hydrolysed.

Crystal structure :

Pure calcium carbide is a colourless solid. The common crystalline form at room temperature is a distorted rock-salt structure with the C₂²⁻ units lying parallel.

Applications :

Production of acetylene

The reaction of calcium carbide with water, producing acetylene and calcium hydroxide, was discovered by Friedrich Wöhler in 1862.

CaC₂ + 2 H₂O → C₂H₂ + Ca(OH)₂

This reaction is the basis of the industrial manufacture of acetylene, and is the major industrial use of calcium carbide.

In China, acetylene derived from calcium carbide remains a raw material for the chemical industry, in particular for the production of polyvinyl chloride. Locally produced acetylene is more economical than using imported oil. Production of calcium carbide in China has been increasing. In 2005 output was 8.94 million tons, with the capacity to produce 17 million tons.

Carbide lamps

Lit carbide lamp

Calcium carbide is used in carbide lamps, in which water drips on the carbide and the acetylene formed is ignited. These lamps were usable but dangerous in coal mines, where the presence of the flammable gas methane made them a serious hazard. The presence of flammable gases in coal mines led to miner safety lamps such as the Davy lamp, in which a wire gauze reduces the risk of methane ignition. However, carbide lamps were still used extensively in slate, copper, and tin mines, where methane is not a serious hazard, but most miner's lamps have now been replaced by electric lamps.

Carbide lamps are still used for mining in some less wealthy countries, for example in the silver mines near Potosí, Bolivia.

Other uses

In the ripening of fruit, calcium carbide is sometimes used as source of acetylene gas, which is a ripening agent similar to ethylene.

Calcium carbide is used in toy cannons such as the Big-Bang Cannon, as well as in bamboo cannons.

Calcium carbide, together with calcium phosphide, is used in floating, self-igniting naval signal flares, such as those produced by the Holmes' Marine Life Protection Association.

In the Netherlands, calcium carbide is still used in a traditional New Year's Eve custom called Carbidschieten (Carbide Shooting). To create an explosion, carbide and water are put in a milk churn with a lid, with ignition usually done with a torch. Some villages in the Netherlands fire multiple milk churns in a row. The custom may be derived from an old pagan religious practice intended to chase off spirits.

tremely hard like diamond and possess very high melting points.

Boron Carbide :

Boron Carbide is one of the hardest materials known, ranking third behind diamond and cubic boron nitride. It is the hardest material produced in tonnage quantities.  Originally discovered in mid 19^th century as a by-product in the production of metal borides, boron carbide was only studied in detail since 1930.

Boron carbide powder (see figure 1) is mainly produced by reacting carbon with B₂O₃ in an electric arc furnace, through carbothermal reduction or by gas phase reactions.  For commercial use B₄C powders usually need to be milled and purified to remove metallic impurities.

In common with other non-oxide materials boron carbide is difficult to sinter to full density, with hot pressing or sinter HIP being required to achieve greater than 95% of theoretical density. Even using these techniques, in order to achieve sintering at realistic temperatures (e.g. 1900 - 2200°C), small quantities of dopants such as fine carbon, or silicon carbide are usually required.

Property
Density (g.cm-3)	2.52
Melting Point (°C)	2445
Hardness (Knoop 100g) (kg.mm-2)	2900-3580
Fracture Toughness (MPa.m-½)	2.9 - 3.7
Young's Modulus (GPa)	450 - 470
Electrical Conductivity (at 25°C) (S)	140
Thermal Conductivity (at 25°C) (W/m.K)	30 - 42
Thermal Expansion Co-eff. x10-6 (°C)	5
Thermal neutron capture cross section (barn)	600

As an alternative, B₄C can be formed as a coating on a suitable substrate by vapour phase reaction techniques e.g. using boron halides or di-borane with methane or another chemical carbon source.

Key Properties:

Boron carbide is characterised by its:

•         Extreme hardness

•         Difficult to sinter to high relative densities without the use of sintering aids

•         Good chemical resistance                                                                                                                                                                                                         Table 1. Typical properties of boron carbide

•         Good nuclear properties

•         Low density

Typical properties for boron carbide are listed in table 1.

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Applications :

Abrasives

Due to its high hardness, boron carbide powder is used as an abrasive in polishing and lapping applications, and also as a loose abrasive in cutting applications such as water jet cutting. It can also be used for dressing diamond tools.

Nozzles

The extreme hardness of boron carbide gives it excellent wear and abrasion resistance and as a consequence it finds application as nozzles for slurry pumping, grit blasting and in water jet cutters (see figure 2).

Nuclear applications

Its ability to absorb neutrons without forming long lived radio-nuclides make the material attractive as an absorbent for neutron radiation arising in nuclear power plants. Nuclear applications of boron carbide include shielding, and control rod and shut down pellets.

Ballistic Armour

Boron carbide, in conjunction with other materials also finds use as ballistic armour (including body or personal armour) where the combination of high hardness, high elastic modulus, and low density give the material an exceptionally high specific stopping power to defeat high velocity projectiles.

Other Applications

Other applications include ceramic tooling dies, precision toll parts, evaporating boats for materials testing and mortars and pestles.

Aluminium carbide :

Aluminium carbide, chemical formula Al₄C₃, is a carbide of aluminium. It has the appearance of pale yellow to brown crystals. It is stable up to 1400 °C. It decomposes in water with the production of methane.

Structure

Aluminium carbide has an unusual crystal structure that consists of two types of layers. It is based on AlC₄ tetrahedra of two types and thus two types of carbon atoms. One is surrounded by a deformed octahedron of 6 Al atoms at a distance of 217 pm. The other is surrounded by 4 Al atoms at 190–194 pm and a fifth Al atom at 221 pm. Other carbides (IUPAC nomenclature: methides) also exhibit complex structures.

Reactions

Aluminium carbide hydrolyses with evolution of methane. The reaction proceeds at room temperature but is rapidly accelerated by heating.

Al₄C₃ + 12 H₂O → 4 Al(OH)₃ + 3 CH₄

Similar reactions occur with other protic reagents:

Al₄C₃ + 12 HCl → 4 AlCl₃ + 3 CH₄

Preparation

Aluminium carbide is prepared by direct reaction of aluminium and carbon in an electric furnace.

4 Al + 3 C → Al₄C₃

An alternative reaction begins with alumina, but it is less favorable because of generation of carbon monoxide.

2 Al₂O₃ + 9 C → Al₄C₃ + 6 CO

Silicon carbide also reacts with aluminium to yield Al₄C₃. This conversion limits the mechanical applications of SiC, because Al₄C₃ is more brittle than SiC.

4 Al + 3 SiC → Al₄C₃ + 3 Si

In aluminium-matrix composites reinforced with silicon carbide, the chemical reactions between silicon carbide and molten aluminium generate a layer of aluminium carbide on the silicon carbide particles, which decreases the strength of the material, although it increases the wettability of the SiC particles.  This tendency can be decreased by coating the silicon carbide particles with a suitable oxide or nitride, preoxidation of the particles to form a silica coating, or using a layer of sacrificial metal.

An aluminium-aluminium carbide composite material can be made by mechanical alloying, by milling aluminium powder with graphite particles.

Occurrence :

Small amounts of aluminium carbide are a common impurity of technical calcium carbide. In electrolytic manufacturing of aluminium, aluminium carbide forms as a corrosion product of the graphite electrodes.

In metal matrix composites based on aluminium matrix reinforced with metal carbides (silicon carbide, boron carbide, etc.) or carbon fibers, aluminium carbide often forms as an unwanted product. In case of carbon fiber, it reacts with the aluminium matrix at temperatures above 500 °C; better wetting of the fiber and inhibition of chemical reaction can be achieved by coating it with e.g. titanium boride.

Applications :

Aluminium carbide particles finely dispersed in aluminium matrix lower the tendency of the material to creep, especially in combination with silicon carbide particles.

Aluminium carbide can be used as an abrasive in high-speed cutting tools. It has approximately the same hardness as topaz.

Tungsten carbide :

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

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 :

α-WC structure, carbon atoms are gray.

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 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 dimensionsthe 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

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, as well as 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. W₂C projectiles were first used by German Luftwaffe tank-hunter squadrons 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. 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.

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 K. Daghlian, Jr. accidentally dropped a tungsten carbide brick onto a plutonium sphere, causing the subcritical mass to go supercritical with the reflected neutrons.

Surgical instruments

It 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, also called cemented carbide, has become a popular material in the bridal jewelry industry due to its extreme hardness and high resistance to scratching.