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 C22– (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 C22–. 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 LaIII with the extra electron delocalised into the antibonding orbital on C22−, explaining the metallic conduction.[1]
Sesquicarbides. The polyatomic ion C34–, 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 C34–.
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 | ccp | rock salt | ||||
zirconium | hcp | ccp | rock salt | ||||
hafnium | hcp | ccp | rock salt | ||||
vanadium | cubic body centered | ccp | rock salt | hcp | h/2 | V4C3 | |
niobium | cubic body centered | ccp | rock salt | hcp | h/2 | Nb4C3 | |
tantalum | cubic body centered | ccp | rock salt | hcp | h/2 | Ta4C3 | |
chromium | cubic body centered | Cr23C6, Cr3C, | |||||
molybdenum | cubic body centered | hexagonal | hcp | h/2 | Mo3C2 | ||
tungsten | cubic body centered | 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 Cn 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.
Sports
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.
Domestic
Tungsten carbide is sometimes used to make the rotating ball in the tips of ballpoint pens that disperse ink during writing.
A tungsten carbide ring
Tungsten carbide can now be found in the inventory of some jewelers, most notably as a primary material in men's wedding rings. When used in this application the bands appear with a lustrous dark hue often buffed to a mirror finish. The color is more similar to that of hematite than to that of platinum. The finish is highly resistant to scratches and scuffs, holding its mirror-like shine for years. Although it is possible to inlay precious metals, woods, and other materials, these are less scratch-resistant than tungsten carbide.
A common misconception concerning tungsten carbide rings is that they cannot be removed in the case of emergency medical treatment, requiring the finger to be removed instead. This is not true. Emergency care providers have specialized tools that allow them to easily break tungsten rings into large pieces. As a result, it is easier and less dangerous to remove tungsten carbide rings than gold, silver, or titanium without injuring the hand or finger. An easier way to remove tungsten carbide rings is to use a tool such as a vise, which can be used to shatter the ring.
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.
Toxicity. The primary health risks associated with carbide relate to inhalation of dust, leading to fibrosis. Cobalt–Tungsten Carbide is also reasonably anticipated to be a human carcinogen by the National Toxicology Program.
UNIT 7 | CEMENTED CARBIDES |
A unique material
Cemented Carbide is one of the most successful composite engineering materials ever produced. Cemented Carbide's unique combination of strength, hardness and toughness satisfies the most demanding applications.
A key feature of the Cemented Carbide is the potential to vary its composition so that the resulting physical and chemical properties ensure maximum resistance to wear, deformation, fracture, corrosion, and oxidation. In addition, the wide variety of shapes and sizes that can be produced using modern powder metallurgical processing offers tremendous scope to design cost-effective solutions to many of the problems of component wear and failure encountered in both the engineering and domestic environment.
What are the different types of Cemented Carbide?
The Cemented Carbides are a range of composite materials, which consist of hard carbide particles bonded together by a metallic binder.
The proportion of carbide phase is generally between 70-97% of the total weight of the composite and its grain size averages between 0.4 and 10 μm.
Tungsten carbide (WC), the hard phase, together with cobalt (Co), the binder phase, forms the basic Cemented Carbide structure from which other types of Cemented Carbide have been developed. In addition to the straight tungsten carbide – cobalt compositions – Cemented Carbide may contain varying proportions of titanium carbide (TiC), tantalum carbide (TaC) and niobium carbide (NbC). These carbides are mutually soluble and can also dissolve a high proportion of tungsten carbide. Also, Cemented Carbides are produced which have the cobalt binder phase alloyed with, or completely replaced by, other metals such as iron (Fe), chromium (Cr), nickel (Ni), molybdenum (Mo), or alloys of these elements.
There are three individual phases which make up Cemented Carbide. In metallurgical terms, the tungsten carbide phase (WC) is referred to as the a-phase (alpha), the binder phase (i.e. Co, Ni etc.) as the b-phase (beta), and any other single or combination of carbide phases (TiC, Ta/NbC etc) as the g-phase (gamma). Other than for metal cutting applications, there is no internationally accepted classification of Cemented Carbides. |
|
WC-Co grades
This group of Cemented Carbides contains WC and Co only (i.e. two phases) and a few trace elements. These grades are classified according to their cobalt content and WC grain size. The grades with binder content in the range 10-20% by weight and WC-grain sizes between 1 and 5 μm have high strength and toughness, combined with good wear resistance.
The microstructure of wc-co grade
The grades with binder contents in the range 3-15% and grain sizes below 1 μm have high hardness and compressive strength, combined with exceptionally high wear resistance.The Sandvik grade program also includes WC-Co grades which utilize a range of ultra-fine WC grain sizes (< 0.5 μm). With such fine, uniform grain sizes, a unique combination of hardness, wear resistance and toughness can be achieved.
Microstructure of γ-phase grade
The grades of Cemented Carbides in this group contain WC and Co as the main elements, although small additions or trace levels of other elements are often added to optimize properties. These grades are classified according to their Cobalt content and WC grain size and are often called the "straight grades". They have the widest range of strength and toughness of all the Cemented Carbide types and this is in combination with excellent wear resistance. This range of Cemented Carbides can be subdivided into its major application areas as follows:
How are Cemented Carbides made?
The manufacturing process begins with the composition of a specific tungsten carbide powder mixture - tailored for the application.
The tungsten carbide powder is compacted into a form.
In a high-temperature sintering furnace, the tungsten carbide structure of the blank is shaped at precise temperatures for strictly defined periods. During this heat treatment, the tungsten carbide blank undergoes shrinkage of some 50% in volume.
The sintered Cemented Carbide component gains its final finish by additional grinding, lapping and/or polishing processes.
The main use of tungsten (in the form of tungsten carbide) is in the manufacture of cemented carbides. After Scheele’s discovery of "Tungsten" in 1781, it took an additional 150 years before his successors’ efforts led to the application of tungsten carbide in the industry.
Cemented carbides, or hardmetals as they are often called, are materials made by "cementing" very hard tungsten monocarbide (WC) grains in a binder matrix of tough cobalt metal by liquid phase sintering.
Microstructure of a Wc-Co Cemented Carbide
The combination of WC and metallic cobalt as a binder is a well-adjusted system not only with regard to its properties, but also to its sintering behaviour.
The high solubility of WC in cobalt at high temperatures and a very good wetting of WC by the liquid cobalt binder result in an excellent densification during liquid phase sintering and in a pore-free structure. As a result of this, a material is obtained which combines high strength, toughness and high hardness.
The beginning of tungsten carbide production may be traced to the early 1920’s, when the German electrical bulb company, Osram, looked for alternatives to the expensive diamond drawing dies used in the production of tungsten wire.
These attempts led to the invention of cemented carbide, which was soon produced and marketed by several companies for various applications where its high wear resistance was particularly important. The first tungsten carbide-cobalt grades were soon successfully applied in the turning and milling of cast iron and, in the early 1930’s, the pioneering cemented carbide companies launched the first steel-milling grades which, in addition to tungsten carbide and cobalt, also contained carbides of titanium and tantalum.
By the addition of titanium carbide and tantalum carbide, the high temperature wear resistance, the hot hardness and the oxidation stability of hardmetals have been considerably improved, and the WC-TiC-(Ta,Nb)C-Co hardmetals are excellent cutting tools for the machining of steel. Compared to high speed steel, the cutting speed increased from 25 to 50 m/min to 250 m/min for turning and milling of steel, which revolutionized productivity in many industries.
Shortly afterwards, the revolution in mining tools began. The first mining tools with cemented carbide tips increased the lifetime of rock drills by a factor of at least ten compared to a steel-based drilling tool.
In all these applications, there has been a continuous expansion in the consumption of cemented carbide from an annual world total of 10 tons in 1930; to 100 tons around 1935; 1,000 tons in the early 1940’s; through 10,000 tons in the early 1960’s and up to nearly 30,000 tons at present.
The development of metal cutting tools has been very rapid over the last four decades, having been greatly stimulated by much improved design and manufacturing techniques, e.g. the introduction of indexable inserts in the 1950’s and the invention of coated grades around 1970.
The first coating was with a thin layer (~5 µm thick) of titanium carbide made by a Chemical Vapour Deposition (CVD) process. It improved the lifetime of tools by a factor of 2 to 5.
This technique has since been improved by multilayer coatings, where layers of alumina, titanium nitride, titanium aluminium nitride and other materials have been added which have further improved the lifetimes by 5 to 10 times.
However, coating and improved design are only one side of the coin. Continuous improvement of intermediates and manufacturing techniques led to improved performance of hardmetals and opened new areas of applications. The introduction of solvent extraction in tungsten chemistry, new techniques in hydrogen reduction and carburization improved the purity and uniformity of tungsten and tungsten carbide powder.
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