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� 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
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Figure
1.Survey
ofbinarycompoundsofcarbonwiththeelem
ents
566 Carbides Vol. 6
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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(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.
8 H. Nowotny, F. Benesovsky, Planseeber. Pulvermetall.
16 (1968) 204 – 214.
9 H. Nowotny, Angew. Chem. 84 (1972) 973 – 982; An-
gew. Chem. Int. Ed. Engl. 11 (1972) 906 – 915.
10 D. N. French, D. A. Thomas, Trans. Metall. Soc. AIME
233 (1965) 950 – 952.
11 O. R€udiger, G. Ostermann, H. Kolaska, Tech. Mitt. Krupp
Forschungsber. 28 (1970) no. 2, 33 – 54.
12 E. Neuenschwander, J. Less-Common Met. 11 (1966)
365 – 375.
13 L. Ramquist in H. H. Hausner (ed.): Modern Develop-
ments in Powder Metallurgy Processes, vol. 4, Plenum
Press, New York 1970, pp. 75 – 84.
14 M. Miyake, Prepr. Eur. Symp. Powder Metall. 5th 1978
1978 – 1979, 93 – 98.
15 K. Ushiyima, Powder Metall. Int. 11 (1979) 158 – 160.
16 G. Jangg, R. Kieffer, L. Usner, J. Less-Common Met. 14
(1968) 269 – 277.
17 G. Brauer, R. Lesser, Z. Metallkd. 50 (1959) 8.
18 Ch. Chatfield, Powder Metall. Int. 15 (1983) 18 – 19.
19 H. Holleck, Metall (Berlin) 35 (1981) 999 – 1004.
20 H. Holleck, Metall (Berlin) 35 (1981) 1246 – 1253.
21 R. Kieffer, P. Ettmayer, M. Freudhofmeier, Metall (Ber-
lin) 25 (1971) 1335 – 1342.
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.
582 Carbides Vol. 6