� 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Article No : a05_061

Carbides

HELMUT TULHOFF, Hermann C. Starck Berlin, Werk Goslar, Goslar, Federal Republic

of Germany

1. Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565

1.1. Saltlike Carbides . . . . . . . . . . . . . . . . . . . . . 565

1.2. Metal-like Carbides . . . . . . . . . . . . . . . . . . . 567

1.3. Diamond-like Carbides . . . . . . . . . . . . . . . . 567

1.4. Carbides of Nonmetallic Elements . . . . . . . . 567

1.5. Crystal Structure . . . . . . . . . . . . . . . . . . . . . 567

1.6. General Production Processes . . . . . . . . . . . 568

1.7. Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 569

2. Metal-like Carbides of Industrial Importance 569

2.1. Tungsten Carbide . . . . . . . . . . . . . . . . . . . . 569

2.2. Titanium Carbide . . . . . . . . . . . . . . . . . . . . 573

2.3. Tantalum Carbide . . . . . . . . . . . . . . . . . . . . 574

2.4. Niobium Carbide . . . . . . . . . . . . . . . . . . . . . 575

2.5. Zirconium Carbide . . . . . . . . . . . . . . . . . . . 576

2.6. Hafnium Carbide. . . . . . . . . . . . . . . . . . . . . 576

2.7. Vanadium Carbide . . . . . . . . . . . . . . . . . . . 576

2.8. Chromium Carbide . . . . . . . . . . . . . . . . . . . 577

2.9. Molybdenum Carbide . . . . . . . . . . . . . . . . . 578

3. Mixed Carbides . . . . . . . . . . . . . . . . . . . . . . 579

3.1. Tungsten – Titanium Carbide . . . . . . . . . . . 580

3.2. Other Mixed Carbides . . . . . . . . . . . . . . . . . 580

3.3. Carbonitrides. . . . . . . . . . . . . . . . . . . . . . . . 581

3.4. Mixed Carbonitrides . . . . . . . . . . . . . . . . . . 581

4. Carbides of the Iron Group and Manganese 581

5. Complex Carbides . . . . . . . . . . . . . . . . . . . . 581

References . . . . . . . . . . . . . . . . . . . . . . . . . . 582

1. Survey

Most of the elements form binary compoundswith carbon, all of which can be called carbides.The properties of these carbides are very differ-ent; therefore, like binary hydrides and nitrides,the carbides should be classified into groups. Toavoid too many subdivisions, the following fourtypes of carbides may be defined:

1. saltlike carbides of metallic elements, e.g.,CaC2

2. metal-like carbides of metallic elements, e.g.,WC

3. diamond-like carbides, e.g., B4C4. carbides of nonmetallic elements, e.g., CO

This classification suggests another group: theelements that do not react with carbon, e.g., Sn.

Generally, the four groups of carbides can notbe strictly separated from each other. Numerouscarbides are in intermediate positions betweenthese groups. One example is BeC2 [57788-94-0]. It is a typical saltlike carbide and is decom-posed by water. On the other hand, it may be

viewed as a diamond-like carbide because of itshardness and other properties resembling thoseof SiC.

Figure 1 surveys the four types of carbides inthe form of a periodic table. Elements that do notform binary compounds with carbon, or are notknown to form carbides, are not shown. Thecarbides of the iron group and manganese area subgroup of the metal-like carbides.

1.1. Saltlike Carbides

Saltlike carbides of metallic elements are thecarbides of the elements of groups 1 – 3 and11 – 13 (I – III, bothA’s andB’s) of the periodictable, the lanthanides and actinides included.Exceptions are Ga, In, and Tl, which do not formcarbides, and B4C, which is a typical diamond-like carbide.

The saltlike carbides – also called ionic carbides– are attacked by water to form hydrocarbons.Most of these carbides form acetylene, e.g.:

CaC2þ2 H2O!CaðOHÞ2þC2H2

DOI: 10.1002/14356007.a05_061

Figure

1.Survey

ofbinarycompoundsofcarbonwiththeelem

ents

566 Carbides Vol. 6

These carbides can be viewed as salts ofacetylene and may be called acetylides. Thecrystals contain C2�2 anions.

The carbides Be2C and Al4C3 form puremethane when hydrolyzed:

Be2Cþ4 H2O! 2 BeðOHÞ2þCH4Al4C3þ12 H2O! 4 AlðOHÞ3þ3 CH4

In the crystal lattice of these carbides, thecarbon atoms are isolated from each other, incontrast to the C2 groups of the acetylides. TheBe2C lattice is antiisotypical to that of CaF2.

The carbide MgC2 can be decomposed byheating to form Mg2C3 and graphite. Hydrolysisof Mg2C3 yields propyne:

Mg2C3þ4 H2O! 2MgðOHÞ2þCH3�C � CHIn their carbides the lanthanides and actinides

are mainly divalent. During hydrolysis they be-come trivalent, and hydrogen is formed in thisreaction:

M2þþHþ !M3þþHThis hydrogen reacts with the acetylene also

formed to produce a mixture of acetylene, meth-ane, ethylene, and hydrogen.

Whereas the saltlike carbides of groups 1 and2 are transparent and are not electrical conduc-tors, the lanthanide and actinide carbides showsome metallic behavior, an indication of a stateintermediate between saltlike and metal-like car-bides. The electrical conductivity and metallicluster may be due to the fact that the metals aredivalent in their carbides and the third valenceelectron is available for metallic bonding.

One other subgroup of saltlike carbides shouldbe mentioned: the alkali-metal – graphite com-pounds. They are formed by absorption ofmoltenNa, K, Rb, and Cs by graphite. Compositionssuch as MC8, MC16, andMC60 are known. Thesecompounds are quite likely not chemical com-pounds, butmerely adsorptional compounds, andperhaps better not called carbides.

1.2. Metal-like Carbides

Metal-like carbides of metallic elements are thecarbides of the transition elements of groups 4, 5,and 6 of the periodic table. These carbides, alsocalled metallic carbides, are not attacked by

water. The metallic character of these com-pounds is shown in their high thermal and elec-trical conductivity as well as in their metallicluster.

All the metallic carbides are stable at roomtemperature and resist attack by dilute acids aswell as by alkaline and organic liquids. Theirhardness and wear resistance are utilized in thecemented carbides (! Hard Materials), whichare sintered products of the carbides with cobaltor other metals. Because of their industrial sig-nificance, these carbides are described in moredetail in Chapter 2.

The carbides of Mn, Fe, Co, and Ni aregenerally included in the metal-like carbides,although they are really better classified as agroup on their own. These carbides are in anintermediate position between the metal-likecarbides and the saltlike carbides. Their crystalstructures are quite different from the structuresof the metal-like carbides and the saltlike car-bides. The pure compounds are attacked bywateror dilute acids.

1.3. Diamond-like Carbides

Diamond-like carbides include, strictly speak-ing, only B4C and SiC. They are called diamond-like because of their extreme hardness, which isexceeded only by diamond itself. Sometimes thevery hard Be2C is included in the diamond-likecarbides. However, its hardness cannot be usedindustrially, because of its decomposition bywater.

1.4. Carbides of Nonmetallic Elements

Such carbides as CO, CS2, and CCl4, the carbidesof nonmetallic elements, have covalent, molecu-lar character and are not discussed in this article.

1.5. Crystal Structure

The lattice structure of most carbides can bededuced from the structure of their most impor-tant group, the metal-like transition-metal car-bides. Basically these carbides are cubic or hex-agonal closest packings of metal atoms with thesmaller carbon atoms in the interstitial sites.

Vol. 6 Carbides 567

Therefore, the transition-metal carbides can alsobe called interstitial carbides.

In 1931H€AGG [7] reported that the structure ofthe transition-metal carbides is determined by theradius ratio r of the metal and carbon atomsr ¼ rC/rmetal. When r is less than 0.59, the metalsform the simple structures just described, withthe carbon atoms located at the octahedral inter-stices. If all interstices are occupied in a body-centered cubic (bcc)metal lattice, the result is theface-centered cubic (fcc) sodium chloride struc-ture. All the carbides of transition-metalgroups 4 and 5 crystallize in this B1 lattice.Tungsten carbide has a simple hexagonal struc-ture with all of the trigonal prismatic interstitialsites occupied by carbon.

The B1 carbides, principally TiC, ZrC, HfC,and VC, tend to form defect structures in whichthe interstitial sites are not completely filled.Broad homogeneity ranges are the result, butsome substructures with overlapping homogene-ity ranges are indicated [8].

When only one-half of the octahedral intersti-tial sites are occupied in an hexagonal-closest-packed (hcp) metal structure, the subcarbides –V2C, Nb2C, Ta2C, Mo2C, and W2C – are ob-tained. This is a simplified interpretation, and infact the subcarbides aremore complex structures,as was shown by NOWOTNY [9]. Indeed, thesestructures are sometimes calledNowotny phases,to contrast them with the simpler H€agg phases.

When H€agg’s ratio exceeds 0.59, the simplephases can no longer be formed as before. Closeto 0.59 and in the case of low carbon content,there are the compounds Cr23C6 and Mn23C6,which can still be viewed as interstitial structures.For higher values of r and higher carbon content,more complex structures, no longer interstitialcompounds, are formed: M3C, M7C3, M3C2.These stoichiometries are primarily found in theiron group. These more complex structures areless metallic than the H€agg phases. Hardness,melting point, and chemical resistance aremarkedly lower.

The structures of the saltlike carbides can alsobe deduced from the H€agg phases. When thereare more carbon atoms than octahedral intersti-tial sites in themetal lattice, pairs of carbon atomsare formed. The CaC2 type is a tetragonallydeformed B1 structure. The dicarbides of thelanthanides and actinides crystallize in this sys-tem. They lack metallic characteristics. The bcc

carbides, M2C3, also contain C2 groups, e.g.,U2C3.

The structures of the diamond-like carbidesSiC and B4C differ from all structures describedthus far. The carbide SiC has an expanded dia-mond lattice, whereasB4C crystallizes in a rhom-bic lattice containing B12 icosahedrons and C3chains.

1.6. General Production Processes

There are a number of general methods of pro-ducing carbides:

1. Nearly all carbides can be prepared at hightemperature by direct reaction from the metalpowdermixedwith lampblackorgraphite, e.g.:

WþC!WCGenerally the temperature is in the range

1000 – 1500 �C, and special furnaces are used.A protective atmosphere or vacuum is needed.

2. Instead of the pure metal, the oxide or hydridecan be carburized with solid carbon:

Ta2O5þ7 C! 2 TaCþ5 COLarge amounts of gas result from this

reaction. Both processes 1 and 2 are solid-state reactions.

3. Carbides with high melting points can beprepared by a modified aluminothermic pro-cess:

3 Cr2O3þ6 Alþ4 C! 2 Cr3C2þ3 Al2O3

4. Instead of solid carbon, gaseous carbon com-pounds, such as CO or CH4, can be used. Thisprocess is important in the steel industry,where mainly iron, chromium, and manga-nese carbides are formed during fusion:

3 Feþ2 CO! Fe3CþCO2

5. Reaction of metal chlorides with hydrocar-bons in a hydrogen atmosphere produces car-bides:

ZrCl4þCH4 ! ZrCþ4 HClThis method is used to produce layers of

carbides on other materials by chemical vapordeposition (CVD ! Thin Films).

568 Carbides Vol. 6

Most of the saltlike carbides are prepared byprocesses 1 and 2, by heating themetals or oxideswith carbon, e.g.:

CaOþ3 C!CaC2þCOHowever, another, quite different process can

also be used:Reaction of acetylene with metals or salts

dissolved in water or liquid ammonia or sus-pended in inert organic fluids forms simple orcomplex acetylides. Most are metastable but arestabilized by H2O, NH3, or acetylene itself.Cautious decomposition produces individualcarbides:

LiðsÞþNH3ðlÞþC2H2 ! LiHC2�NH3þ0:5 H2

LiHC2�NH3 ! LiHC2 ! Li2C2Today this last method is primarily of labora-

tory interest.

1.7. Uses

The uses of carbides are as diverse as the types ofcarbides. Most important from an economicpoint of view are the carbides of the transitionmetals of groups 4, 5, and 6. The most importantof these is tungsten carbide. The hardness andchemical resistance of these carbides are thebasis for their use by the tool and machineindustry in multitudinous applications as cemen-ted carbides. The terms hardmetals and cemen-ted carbides are synonymous. The term cermet isalso used for some or all of these composites.Furthermore the term tungsten carbide is usedbecause WC is the main constituent in most ofthese materials. These cemented carbides aresintered products of one or more carbides witha metallic binder, preferably the metal cobalt.There are many different combinations of car-bides and binder metals. Factories producingcemented carbides are found in every industrialcountry; the world’s annual production is esti-mated at ca. 20 000 t in 1985. Some tungstencarbide combined with Cu or Ag is used inelectrical contacts and in fuel cells.

The carbides of manganese and iron are neverused alone like the harder transition-metal car-bides, but rather are formed in alloys duringfusion. These carbides, especially cementite,Fe3C, are of fundamental importance because

the individual carbides and the binary mixedcarbides with V, Cr, Mo, and W are responsiblefor the hardness of steel, Stellites, and relatedalloys.

The most important saltlike carbide is CaC2(! Calcium Carbide). One-half of the world’sannual production, several million tons, is con-verted to cyanamide (! Cyanamides), which isused as fertilizer. Some 20% is used for acety-lene production (! Acetylene, Section 4.3.4.),and the remainder is used in steelmaking as acarburizing additive.

The monocarbides and dicarbides of uraniumand thorium are used as nuclear fuels in high-temperature reactors. These carbides are not usedas hard materials, although they do have somemetallic character. Other saltlike carbides do nothave industrial importance.

The carbides B4C and SiC are used in largequantities as abrasives (! Abrasives; ! BoronCarbide, Boron Nitride, and Metal Borides !Silicon Carbide). Heating elements and manyheat-resistant parts are made from SiC.

2. Metal-like Carbides of IndustrialImportance

The important individual carbides of the transi-tion metals and the mixed carbides of thesemetals are described in detail in the following.Tungsten carbide,WC, because of its importancein cemented carbides, or hardmetals, is describedfirst. Thereafter, TiC, TaC, and NbC, which arealso basic hard carbides, are described. Thephysical properties of these four carbides aregiven in Table 1. Finally, the carbides of Zr, Hf,V, Cr, and Mo are described. These last carbidesare used only as additives in cemented carbidesand have less importance. Their physical prop-erties are given in Table 2.

2.1. Tungsten Carbide

There are two hexagonal carbides in the tung-sten – carbon system (Fig. 2): the monocarbide,WC, and the subcarbide [12070-13-2],W2C. Thehexagonal WC, also called a-WC, decomposesat its incongruent melting point of 2776 �C. Itsrange of homogeneity is extremely narrow: from49.5 to 50.5 mol% C.

Vol. 6 Carbides 569

The subcarbide, W2C, probably has threemodifications. The highest temperature modi-fication melts without decomposition at� 2800 �C. The ‘‘eutectic’’ of W2C and WC isknown as cast or fused tungsten carbide. Thesolid eutectic mixture is sometimes incorrectlycalled W2C. In addition to these two industrialcarbides, there is a substoichiometric face-

centered cubic WC1�x phase, also called b-WC,which is unstable at room temperature, formingonly above 2530 �C. The phase diagram ismademore complicated by this additional compoundand the W2C modifications, mainly in the high-temperature range. Although many have inves-tigated the tungsten – carbon system, unan-swered questions remain.

Table 1. Physical properties* of WC, TiC, TaC, and NbC

Property WC TiC TaC NbC

CAS Registry Number [12070-12-1] [12070-08-5] [12070-06-3] [12069-94-2]

Molecular mass Mr 195.87 59.91 192.96 104.92

Carbon content (theory), wt% 6.13 20.05 6.23 11.45

Crystal structure hex., Bh fcc, B1 fcc, B1 fcc, B1

Lattice constants, pm a 291 432.8 445.5 447.0

c 284

Density, g/cm3 15.7 4.93 14.48 7.78

Melting point mp, �C 2776 3067 3985 3610Microhardness, kg/mm2 1200 – 2000 � 3000 1800 2000Transverse rupture strength, MPa 550 240 – 390 350 – 400 300 – 400

Modulus of elasticity, GPa 696 451 285 338

Specific heat, J mol�1 K�1 39.8 47.7 36.4 36.8Heat of formation DH298, kJ/mol � 40.5 � 183.7 � 148.9 � 141.0Coefficient of thermal conductivity, W m�1 K�1 121 21 22 14Coefficient of thermal expansion, 10�6 K�1 a 5.2 7.74 6.29 6.65

c 7.3

Electrical resistivity, mW � cm 22 68 25 35Superconductive transition temperature, K 10.0 1.15 10.3 11.1

Hall constant, 10�4 cm3 A�1 s�1 � 21.8 � 15.0 � 1.1 � 1.3Magnetic susceptibility, 10�6 emu/mol þ 10.0 þ 6.7 þ 9.3 þ 15.3*Properties given without a temperature are for room temperature.

Table 2. Physical properties* of ZrC, HfC, VC, Cr3C2, and Mo2C

Properties ZrC HfC VC Cr3C2 Mo2C

CAS Registry Number [12020-14-3] [12069-85-1] [12070-10-9] [12012-35-0] [12069-89-5]

Molecular mass Mr 103.23 190.51 62.96 180.05 203.91

Carbon content (theory), wt% 11.64 6.30 19.08 13.33 5.89

Crystal structure fcc, B1 fcc, B1 fcc, B1 orthorh., D510 hex. L03

Lattice constants, pm 469.8 464.8 416.5 a 1147 a 300

b 554 c 473

c 283

Density, g/cm3 6.46 12.3 5.36 6.68 9.18

Melting point mp, �C 3420 3930 2650 1810 2520Microhardness, kg/mm2 2700 2600 2900 1400 1500

Modulus of elasticity, GPa 348 352 422 373 533

Specific heat, J mol�1 K�1 37.8 37.4 32.3 32.7 30.3Heat of formation DH298, kJ/mol �196.8 �209.6 �100.8 �94.2 �46.0Coefficient of thermal conductivity,

W m�1 K�1 20.5 20.0 38.9 19.1 21.5Coefficient of thermal expansion,

10�6 K�1 6.73 6.59 7.2 10.3 7.8Electrical resistivity, mW � cm 42 37 60 75 71Superconductive transition

temperature, K >1.2 >1.2 >1.2 >1.2 2.78

Hall constant, 10�4 cm3 A�1 s�1 �9.42 �12.4 �0.48 �0.47 �0.85Magnetic susceptibility, 10�6 emu/mol �23 �25.2 þ26.2 — —*Properties given without a temperature are for room temperature.

570 Carbides Vol. 6

Properties. Commercial monocarbide,WC, the raw material for the powder-metallurgyindustry, is a gray metallic powder. Its averagegrain size is between 0.5 and 20 mm. In addition,very small quantities are preparedwith smaller orcoarser size for special applications. The carbideis insoluble in water and dilute acids, but isdissolved by hot mixtures of HNO3 and HF. Itis oxidized in air above 600 �C. Although it isstable in dry hydrogen up to its melting tempera-ture, wet hydrogen decarburizes it. ChlorineattacksWC above 400 �C, while fluorine attacksWC at room temperature.

Hardness, combined with high modulus ofelasticity, is themost importantmechanical prop-erty of WC. The microhardness is anisotropic[10], [11] and, because of this, values rangingbetween 1000 and 2500 kg/mm2 can be found inthe literature.

The chemical resistance of the subcarbideW2C is less than that of WC. The subcarbide isdissolved by HNO3 – HF mixtures even at roomtemperature. Itmay be distinguished fromWCbyits reaction with alkaline potassium hexacyano-ferrate (Murakami’s reagent): W2C turns yellowto brown, whereas WC remains gray. The micro-hardness of the subcarbide is higher than that of

WC, but W2C is not used alone industriallybecause it is too brittle.

Because eutectic W2C – WC is prepared by afusion process, it is not produced as a powder inthe micron range. The grains are much coarser,up to several millimeters. The carbon content canvary from 3.5 to 4.5 wt%, corresponding to50 – 90% of W2C or 10 – 50% of WC in this‘‘eutectic’’.

Preparation. Most of the world’s annualproduction of 15 000 – 20 000 t of WC is madeby direct carburization of tungsten metal withcarbon. Mixtures of metal and lamp black, oreven graphite, are heated to temperatures be-tween 1400 and 2000 �C in a hydrogen atmo-sphere or vacuum. Electrical walking-beam orpusher-type furnaces or gas-firedmuffle furnacesare used. Carbon tube furnaces are needed for thehigh-temperature range, and batch-type induc-tion furnaces are needed for vacuum processing.

After purity, the most important property ofthe carbide is grain size, because the grain sizesignificantly affects the mechanical properties ofWC products. Fine-grained powders cannot bemade from coarser powders only by milling.Intensive milling changes carbon and oxygencontents, the shape of the grains, and the grainsize distribution. Therefore, the grain size isbetter determined by the processing parametersduring reduction and carburization: temperature,reaction time, humidity, flow rate of the hydro-gen, and several other factors. Most importantly,the grain size of the starting material must beselected to produce the desired end product.Generally, powders become coarser when con-verted from oxide to metal to carbide.

The chemical nature of the starting materialsand the intermediate steps also affect the physicalproperties of the final carbide. Possible startingmaterials are tungstic acid [7783-03-1], H2WO4,and ammonium paratungstate or APT [11120-25-5], (NH4 )10W12O41 � 5 H2O. The intermedi-ates are yellow oxide (WO3 ), blue oxide (W4O11,simplified), and brown oxide (WO2 ). Variousways of processing are illustrated in Figure 3 bythe flow sheet, which contains 12 different pro-duction lines.

The following are typical production lines:

1. Fine tungstic acid powder is reduced directlyto metal by dry hydrogen at 750 �C. Metal

Figure 2. Tungsten – carbon phase diagram

Vol. 6 Carbides 571

particles with an average size of 0.7 – 0.8 mmare obtained. These are carburized at 1400 �Cto produce 1-mm carbide.

2. Ammonium paratungstate is calcined at700 �C in a stream of nitrogen to give blueoxide, which is reduced at 800 �C. The metalis carburized at 1400 �C to produce carbide of2 – 5 mm.

3. Ammonium paratungstate is roasted at800 �C in air to produceWO3. This is reducedby wet hydrogen at 950 �C. Carburizing at1600 �C produces carbide of 10 – 20 mm.

Tungsten oxides can also be carburized di-rectly with carbon, e.g.,

WO3þ4 C!WCþ3 CO

In this case, the intermediate metal step isomitted. The disadvantage is the difficulty inobtaining the correct carbon content in the car-bide, since the CO itself reacts with oxide to formmetal to a degree that cannot be calculated.Therefore, this process is used only to producetechnical grades. Tungstic acid, ammoniumparatungstate, or even scheelite (natural or artifi-cial) can be treated in the same way.

However, when scheelite is heated with car-bon, the resulting cake must be leached with acidto isolate the carbide, and this carbide is of lowquality because of its high level of impurities.

Tungsten metal can be carburized by carbon-containing gases, usually carbon monoxide ormethane. Gas-phase carburization is done pref-erably in the temperature range 800 – 900 �C.Therefore, the grains do not become muchcoarser. For example, WC of 0.3 – 0.4 mm isobtained from 0.3-mm metal. Such fine carbidesare often called submicron carbides. Tungsten

oxide can also be reduced and carburized in onestep by CO or CH4, but the product is alwaysslightly deficient in combined carbon. When COis used, water is not present in the furnaceatmosphere as a byproduct of reduction. Becausegrain growth of tungsten during reduction isinduced mainly by water vapor and high temper-ature, extremely fine WC can be made. Thesepowders can be used as catalysts in fuel cells.

Another method of preparing fine WC is thereaction of tungsten metal or oxide with CH4 andH2 in a plasma reactor [12]. Carbide having agrain size of 0.1 mm or less, sometimes calledultrafine carbide, is obtained. The plasma tech-nology and the use of such ultrafine WC are stillbeing developed.

Numerous other ways to prepare WC havebeen developed. Some are modifications of theprocesses just described; others are entirely dif-ferent. Most still need to be improved and are notyet in use on an industrial scale:

1. In the Axel – Johnson process, tungsten ore,ferrotungsten, or tungsten scrap is treatedwithchlorine to form WCl6, which is reduced byH2 in a gas-phase reaction. The fine metalformed by this reaction is carburized by aconventional process [13].

2. A mixture of WO3 and carbon is heated in atwo-stage rotary furnace. In the first stage theoxide is reduced tometal in a streamofnitrogen,and in the second stageWC is formed at highertemperature in a stream of hydrogen [14].

3. A mixture of WO3, Co3O4, and carbon isreduced in H2. After carburization, the mix-ture of WC and cobalt metal can be sintereddirectly to cemented carbides. TheWC grainshave a uniform, fine size, which is a result ofthe coreduction of the oxides [15].

All methods of preparation described thus farare solid-state or gas-phase reactions. The prep-aration of cast tungsten carbide is the only meth-od involving fusion. A mixture of tungsten metaland carbon or tungsten carbide is heated in acarbon tube or high-frequency furnace to ca.2800 �C. The molten eutectic is quenched inwater or otherwise cooled rapidly to produce afine crystalline structure.

Uses. Tungsten carbide is by far the carbidemost used in cemented carbides: About 90% of

Figure 3. Various production lines leading to WCAPT stands for ammonium paratungstate

572 Carbides Vol. 6

the world’s production of carbide tools are tung-sten carbide-based sinter alloys. Of these, 50%are so-called straight grades, tungsten carbide –cobalt products consisting of 70 – 95 wt% oftungsten carbide. There are many, many uses forcemented carbides of various compositions. Thegreatest demand is for cutting and drilling tools,miningmachinery, and wear-resistant parts of allkinds. Some examples illustrate the broad field ofapplications: milling cutters, cutting tips anddrills, sawing teeth and blades, drawing andheading dies, rolls, nozzles, sealing rings, ballsfor ball mills, balls for ballpoint pens, tire studs,and even scratch-proof watchcases. Protectivesurface coatings are made from cast tungstencarbide.

The mechanical properties of the cementedcarbides depend primarily on the grain size of thetungsten carbide. Generally speaking, smallergrain sizes produce greater hardness but lowercrack resistance. The cobalt content also affectsthe mechanical properties, and the properties ofthe cemented carbide can be adjusted to themechanical requirements over a wide range.

Toxicology. Tungsten carbide and the othercarbides of the transition metals are not known tobe toxic in themselves. However, nearly all ofthese carbides are used in combination withcobalt metal, and cobalt dust is carcinogenic.Therefore, mixtures for the powder-metallurgi-cal preparation of cemented carbides are classi-fied as dangerous materials in some countries.

2.2. Titanium Carbide

The face-centered cubic monocarbide TiC is theonly carbide in the titanium – carbon system. Itmelts without decomposition at ca. 3000 �C. Itsrange of homogeneity is very broad, rangingfrom 35 mol% to just below 50 mol% carbon.The composition with the highest melting pointand the largest lattice constant contains lesscarbon than stoichiometric TiC (see Fig. 4). Be-cause of this, undesired low-carbon phases can-not be formed during sintering of cementedcarbides when TiC is present, unlike the casefor the straight WC – Co grades. Titanium car-bide forms solid solutions with all other cubictransition-metal carbides of groups 4 and 5. Inaddition, it is the host lattice for hexagonalWC in

the most important industrial solid-solution car-bide, (W,Ti)C (see Section 3.1).

At elevated temperatures, TiC and Ti metalreact with oxygen and nitrogen to form TiO andTiN, the structures of which are isotypical to thelattice of TiC. Therefore, many TiC powderscontain small amounts of N and O, to an extentof 1%ormore, andmay be viewed as Ti(C, N, O)mixed crystals.

Commercial TiC is a gray powder usuallyhaving an average grain size of 2 – 10 mm. It isvery resistant to acids, oxidation, and heat. How-ever, it is dissolved easily by mixtures of HNO3and HF. In hydrogen it can be heated to itsmelting point without decomposition. Titaniumcarbide is the hardest of all the transition-metalcarbides.

Preparation. Most commercial TiC ismadeby the reaction of TiO2 with carbon. Intimatemixtures of pure TiO2 and carbon are heated to2000 �C or above in a hydrogen atmosphere.Large quantities of CO are produced. After theresulting cake is milled, the material contains upto 1 wt% each of free carbon, nitrogen, andoxygen. The amounts of these elements must bereduced in a second step, usually a vacuum

Figure 4. Titanium – carbon phase diagram [3]

Vol. 6 Carbides 573

heating process. Nitrogen and oxygen contentsare decreased to less than 0.1 wt% each. The freecarbon content is usually in the range 0.2 –0.4 wt%, and the combined carbon is 19.5 wt%max., somewhat less than the stoichiometriccontent of 20.05 wt%.

Titanium metal can be carburized with car-bon. Titanium sponge, or even finely dividedscrap, is used. The process is exothermic, andtherefore, exact temperature control is not possi-ble. As a result, the cake is sometimes extremelydense and can merely be broken down or milledonly with difficulty. Carbon, nitrogen, and oxy-gen contents must be adjusted in a subsequentprocess.

Very coarse, comparatively pure TiC is pre-pared by the auxiliary metal bath technique [16].Titanium metal, ferrotitanium, or even titaniumalloy scrap is dissolved along with graphite inmoltenmetal, preferably iron or nickel. After thismixture is cooled, TiC is isolated by dissolvingthe auxiliary metal with a nonoxidizing acid(menstruum process).

Extremely fine TiC is made in a plasma reac-tion of TiCl4, H2, and CH4.

Layers of TiC on other materials may beproduced by controlled vapor deposition froma mixture of TiCl4, H2, and CH4.

Uses. Titanium carbide is the hardest car-bide of the commercial transition-metal carbides,but it is too brittle to be used alone. However, it isthe most important additional carbide in tung-sten-based cemented carbides for cutting steel.Although toughness is decreased a little by theaddition of TiC, the hardness and especially theheat resistance are increased significantly. Nor-mal steel-cutting grades contain 5 – 30 wt% ofTiC. Furthermore, TiC is the basic carbide for theformation of solid solutions with all other transi-tion-metal carbides used in cemented carbides.In tungsten-free hardmetals, TiC is used in com-bination with molybdenum carbide and nickelbinder metal.

Titanium carbide in combination with steelalloy forms a special type of hard alloy calledFerro-TiC. Tungsten-based cemented carbidescan be replaced by this material in some cases.TiC was the first carbide material used for coat-ings on cutting tips made from normal cementedcarbides. Even though the thickness of such alayer is in the range of only a fewmicrons, the life

of the cutting tools is increased markedly. SomeTiC is used in combination with oxides in ce-ramic cutting tools (Al2O3 – TiC).

2.3. Tantalum Carbide

In general, the phase relationships in the systemsof group 5 metals and carbon are complex. Thesystem Ta – C (Fig. 5) is typical for the group.This system is characterized by several subcar-bides, with lower carbon contents, in addition tothe monocarbide, TaC. The face-centered cubicmonocarbide melts without decomposition near4000 �C, one of the highest melting pointsknown. The broad range of homogeneity extendsfrom 43 to 50 mol% C. The subcarbide Ta2Cdecomposes at its incongruent melting point.There are two modifications, a high-temperaturephase, with disordered L 0 3 structure, and a low-temperature phase, of C6 type. The transforma-tion temperature is near 2000 �C. In addition,there is a metastable Ta3C2, which is sometimescalled the Brauer or z-phase. Similar phases arealso found in the V – C and Nb – C systems,although the structure of the carbides is still notcompletely resolved [17].

Figure 5. Tantalum – carbon phase diagram [3]

574 Carbides Vol. 6

The monocarbide, TaC, is the only phase ofcommercial interest. It is a brown powder, usu-ally of 1 – 5 mm average grain size. Sintered andpolished pieces have a yellow-golden sheen. Thechemical resistance is high. The monocarbide isstable in nonoxidizing acids, although it is at-tacked easily by a mixture of HNO3 and HF andby oxidizing salt melts. It can be heated up to3000 �C in hydrogen or nitrogen, but it is oxi-dized rapidly in air at 800 �C.

Preparation. The method most used forpreparation is based on the reaction of tantalumoxide with carbon:

Ta2O5þ7 C! 2 TaCþ5 COIntimate mixtures of oxide and carbon are

pressed into graphite boats and heated at 1700 �Cin hydrogen. Usually the product is deficient inbound carbon, and this must be adjusted in asecond step. Tantalummetal can also be used fordirect carburization with carbon. Very puremonocarbide for scientific use is obtained by thereaction of tantalum hydride with carbon.

A commercial grade of lower purity can bemade by melting ferrotantalum or tantalum-con-taining scrap and slag in a metal bath with anexcess of carbon. After the mixture is cooled, theauxiliary metal is dissolved with acid to freetantalum carbide. An additional step to adjust thecarbon content is also necessary in this process.

Uses. Because of its extremely high meltingpoint, some TaC is used in high-temperaturetechniques, but the main application is in hard-metals. AlthoughTaC is themost expensive of allthe carbides normally used in cemented carbides,consumption is still increasing because of themarked improvement in the properties of cemen-ted carbides containing TaC. The world’s annualdemand can be estimated to be ca. 500 t.

There are two quite different reasons for thisuse of TaC. First, small amounts of TaC, in therange of 0.2 – 2.0 wt%, are added to straightWC – Co grades in which fine-grained WC,1.5 mm or less, is used. In these grades, oftenthere is an undesired grain growth of the carbidephase during sintering because of the sinteringtime and temperature and probably some stillunknown factors. This grain growth is inhibitedto a great extent by TaC. Although there areother, cheaper compounds that retard grain

growth, TaC is preferred because it is the onlycompound known to have no negative effects.The second reason for using TaC in cementedcarbides is based on the great improvement incutting tools, mainly in long-chipping steel cut-ting grades. In this second case the TaC contentranges from 2 to 15 wt%, and evenmore in somespecial cases. Thermal shock resistance, hothardness, and resistance against cratering andoxidation are all increased markedly.

2.4. Niobium Carbide

The phase relationships in the system niobium –carbon are quite similar to those in the systemtantalum – carbon. However, because of theclose similarity, there is some doubt about thecorrectness of the phase diagram. The face-cen-tered cubic monocarbide, NbC, melts withoutdecomposition at ca. 3600 �C. It has a range ofhomogeneity from ca. 40 to almost 50 mol%; thestoichiometric value of 50% is never reached.The subcarbide Nb2C decomposes at its meltingpoint of ca. 3000 �C. It exists in several mod-ifications, the number and structure of which area point of uncertainty. The other open question isthe existence of an additional z-phase, Nb3C2.

Themonocarbide, NbC, is the only phase usedindustrially. It is a gray-brown powder of nor-mally less than 10 mm average grain size. Sin-tered pieces show a lavender tint. The chemicalreactivity is similar to that of TaC, butNbC is lessresistant to nitrogen. Heating the carbide in am-monia produces the nitride.

Preparation. Niobium monocarbide, NbC,is made in the sameway as TaC: by carburizationof the oxide, hydride, or metal at 1500 �C. Theauxiliary metal bath technique can also be used.Most NbC is not produced as a single purecompound because demand for pure NbC issmall. Nearly all of the NbC is used in combina-tion with TaC; therefore, mixtures of Nb2O5 andTa2O5, in various ratios, are carburized to-gether.The resulting products are true mixed crystals ofNbC and TaC. Any ratio can be prepared, but theusual commercial compositions contain 10, 20,or 50 wt% NbC.

Uses. Only small quantities of pure NbCare needed. Some is used in special grades of

Vol. 6 Carbides 575

cemented carbides in combination with Al2O3(cermets). Another use is the reduction of Nb2O5byNbC to niobiummetal, a process carried out at1600 �C under vacuum or hydrogen.

Most of the NbC is used in combination withTaC in hardmetals. When used with TaC, theNbC improves the properties of the sinteredmaterial just like pure TaC does. At the sametime, NbC is much less expensive than TaC.However, NbC is said to decrease the strengthwhen it is added in large amounts. The limit andthe degree of toughness loss are not knownexactly. In any case, NbC is never used alone,and the content of NbC in the TaC ranges fromless than 1 to 50 wt%.

2.5. Zirconium Carbide

The monocarbide ZrC is the only compound inthe zirconium – carbon system. It has a face-centered cubic crystal structure, and the range ofhomogeneity reaches from 38 to 50 mol% C.Zirconium carbide melts without decompositionat ca. 3400 �C. The phase boundaries in the Zr –C system are extremely sensitive to oxygen,nitrogen, and probably even more to other impu-rities. On the other hand, ZrC is difficult toprepare free from oxygen and nitrogen becausethe lattices of ZrC, ZrO, and ZrN are isotypical.As a result there have been many misinterpreta-tions of the phase diagram in the past, and somedisagreements still must be clarified.

The carbide is a gray powder. Its chemicalresistance is somewhat lower than that of TiC. Itis dissolved by cold HNO3 or a cold mixture ofH3PO4 and dilute H2SO4. It can be heated inhydrogen up to its melting point, but it is attackedby oxygen at 500 �C.When the carbide is heatedin nitrogen above 1500 �C, the nitride is formed.

Preparation. Zirconium carbide is mademostly by heating mixtures of ZrO2 and carbonat 1800 – 2400 �C in hydrogen or under vacuum.The carbon content must be adjusted in a secondstep. The metal, in the form of a sponge, or thehydride can be carburized with carbon at a tem-perature as low as 1400 – 1600 �C. Up to1800 �C, the carbide getters oxygen, and oxy-gen-free material is difficult to obtain. Very pureZrC, for scientific use, can be made by the gas-phase reaction of ZrCl4, H2, and a hydrocarbon.

Use. Up to now only small quantities of ZrChave been used in hardmetals. This may be due tothe comparatively high price of ZrC as well as toits insufficient heat resistance. ZrC forms solidsolutions with all other transition-metal carbides.Therefore, the demand of ZrC may increase.Other than use in cemented carbides, there is nouse of importance.

2.6. Hafnium Carbide

The hafnium – carbon system and the propertiesof the carbide are similar to those of zirconium.The only phase is the face-centered cubic mono-carbide, HfC. The broad range of homogeneityextends from 37.5 to 50 mol% C. The carbidemelts at ca. 3900 �C without decomposition. Itschemical reactivity seems to be similar to that ofZrC, but little information is to be found in theliterature.

Preparation. Hafniumdioxide can be carbu-rized like ZrO2 in hydrogen or under vacuum at1800 – 2200 �C. If hafnium metal or hydride isused for carburization, a temperature of 1600 –1700 �C is sufficient. When the oxide is the start-ingmaterial, a second step foradjustmentofcarboncontent and reduction of the oxygen content isnecessary. This second step is often not neces-saryif the metal or hydride is the starting material.

Uses. Hafnium oxide ormetal is a byproductin the production of zirconium for nuclear reac-tors. Therefore, hafnium is available in sufficientquantities, and HfC has become attractive forcemented carbides. Tantalum carbide, TaC, insteel-cutting tools or as grain-growth inhibitormay be replaced by HfC. An HfC – NbC solidsolution seems to be the most effective. Coatingsof HfC on normal hardmetal tools increase theoxidation resistance. Such coatings are made bychemical vapor deposition (CVD) with HfCl4and a carbonizing gas.

2.7. Vanadium Carbide

Vanadium forms the same phases with carbon astantalum and niobium. The face-centered cubicmonocarbide, VC, exists over a broad range ofhomogeneity, from 43 to 49 mol% C. It melts

576 Carbides Vol. 6

without decomposition at ca. 2800 �C. There aretwo modifications of the subcarbide, V2C, theorthorhombic low-temperature a-phase and thehexagonal high-temperature b-phase. The latterdecomposes on melting. Furthermore, there is ametastable z-phase, V3C2. Often a V4C3 phasehas been described in the literature, but this phaseis only found in vanadium-containing steel al-loys. Probably it is not a distinct phase, but rathera solid solution of carbide, oxide, and nitride, thestructures of which are isotypical.

The only phase of commercial interest is themonocarbide,VC. It is a gray powderwith a grainsize usually of several micrometers. It is resistantto cold acids, except HNO3, but it is easilydissolved by hot oxidizing acids. The monocar-bide can be heated to its melting point in hydro-gen, but in air it is oxidized rapidly at 800 �C.

Preparation. Vanadium monocarbide ismade mostly by heating V2O3 or V2O5 withcarbon at 1500 – 1700 �C in hydrogen. Ammo-nium vanadate also can be used as the startingmaterial. A second treatment under vacuum isnecessary in every case to adjust the carboncontent and to reduce the oxygen level. Becauseof the great stability of the V(C,O,N) mixedcrystal, oxygen-free material is difficult to pre-

pare. Very pure VC is best made by the reactionof vanadium metal with carbon under vacuum.

Uses. Vanadium monocarbide is too brittleto be used alone in cemented carbides. Somespecial grades were made with VC – TiC mixedcrystals with nickel or iron binder, but this wasdone temporarily only when there was a shortageof tungsten. Small quantities of VC are used toinhibit grain growth in tungsten carbide – cobalthardmetals. The effectiveness is higher than thatof TaC, but the toughness of the sinteredmaterialis lower when > 0.5% VC is added. Largequantities of VC are contained in steel alloyswhere it forms during melting.

2.8. Chromium Carbide

The phase relationships in the chromium – car-bon system are quite different from those ofthe other metals of group 6 as well as those ofthe metals of groups 4 and 5. H€agg’s ratio of theatomic radii is 0.609 in the case of chromium andcarbon; thus, the critical figure of 0.59 is ex-ceeded and a simple closest packed structure canno longer form. There are three chromium car-bides in the system (Fig. 6). The cubic carbide

Figure 6. Chromium – carbon phase diagram [6]

Vol. 6 Carbides 577

Cr23C6 [12105-81-6] is a complex D84 type with116 atoms in the unit cell. It decomposes onmelt-ing at ca. 1500 �C. This Cr23C6 is sometimesformulated incorrectly as Cr4C. The hexagonalCr7C3 [12075-40-0] melts without decomposi-tion at ca. 1800 �C, whereas the orthorhombicCr3C2 [12012-35-0] decomposes at its meltingtemperature of ca. 1900 �C. All three phaseshave very narrow ranges of homogeneity. Thereare hints of the existence of one or more addi-tional phases in the high-temperature range.These phases and some uncertainties in the phaseboundaries between the known carbides are still amatter of discussion. Very little is known aboutthe properties of Cr23C6 and Cr7C3 because thesecarbides are not prepared as pure compounds andare never used alone. They form during meltingof steel and ferrous alloys, and they exist proba-bly in the form of mixed crystals with iron andother metallic carbides. The carbide Cr3C2 is theonly phase produced as such. It is a gray powderof a grain size normally less than 10 mm. It isinsoluble in cold HCl, but dissolves in hot oxi-dizing acids and in H2O2. It has the greatestresistance to oxidation of all metal-like carbides.It is stable in air up to 1000 �C because of a verydense and firm oxide layer that forms on itssurface.

Preparation. The carbide Cr3C2 is made byheating mixtures of Cr2O3 and carbon up to1600 �C in hydrogen. Below 1300 �C, primarilyCr7C3 is formed. The following equations dem-onstrate how complicated the process of carburi-zation is:

3 Cr2O3þ13 C! 2 Cr3C2þ9 CO

5 Cr2O3þ27 Cr3C2 ! 13 Cr7C3þ15 CO

Cr2O3þ3 Cr7C3 ! Cr23C6þ3 CO

3 Cr2O3þ3 Cr3C2 ! 13 Crþ6 CO

Oxygen-free Cr3C2 with the stoichiometriccarbon content is difficult to obtain if the startingmaterial is the oxide. Very pure Cr3C2 can bemade by the carburization of chromium metalpowder.

Uses. Some Cr3C2 is used in hardmetals inspecial tools with great resistance to acids and

salts. In these grades the carbide is bound withnickel. Small quantities of Cr3C2 are used as agrain-growth inhibitor in WC – Co cementedcarbides. Considerable amounts of the eutecticCr7C3 – Cr3C2 are used inwelding electrodes forhard facing.

The greatest demand for chromiumcarbides isin steel, Stellites, and related alloys. In suchcases, pure chromium carbides are not used;instead, chromium metal is added to the melttogether with carbon-containing additives.

2.9. Molybdenum Carbide

Although much work has been done on themolybdenum – carbon system, there are stilluncertainties and disagreements. The existenceof at least four phases seems to be assured(Fig. 7). The hexagonal Mo2C is the only phasestable at room temperature. Its range of homo-geneity is very narrow and lies between 33 and34 mol% C. The orthorhombic Mo2C phase isstable only above 1475 �C. It melts withoutdecomposition at ca. 2400 �C. Two carbon-richphases, ca. 39 mol% C, exist only at high tem-perature, a hexagonal one above 1655 �C and acubic one above 1960 �C. Both phases are des-ignated as MoC1�x by some authors and as MoCor Mo3C2 by other authors. Below their decom-position temperatures these phases break downinto Mo2C and C. Hexagonal MoC1�x is isoty-pical with WC and can be stabilized by theinclusion of tungsten. More phases have beenobserved in the system, but probably all of themwere oxygen-containing mixed phases.

The only phase of commercial interest is thehexagonal Mo2C. It is a gray powder in themicron range. It is resistant to nonoxidizing acidsbut is dissolved by HNO3 or by hot H2SO4. It isstable in hydrogen, but it is oxidized in air at500 �C.

Preparation. Although MoO3 or MoO2 canbe carburized with carbon at 1500 �C, a carbidewith the correct carbon content and a low oxygencontent is difficult to obtain. Pure Mo2C is bestmade by heating molybdenum metal powderwith carbon in hydrogen at ca. 1500 �C.

Uses. Mo2C is used in special cementedcarbide grades containing TiC and nickel metal.

578 Carbides Vol. 6

Such grades were the first tungsten-free hard-metals. Attempts have been made to replacetungsten partially with Mo – W mixed crystals.Most Mo2C is used in steel alloys, where it formsduring melting.

3. Mixed Carbides

The commercial carbides of groups 4, 5, and 6form numerous mixed carbides with each other.The formation of these solid solutions depends onthe lattice constants of the carbides and corre-sponds to theHume – Rothery rule on the atomicvolumes. Only a few of the metallic carbides donot conform to these conditions and, thus, do notform a continuous series of solid solutions. Thecubic monocarbides of the metals of groups 4and 5 are completely miscible, except the pairsZrC – VC and HfC – VC, which are soluble in

each other only to a limited extent. Limitedmiscibility is found betweenTiC and the carbidesof Cr, Mo, and W, but there are still someuncertainties about the TiC – WC system. Solidsolutions of three ormore carbides also exist. Thehost lattice of the mixed carbides is usually TiC.The hexagonal WC has only a negligible capa-bility to receive cubic carbides into solidsolution.

The use of mixed crystals in cemented car-bides offers several advantages. Mixed crystalsare harder and tougher than single, unalloyedcarbides. The contents of oxygen, nitrogen, andfree graphite are distinctly lowered by autopur-ification during the diffusion process; the wetta-bility by cobalt and other binder metals isincreased.

The temperatures for the preparation of mixedcrystals are ca. 500 �C higher than the normalsintering temperatures for hardmetals. Therefore,

Figure 7. Molybdenum – carbon phase diagram [6]

Vol. 6 Carbides 579

an exact formation of mixed crystals cannot beattained by mixing the individual carbides withthe binder metal before sintering: the mixedcarbides must be prepared in a separate process.

The mixed carbides can be prepared by theprocesses used for the preparation of the singlecarbides. Mixtures of the oxides and carbon areheated up to 1800 – 2000 �C in hydrogen, most-ly in high-frequency induction furnaces. A sec-ond step under vacuum is always necessary toadjust the carbon content. The mixed metalpowders can be treated with carbon in the sameway. In addition, the reaction of metal oxide withanother metal carbide and additional carbon isused, for example:

TiO2þWCþ3 C! 2ðTi;WÞCþ2 CO

Very pure mixed carbides are best made byheating mixtures of the single carbides at2000 �C under vacuum. The process of diffusioncan be accelerated by the addition of cobalt,nickel, iron, or chromium metal in the range0.5 –1.0 wt%:

Pure mixed crystals are also made by theauxiliary metal bath technique (menstruum pro-cess, see Preparation).

3.1. Tungsten – Titanium Carbide

The most important mixed carbide in cementedcarbides is tungsten – titanium carbide. Theternary system is not yet known in all its details.The uncertainties in the Ti – C system caused bythe isotypical TiO andTiNphases are observed inthe Ti – W – C system as well. The solid solu-bility of WC in the cubic lattice of TiC is limitedand depends on the temperature:

Temperature, �C 1500 2000 2400

WC, wt% 60 80 90

WC, mol% 31 55 73

Above 2600 �C, there is probably completemiscibility.

Saturated mixed crystals prepared at hightemperatures become supersaturated when used

in hardmetal production because of the compar-atively low sintering temperatures, only 1400 –1500 �C. Therefore, very fine WC crystals pre-cipitate in the metal binder phase, thus stronglyinfluencing the properties of the hardmetal. Themechanisms are still a matter of discussion. TheW – Ti mixed crystals used in industry generallycontain 50 or 70 wt% W.

3.2. Other Mixed Carbides

Another important mixed carbide for cementedcarbides is tungsten – tantalum carbide. Thesolubility of cubic TaC in hexagonal WC isnegligible, but WC has a limited solubility inTaC that depends strongly on the temperature:

Temperature, �C 1500 1800 2000 2500WC, wt% 10 20 27 70

Because the solubility of WC in TaC decreasesrapidly as the temperature is lowered, WC crys-tals always precipitate from the solid solutionsduring cooling. Therefore, preparation of single-phase (W,Ta)C mixed crystals is almost impos-sible, and unlike othermixed carbides, (W, Ta) Cis usually marketed with the additional designa-tion double phase.

The (W,Ti,Ta)Cmixed crystal may be viewedas a combination of (W,Ti)C and (W,Ta)Cmixedcrystals. It is used in considerable quantities incutting tools for steel and related long-chippingmaterials. Sometimes this mixed crystal is called‘‘triple carbide’’, which is incorrect because innearly every case, TaC – NbCmixtures, not pureTaC, are used. The systemW – Ti – Ta – Nb –C is not yet known in all its details. There aresome isothermal sections in the quasiternarysystem WC – TiC – TaC [18], but the effects ofthe addition of NbC to this system are not knownexactly.

Another important mixed crystal is titanium –molybdenum carbide (Ti,Mo)C. It is used intungsten-free hardmetals with nickel binder forspecial steel cutting tools.

From the nine commercial carbides of thetransition metals, 36 combinations of doublecarbides can be formed. Some of these are at-tracting growing interest, for example (W,Mo)Cas a partial substitute for WC, or (Zr,Hf)C and

580 Carbides Vol. 6

(Zr,Nb)C as substitutes for TaC. All the doublecarbides are being intensively investigated [19],[20]. Mixed crystals of three transition-metalcarbides, such as (Ti,Nb,Ta)C, number a totalof 84, and 126 combinations are possible fromfour carbides, for example, (Ti,W,Hf,Zr)C. Theknowledge about most of these systems is stillquite limited.

3.3. Carbonitrides

The face-centered cubic monocarbides ofgroups 4 and 5 are isotypical with the face-centered cubic nitrides of the same groups, andbecause of this, there is complete miscibilitybetween a metal carbide and metal nitride. Thesituation for Ta (C,N) is more complex becausethe usual form of TaN is hexagonal, the cubicTaN being stable only at high temperature andpressure.

With some limitations all carbonitride com-pounds can be used in cemented carbides [21]. Aspecial application is scratchproof watchcases,because some carbonitrides are colored. The tintdepends on the C :N ratio. For example, Nb (C,N) is violet at high carbon contents but yellow athigh nitrogen contents. Another special applica-tion is coating of normal cemented carbides withlayers of carbonitrides, usually Ti (C,N), bychemical vapor deposition (CVD). Carbonitridescan best be made by heating mixtures of thesingle carbides and nitrides in argon or undervacuum at 1600 – 1800 �C.

3.4. Mixed Carbonitrides

By methods similar to those used in the produc-tion of solid solutions of carbides or carboni-trides, a large number of mixed crystals can beprepared with various metals and nonmetals inone lattice. Only a few of them have been inves-tigated up to now, and still fewer are used com-mercially. However, interest is growing. Forexample, (W,Mo)(C,N) with a nickel binder hasproperties comparable to those of WC – Co ce-mented carbides and can be used as a partialsubstitute for tungsten. The mixed carbonitride(Ti,Mo)(C,N) with a nickel alloy binder can bemade to have an extremely fine carbide structurebecause of a spinodal decomposition of the car-

bonitrides. Both (Ti,W)(C,N) and (Ti,Ta)(C,N)have excellent heat-resistant properties.

4. Carbides of the Iron Group andManganese

The carbides of Fe, Co, Ni, and Mn are usuallyclassified as metallic carbides or metal-like car-bides like the carbides of groups 4, 5, and 6.However, in fact, they are different from thetransition-metal carbides, and their metalliccharacteristics are less pronounced. Hardness,melting points, and electrical conductivity areall distinctly lower. The crystal structures are notthe simple interstitial H€agg phases, but rathermuch more complex structures, similar to thoseof the chromium carbides.

The carbides of Fe, Co, Ni, andMn are neitherprepared nor used alone. They are formed in ironand steel alloys during the melting process, andthey can be isolated from these products byanodic oxidation of the metals. The carbides areimportant because they are the hardening phasesin steel alloys, Stellites, cast iron, and relatedmaterials.

In the iron – carbon system there is probablyonly one phase, the orthorhombic Fe3C, which iscalled cementite [12011-67-5]. Preparation ofthe pure carbide from the elements has not beenachieved up to now. When Fe3C is isolatedelectrolytically from alloys and sintered withcobalt metal, it decomposes. In alloy steels, the‘‘iron carbide’’ is mostly included in mixed crys-tals with chromium carbides: (Fe,Cr)23C6, (Fe,Cr)7C3, and (Fe,Cr)3C2.

Cobalt and nickel form only the carbidesCo3C and Ni3C, which are isotypical to Fe3C.In the manganese – carbon system, three car-bides are formed: Mn3C is isotypical to Fe3C,and Mn7C3 and Mn23C6 are isotypical to thecorresponding chromium carbides.

5. Complex Carbides

A great number of ternary and quaternary phasescan be formed between carbon and two or threemetals, one a transition metal. In addition, theelements S, P, and As can be included.

These so-called complex carbides are a groupof their own. They are not solid solutions of one

Vol. 6 Carbides 581

carbide in the lattice of another carbide. Each hasits own typical structure, which in all cases ismuch more complicated than the simple H€aggphases of the transition-metal carbides.

Numerous complex carbides have been inves-tigated [9], and all were found to contain octahe-dral or, less often, trigonal prismaticM6Cgroups.M is always a transitionmetal, and six such atomssurround a central carbon atom. The octahedronsare linked by common corners, edges, or faces.The resulting interstitial sites can be occupied byother metals. Many distinct crystal structures canbe formed under these conditions. The most im-portant are perowskite carbides such as Ti3AlC,b-Mn carbides such as Ta3Al2C, k-carbidessuch as W16Ni3C6, h-carbides such as W3Co3C,H-phases or Cr2AlC-type carbides such as Zr2SC,V3AsC-type carbides such as Cr3PC, and Mn5Si3-type carbides such as Nb5Ga3C0.2. Complex car-bides can be best prepared by heating mixtures ofthe single carbides and metals for an extendedperiodof time.Mechanical pressureorgaspressureis helpful.

Of commercial interest are mainly the h-car-bides, which are formed in alloy steels and inStellites. In hardmetals, h-carbides, such asW3Co3C and W4Co2C, form because of carbondeficiency, these phases causing a decrease intoughness.

References

General References1 R. Kieffer, F. Benesovsky: Hartstoffe, Springer Verlag,

Wien 1963.

2 R. Kieffer, F. Benesovsky:Hartmetalle, Springer Verlag,

Wien 1965.

3 E. K. Storms: The Refractory Carbides, Academic Press,

New York 1967.

4 L. Toth: Transition Metal Carbides and Nitrides, Aca-

demic Press, New York 1971.

5 W. B. Pearson: Handbook of Lattice Spacings and Struc-

tures of Metals and Alloys, vol. 1 and 2, Pergamon Press,

Oxford 1958 (vol. 1) and 1967 (vol. 2).

6 E. Rudy: Compendium of Phase Diagram Data, AFML-

TR-65–2, ‘‘part 5’’, 1969.

General References7 G. H€agg, Z. Phys. Chem. Abt. B 12 (1931) 33 – 56.

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Further Reading

R.M. Feenstra, C. E. C.Wood (eds.):Porous Silicon Carbide

and Gallium Nitride, Wiley, Chichester 2008.

Z. C. Feng, J. H. Zhao: Silicon Carbide, Taylor & Francis,

New York, NY 2004.

A. Kr€uger: Carbon Materials and Nanotechnology, Wiley-

VCH, Weinheim 2010.

S. T. Oyama (ed.): The Chemistry of Transition Metal Car-

bides and Nitrides, 1st. ed., Blackie Academic & Profes-

sional, London 1996.

S. T. Oyama, R. Kieffer: Carbides, Survey, ‘‘Kirk Othmer

Encyclopedia of Chemical Technology’’, 5th edition, vol.

4, p. 647–655, John Wiley & Sons, Hoboken, NJ, 2004,

online: DOI: 10.1002/0471238961.1921182215250113.

a01.pub2.

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