Preparation of tungsten carbides by reducing and carbonizing WO2 with CO

Article abstract of DOI:10.1016/j.jallcom.2018.02.224

Tungsten carbides, which can be widely used as cutting and drilling tools, chemical catalysts and aerospace coatings, have attracted widespread attentions. The present work reported a simple method to prepare tungsten carbides by using WO₂ as the raw materials and CO as the reduction and carbonization agent. It was found that at lower and moderate temperatures, all the final product was WC, while at higher temperatures, the final products were W₂C or W. The reduction and carbonization mechanism was also studied theoretically and experimentally. WO₂ was reduced and carburized to WC in one step at lower temperatures; WO₂ was first reduced and carburized to W₂C and then to WC at moderate temperatures; while at higher temperatures, WO₂ was first reduced to metallic W and then carbonized to WC or W₂C, but with the temperature increasing, the carbonization became difficult and required much higher CO content.

Full text of DOI:10.1016/j.jallcom.2018.02.224

Journal of Alloys and Compounds 745 (2018) 421e429
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Preparation of tungsten carbides by reducing and carbonizing WO2
with CO
*
Jie Dang , Yijie Wu, Zepeng Lv, Xuewei Lv
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 November 2017
Received in revised form
Tungsten carbides, which can be widely used as cutting and drilling tools, chemical catalysts and
aerospace coatings, have attracted widespread attentions. The present work reported a simple method to
prepare tungsten carbides by using WO
agent. It was found that at lower and moderate temperatures, all the final product was WC, while at
higher temperatures, the final products were W C or W. The reduction and carbonization mechanism
was also studied theoretically and experimentally. WO was reduced and carburized to WC in one step at
lower temperatures; WO was first reduced and carburized to W C and then to WC at moderate tem-
peratures; while at higher temperatures, WO
or W
higher CO content.
2
as the raw materials and CO as the reduction and carbonization
19 January 2018
Accepted 18 February 2018
Available online 20 February 2018
2
2
2
2
Keywords:
2
was first reduced to metallic W and then carbonized to WC
WO
2
2
C, but with the temperature increasing, the carbonization became difficult and required much
Carbon monoxide
Tungsten carbides
Reduction
© 2018 Elsevier B.V. All rights reserved.
Carbonization
1
. Introduction
There are many methods for preparation of tungsten carbides.
These methods include direct carburization of tungsten powder,
high-energy mechanical ball milling[10], the spray conversion
process[11], solid-gas reaction[12], sol-gel and in-situ carburization
[13], and combustion synthesis[14]. The traditional method is to
direct carburization of tungsten powders: tungsten powders are
mixed with carbon black by ball-milling for an extender time, and
Tungsten carbides, which belong to the group of cemented
carbides and refractory carbides, have advantages of high melting
point, high hardness, low friction coefficients, high thermal sta-
bility and low thermal expansion coefficient[1]. These advantages
make tungsten carbides appropriate for various engineering ap-
plications such as manufacture of cutting tools, rock drill tips and
general wear parts[2-5]. Besides, tungsten carbides have been used
as the Pt electro-catalyst support for various processes[6]. A wide
variety of applications have been recognized for bulk nano-
ceramics and nano-ceramic composites, for instance, the durable
ceramic parts for automotive engines, ultrafine filters, aerospace
erosion resistant coatings, flexible superconducting wire and fibre
optic connector components[7].
ꢀ
then carbonized at 1400e1600 C (1673e1873K)[15]. Because the
tungsten powders and carbon black can not contact inadequately,
the reaction proceeds slowly. The application of different sources of
tungsten containing precursor and carbon-containing gas for
preparation of tungsten carbides has been reported in litera-
tures.[16,17]. These methods successfully prepared tungsten car-
bides. However, some of the preparation processes can be
complicated. Because in these processes, a special precursor must
be prepared before reduction and carbonization, such as the
coating of precursor with carbon. Table 1 summarized the typical
specific methods which were reported in the literatures, for prep-
aration of WC. In all of the specific methods of preparation of
tungsten carbide, the processes required either a higher tempera-
ture or a long time. Venables and Brown[22] studied the reduction
Tungsten carbides mainly include two phases, namely, WC and
2
W C. In general, higher tungsten carbide WC has one common
structure, hexagonal structure. However, lower tungsten carbides
W
W
distribution causes the possibility of the formation of several
structural modifications of W C[8,9].
0
2
C have several structural modifications:
Cand ε-W
b
-W
2
C,
b
2
-W C,
b
”-
2
2
C. The presence of different types of carbon atoms
2
of WO
be reduced.
The aim of this work is to prepare tungsten carbides by using
WO as the raw materials and CO as the reduction and carboniza-
tion agent. The reduction and carbonization mechanisms were
3
with CO, which showed that the reaction temperature can
2
*
Corresponding author.
E-mail address: jiedang@cqu.edu.cn (J. Dang).
https://doi.org/10.1016/j.jallcom.2018.02.224
925-8388/© 2018 Elsevier B.V. All rights reserved.
0

422
J. Dang et al. / Journal of Alloys and Compounds 745 (2018) 421e429
studied theoretically and experimentally as well.
. Material and experimental
WO powders (larger than 100 mesh, 99.9% metals basis) from
2
2
Aladdin Industrial Corporation were employed as the raw material.
Co powders (40e100 mesh, 99.9% metals basis) from Chengdu
Jinchun metal material Co., Ltd. were also used as raw materials.
An HCT-2 Thermo-Gravimetric Analyzer (TGA) was used to
monitor the weight change of samples during reduction and the
experimental apparatus was described elsewhere. [23] A sample of
around 100 mg was used in each experimental run and after an
alumina crucible (8 ꢁ 8mm) with samples was put into the TGA,
high-purity argon was introduced to flush air out of the furnace. In
isothermal reduction, the furnace was first heated from room
temperature up to a desired reduction temperature at a rate of
ꢀ
1
0 C/min while maintaining the argon atmosphere. Once the
thermal balance stabilized, the argon gas was switched to reaction
gas, and the weight loss was then monitored continuously by the
TGA. After reacting for a certain time, the reaction gas was switched
to argon again, and samples were cooled down to room tempera-
ture. In non-isothermal experiments, CO was first used to flush air
from the furnace, and then the furnace was heated from room
ꢀ
temperature to 1200 C at heating rates of 4, 6 and 8 K/min, in
different runs. In all experimental runs, a constant gas flow rate of
ꢂ2
ꢀ
6
0 ml/min (about 0.318 ꢁ 10 m/s at 25 C) was maintained. The
flow rate of gas was controlled by gas flow controllers (Alicant,
Model MC-500SCCM-D).
To study the mechanical performance of the prepared WC, WC-6
wt% Co composites were prepared using the hot-press sintering
technique. Around 10 g WC powders and Co powders (with mass
fraction of 6%) were mixed first and then pressed into cylinder
under an axial mechanical pressure of 11 MPa. The hot pressing was
processed in high strength graphite die, high purity argon gas un-
ꢀ
der an axial mechanical pressure of 13 MPa at 1500 C for 3 h.
X-Ray Diffraction (XRD) (PANalytical X'Pert Powder, Panalytical
B.V.) measurements were conducted for samples. Morphologies of
these samples were observed using SEM (TESCAN VEGA 3 LMH,
Czech Republic) technique. The Brunauere EmmetteTeller (BET)
method was employed to determine the surface areas of samples at
7
7 K (Micromeritics ASAP 2020, American). The hardness of WC-6
wt% Co composites sample was determined using hardometer
Hengyi MH-6, China). The thermodynamics calculations were
performed by using FactSage 6.2 with pure substances database.
(
3
. Expected reactions
Equilibrium calculations were carried out for an indication of
the expected reactions during the reduction and carburization.
Fig. 1 gives the equilibrium diagram of CO and WO system, which
clearly shows the reaction products at different temperatures and
CO/(CO þ WO ) ratios. It can be obtained that at low temperatures,
WC is the most stable phase, but W C and W would be formed with
increasing the temperature, and the ratio of CO/(CO þ WO ) must
be rather close to 1 to form W C and WC at high temperatures.
Figs. 2 and 3 give the calculated results for three typical experi-
2
2
2
2
2
ꢀ
ꢀ
mentally used temperatures: 813 C (1086 K), 908 C (1181 K) and
1
ꢀ
179 C (1452 K). Fig. 2 shows the WeOeC predominance diagrams
(
for the three temperatures). The line of crosses in Fig. 2 represents
a total gas pressure of 1 atm; equilibrium gas compositions (con-
sisting largely of CO and CO ) would lie along this line if WO was to
react with pure CO in different molar ratios, for a total pressure of
atm. The formation of WC would require a more CO-rich gas,
2
2
1
corresponding to gas compositions towards the lower right in the
predominance diagrams (along the lines of crosses). Fig. 2 indicates

J. Dang et al. / Journal of Alloys and Compounds 745 (2018) 421e429
423
that different reaction sequences should be expected for reaction at
ꢀ
higher and lower temperatures. At the lower temperatures (813 C
ꢀ
and 908 C in this example), WO
2
can be simultaneously reduced
and carburized, but first forming W
2
C and then forming WC, ac-
cording to reaction (1a) and (1b):
2
WO
2
þ 6CO ¼ W
C þ2CO ¼ 2WC þ CO
However, at the higher temperature, WO
2
C þ 5CO
2
(1a)
(1b)
W
2
2
2
would be reduced to
C, and even WC at
W first, followed by carburization to form W
much higher CO/CO ratios, according to the three steps of reaction
2):
2
2
(
2
WO
2
þ 4CO ¼ 2W þ 4CO
W þ 2CO ¼ W C þ CO
C þ2CO ¼ 2WC þ CO
2
(2a)
(2b)
(2c)
2
2
2
Fig. 1. Equilibrium diagram showing solid phase fields as a function of temperature
and input CO mole fraction in theWO -CO system.
2
W
2
2
by reaction with CO at 813 C, 908 C and 1179 ꢀC, total gas pressure of 1 atm given by crosses (þ).
ꢀ
ꢀ
Fig. 2. Predominance diagrams for reduction and carburization of WO
2

424
J. Dang et al. / Journal of Alloys and Compounds 745 (2018) 421e429
ꢀ ꢀ ꢀ
2
Fig. 3. Stable gaseous and solid reaction products after equilibration of different molar ratios of WO and CO at 813 C, 908 C and 1179 C. (1 atm total pressure).

J. Dang et al. / Journal of Alloys and Compounds 745 (2018) 421e429
425
This difference in expected reaction sequence is also illustrated
by Fig. 3, which shows the equilibrium reaction products for reac-
tion of WO and CO in different molar ratios. In Fig. 3, the partial
pressure of CO - balance is CO - is shown in the upper graphs; the
4. Results and discussion
2
4.1. Non-isothermal reaction
2
relative molar amounts of solid phases are shown in the lower
graphs. Fig. 3 confirms the difference in expected reaction
sequence: simultaneous reduction and carburization [reaction (1)]
at the lower temperature, and reduction to W, followed by carbu-
To clarify the onset temperature of reduction and the effect of
ramping rates on the reaction, non-isothermal reduction of WO
powders was performed by using pure carbon monoxide at three
2
ꢀ
ꢀ
ꢀ
different ramping rates, 4 C/min, 6 C/min and 8 C/min. The ob-
tained results are shown as Fig. 4. Fig. 4a shows the temperature
dependences of the weight changes during non-isothermal reac-
tion, which demonstrates that different ramping rates have a sig-
nificant effect on the reduction rate and the reaction route. It is
rization to W
2
C at the higher temperature [reaction (2)]. However,
ꢀ
it should be noted that the final product is W
2
C at 1179 C even the
in
in
ratio of nCO/nWO2 is as much as 40 or even higher. Fig. 3 also il-
lustrates that the CO partial pressure required to yield W C as
stable product is much lower at the higher temperature.
2
2
ꢀ
shown that the reaction became apparent at around 730 C
These predicted reaction sequences were tested experimentally
by measuring the phases in partially reduced materials, and by
monitoring the mass loss during reaction (thermogravimetry). The
difference in mass loss for the different reaction sequences was
readily detectable: completion of reaction (1) (reduction and
carburization, forming WC) would give a reduction in mass of the
(1003 K), and a higher heating rate led to a faster reaction (Fig. 4b).
Based on the weight change, the reaction is divided into two steps,
loss-mass reaction and gain-mass reaction. All the three curves first
go over ꢂ9.27% (corresponding to forming WC) but do not
reach ꢂ12.05% (corresponding to forming W
to ꢂ9.27%. It suggests that only part of WO
carbonized to W C and then to WC, which is confirmed by XRD
analysis (listed in Table 2). This is because at low temperature, WC
is formed directly, but with increasing the temperature, W C will be
2
C), and then go back
2
was first reduced and
solid of 9.27% (relative to the original mass of WO
2
) and completion
2
of reaction (1a) to form W C would give a mass loss of 12.05%,
2
whereas completion of reaction (2a) (reduction to W) would give a
mass loss of 14.83%.
2
ꢀ
formed first. With increasing the ramping rate from 4 C/min to
ꢀ
6
C/min, the amount of formed W
2
C increases dramatically,
C forming temperature;
resulting from the rapid approaching W
while increasing the ramping rate to 8 C/min, the amount of W
2
ꢀ
2
C
drops off, which may be caused by reacting formed W
immediately at high temperature.
2
C to WC
The maximum rate for the first reaction (loss-mass reaction)
ꢀ
ꢀ
ꢀ
appeared at 840 C (1113 K), 945 C (1218 K), and 973 C (1246 K) at
ꢀ
ꢀ
ꢀ
the heating rate of 4 C/min, 6 C/min and 8 C/min (Fig. 4b),
respectively. The slow reaction rates at low temperatures and high
temperatures are due to the relatively small reduction rate constant
and the exhaustion of samples, respectively. The highest reduction
rate has occurred at the temperature when both reduction rate
constant and oxide concentration were at the high levels.
4.2. Isothermal reaction
Isothermal reduction of WO
2
by pure CO was studied in the
ꢀ
ꢀ
temperature range of 813 C (1086 K) to 1179 C (1452 K). Fig. 5
shows the time dependences of the weight changes during
isothermal reduction at eight different temperatures. The figure
ꢀ
shows that at lower temperatures [below 861 C (1134 K)], the mass
loss increased smoothly to 9.27% (corresponding to the weight
ꢀ
change from WO
(
2
to WC); at moderate temperatures [884 C
ꢀ
1157 K) to 908 C (1181 K)], the weight loss ratio first increased
over 9.27% but not up to 12.05% (corresponding to the weight
change from WO
2
to W
2
C), and finally decreased to 9.27%; at
ꢀ
ꢀ
1009 C (1282 K) and 1102 C (1375 K), the weight loss ratio first
increased up to 14.83% (corresponding to the weight change from
WO to W) and then decreased to about 9.27%; however, the weight
loss ratio first increased up to 14.83% and then decreased very slow
2
ꢀ
ꢀ
when the reaction proceeded at 1150 C (1423 K) and 1179 C
1452 K). This reaction behavior is consistent with the expected
(
reaction sequence, as discussed earlier: at lower temperatures,
simultaneous reduction and carburization to form WC directly from
WO
tion and carburization in WO
rization of to form WC are expected; while at high
temperature, reduction of WO to W and then carburization of W to
WC are expected; at even higher temperatures, W C as the most
2
is expected; at the moderate temperatures, two-step reduc-
2
to W C, followed by further carbu-
2
2
W C
2
2
stable phase is expected, however, the carburization proceeds very
slow.
2
Fig. 4. Non-isothermal reduction of WO with pure CO: (a) weight change verse
temperature, (b) DTG verse temperature.
ꢀ
XRD analyses for completely reduced samples at 813 C

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J. Dang et al. / Journal of Alloys and Compounds 745 (2018) 421e429
Table 2
Phase compositions of samples reduced under different conditions (based on XRD analyses).
Reduction of WO
2
by CO under different conditions
Phases composition
major
minor
trace
ꢀ
Non-isothermal reduction (ramping rate of 4 C/min)
WC
W
WC
WO
W
e
e
W
e
e
e
ꢀ
1
1
8
1
179 C for 222min (isothermal reduction)
W
W
2
C
C
ꢀ
102 C for 175min (isothermal reduction)
2
ꢀ
13 C for 30 min (isothermal reduction)
2
2
WC; b”-W C
e
ꢀ
009 C for 47 min (isothermal reduction)
Fig. 5. Reduction curves of WO
2
by pure CO at different temperatures.
ꢀ
2
Fig. 7. The effect of CO content on the reduction of WO at 861 C.
ꢀ
1
009 C (1282 K) were carried out and the obtained phase com-
positions are shown in Table 2. It illustrates the difference in re-
ꢀ
action sequence depending on temperature: at 813 C (1086 K) the
2 2
reduction sequence is WO to b”-W C and then to WC, which is
consistent with the thermodynamics calculations (Figs. 2 and 3),
and the reactions of WO
taneously; while in the sample reduced for 47 min at 1009 C
2
to W
2
C and W
2
C to WC occurred simul-
ꢀ
(
1
1282 K) (corresponding to the lowest point of TG curve for
009 C), only W phase was detected, in agreement with the
ꢀ
2
expectation that at higher temperatures WO would first be
reduced to metallic W.
4.3. The effect of CO content on the reaction
The effect of CO content on the reduction of WO
gated in this study as well. Fig. 7 shows the time dependences of the
2
was investi-
ꢀ
ꢀ
Fig. 6. XRD patterns of completed reduction of WO
2
by CO at 813 C, 861 C and
ꢀ
1009 C.
weight changes in reduction of WO
61 C (1134 K). It can be obtained that CO content has a significant
2
with different CO content
ꢀ
8
ꢀ
ꢀ
(
1086 K), 861 C (1134 K) and 1009 C (1282 K) are shown as Fig. 6,
influence on the reduction and carbonization rate. With increasing
the CO content from 60% to 80%, the reduction and carbonization
rate increases greatly, but from 80% to 100%, the reduction and
carbonization rate has a modest rise.
which confirms that the final product is WC for samples of which
the mass loss is around 9.27% after reaction. It should be noted that
higher temperature results in better crystallization. The phase's
compositions of samples reduced at 1102 C (1375 K) for 175min
and 1179 C (1452 K) for 222min were also detected (Table 2). It
ꢀ
ꢀ
4
.4. Morphology of reacted samples
ꢀ
shows that at 1179 C (1452 K), the final products are almost W
with trace amount of W
be obtained that at higher temperature [above 1150 C (1423 K)],
2
C. Combining the TG curves (Fig. 5), it can
Fig. 8 shows the SEM images of samples completely or partially
ꢀ
reacted at different temperatures. The morphologies of WC ob-
the formation of W
2
C or WC is very difficult and requires higher CO
ꢀ
tained at low and high temperatures are quite different: at 813 C
content, which agrees well with the thermodynamics results
(
1086 K), the obtained WC particles show acicular shape (Fig. 8a);
(Figs. 1 and 2). To confirm the reduction process, X-ray diffraction
ꢀ
while full reaction at 1009 C (1282 K) results in WC particles with
spherical shape (Fig. 8d), which retains the spherical shape of the
ꢀ
detections of samples partially reacted at 813 C (1086 K) and

J. Dang et al. / Journal of Alloys and Compounds 745 (2018) 421e429
427
ꢀ
ꢀ
ꢀ
Fig. 8. SEM images of: (a) sample completely reduced at 813 C (WC); (b) sample reduced at 1009 C for 47min (W); (c) sample reduced at 1179 C for 222 min (W with trace
ꢀ
amount of W
2
C); (d) sample completely reduced at 1009 C (WC).
ꢀ
2
ꢀ
W particles (Fig. 8b). Fig. 8b and d indicate that the fine substruc-
ture forms during initial reduction in WO to W, not during sub-
813 C, 7.5051 m /g (12.219 nm) for that obtained at 861 C, and
2
ꢀ
2
10.3442 m /g (11.391 nm) for that obtained at 1009 C, and the
calculated particle sizes are 202.4 nm, 51.1 nm and 37.1 nm,
respectively.
The surface areas of WC obtained in this study are similar to
those obtained by Ma et al.[24] via direct solid-state synthesis from
mechanically activated tungsten oxide and graphite and Won
et al.[14] by combustion synthesis.
sequent carburization. Fig. 8c represents the SEM image of W
ꢀ
particles produced at 1179 C (1452 K). Comparing with W pro-
ꢀ
ꢀ
duced at 813 C (1086 K) (Fig. 8b), W particles produced at 1179 C
1452 K) are larger and show a quite different shape (largely poly-
hedron). Fig. 9 shows the typical TEM images of raw WO and WC
(
2
powders obtained at different temperatures. Fig. 9A1 and B1 clearly
ꢀ
show that the morphologies of raw WO
2
and WC obtained at 813 C
are acicular shaped and the particle sizes of them are nearly the
same as each other, which are well agreement with the images
4
.5. Hardness of WC-6 wt% Co composites
ꢀ
obtained by the SEM (Fig. 8). That means WC obtained at 813 C
inherited the morphology of the raw material WO . However,
Fig. 9C1 demonstrates that the shape and size of WC prepared at
ꢀ
The prepared WC obtained at 813 C was used to check its
2
property. WC-6 wt% Co composites were prepared using the hot-
press sintering technique. The determined hardness of WC-6 wt%
Co composites is 1748.8 kgf/mm (HV), which is in the range of
ꢀ
ꢀ
1
009 C are quite different from those of WC obtained at 813 C, and
2
ꢀ
the WC obtained at 1009 C are polyhedron shaped. The corre-
those reported in the literatures [25,26].
sponding SAED patterns for these specimens are depicted in
Fig. 9A2, B2 and C2, respectively. The diffraction spots are exhibited
5
. Conclusions
ꢀ
for WO
2
and WC prepared at 1009 C, which suggests that WO
2
and
ꢀ
WC prepared at 1009 C are both single-crystalline structure.
Although the diffraction rings shown in Fig. 9B2 are not very per-
fect, WC prepared at 1009 C has a polycrystalline structure can be
In this study, a method to prepare tungsten carbides was re-
2
ported. The precursor used to produce tungsten carbides was WO ,
ꢀ
and the reduction and carbonization agent was CO. Based on the
thermodynamic analyses and experiments, the reduction and
carbonization mechanisms were obtained: at lower temperatures,
confirmed. The BET surface areas (average mesopore size in
2
parenthesis) were 1.8965 m /g (7.657 nm) for WC obtained at

428
J. Dang et al. / Journal of Alloys and Compounds 745 (2018) 421e429
ꢀ
ꢀ
Fig. 9. Typical TEM images of the particles; A1: raw WO
2
powders, B1: sample completely reduced at 813 C (WC), C1: sample completely reduced at 1009 C (WC). A2, B2 and C2
are the SAED patterns recorded from the ellipses marked in A1, B1 and C1, respectively.
WO
temperatures, WO
then to WC; while at higher temperatures, WO
metallic W and then carbonized to WC or W
perature increasing, the carbonization became difficult and
required much higher CO content. The morphologies of samples
reduced and carbonized were also investigated and it was found
that the morphologies of WC prepared at different temperatures
were quite different.
2
was reduced and carburized to WC in one step; at moderate
was first reduced and carburized to W C and
was first reduced to
C, but with the tem-
Acknowledgements
2
2
2
Thanks are given to the financial supports from the National
Natural Science Foundation of China (51604046, 51674053),
Fundamental and Frontier Research Project of Chongqing
(cstc2017jcyjAX0322), State Key Laboratory of Advanced Metal-
lurgy (KF-17-02), and the Fundamental Research Funds for the
2
Central
Universities
(106112016CDJXY130005
and
106112017CDJXY130002).

J. Dang et al. / Journal of Alloys and Compounds 745 (2018) 421e429
429
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