Nanosized tungsten carbide powder produced by thermal plasma
Cornel Schreuders1, Marc Leparoux1, Jong-Won Shin1, Michael Dvorak2, Stephan
Siegmann1
1
EMPA, Thun, Switzerland
2
DACS, Thun, Switzerland
ABSTRACT
Tungsten carbide (WC) has a large application potential as coating, reinforcement, and bulk material due to its high
hardness and wear resistance. These properties can still be improved if nanosized grains are used.
At EMPA Thun nanosized tungsten carbide particles have been synthesised in an Inductively Coupled Plasma (ICP)
reactor. In this process, commercially available coarse and bulky WC particles in the diameter range of 0.6-22 µm were
used as precursors. These particles were injected into an Ar/H2 plasma using a special powder feeder able to deliver
continuously non flowable particles with low carrier gas amount.
A parameter study has been carried out varying plasma gas composition and flow rates of both process and powder
carrier gas to define a window where full evaporation of the solid precursor is ensured. Therewith high vaporization rates
of microscale WC particles could be achieved. The vapour phase was then quenched leading to the formation of
nanoparticles with an average diameter below 20 nm.
1 INTRODUCTION
The dwarf is becoming a giant. Nanoparticles and
nanostructured materials are getting more and more
important in several application fields like there are
for example micro- and optoelectronics, gas
sensors, catalysts, coatings, etc. Several techniques
are commonly used to produce nanoparticles.
Among the gas phase methods [1], the plasma
processing has proved to be an efficient way to
synthesise oxide as well as non-oxide nanoparticles.
In this paper an Inductively Coupled Plasma (ICP)
process has been investigated. Its advantages are
high temperatures, high energy densities and long
residence times, which allows all kinds of precursors
(solids, liquids, and gases) to be used. The ICP
process can be a single step reaction and does not
need purification like the removal of solvent. Also
the possibility of a controlled atmosphere and the
absence of electrodes are very beneficial for the
purity of the products. Besides the synthesis of
nanoparticles, the process can also be used for
spheroidisation [2] of bulky particles as well as for
coatings.
WC is very attractive due to its high hardness and
wear resistance, especially in combination with
cobalt in coatings. These properties are improved
when smaller hard phases are used [3]. The
production of WC nanoparticles starting from coarse
WC particles leads however to decarburisation of
WC at high temperatures resulting in substoichiometric species [4].
2 EXPERIMENTAL DATA
2.1 Experimental procedure
Figure 1 shows the experimental ICP set-up used
for this study at EMPA.
Figure 1: Overview of the ICP equipment. 1) Plasma torch
(PL-35, Tekna, Canada). 2) Synthesis chamber equipped
with view ports for in-situ monitoring and for in-situ powder
sampling. 3) Collector for micro sized particles. 4) Small
sampling filter for product sampling during processing. 5)
Production filter. 6) Filter control unit. 7) Gas manifold. 8)
RF generator (30 kW, 13.56 MHz).
The solid precursors where introduced in the
process using the Powdercube feeding system
(DACS, Switzerland). This system enables the
precursor dosage by dense conveying mode with a
small carrier gas flow. The (solid) precursor enters
the torch through an axial moveable injection probe
(Fig. 2).
In experiments no. 1-8 WC (1µm, Ceratizit WCCT100, Luxembourg) was used as precursor,
whereas metallic tungsten (3µm, WOKA, Germany)
was used in experiment 9. Methane and Carbon
Black Grade N134 Fluffy (20nm, Cabot, USA) were
used as carburising media in experiments1-4 and 59, respectively. The gases and their purities used in
the experiments were Ar 4.8, N2 4.5, H2 4.5 and CH4
4.5 (Carbagas, Switzerland).
Central gas (Ar, He)
Sheath gas
(Ar, H2, N2, He)
The samples of exp. 5-9 were ball milled for 1¾
hours.
Precursors
(+ carrier/reactive gas)
The plasma processed powder was collected in the
sampling
filter
[5]
for
further
ex-situ
characterisations.
injector position
2.2 Characterisation methods
quenching
gas
The BET specific surface of the synthesised
samples was measured with Tristar and Gemini gas
adsorbing systems of Micromeritics, USA. The BET
diameter was calculated with the assumptions that
the particles were spherical and had the theoretical
WC bulk density.
Phase characterisations were carried out using Xray diffraction on either a Siemens, Diffraktometer
D5000 operating with Mo(Kα) radiation or a Bruker,
AXS D8 Advance diffractometer operating with
Cu(Kα) radiation. For quantification of the XRD
spectra TOPAS 2 software from Bruker AXS,
Germany was used based on Rietveld analysis.
synthesis
chamber
Figure 2: Schematic drawing of the ICP torch.
For all the experiments, the process pressure was
400 mbar, the central gas flow rate 4 slpm Ar and
the sheath gas flow rate 80 slpm Ar and 6 slpm H2.
Moreover, the quenching position was at 2.5 cm
underneath the top of the synthesis chamber and
the power input to the plasma was about 20 kW.
Table 1. Experimental conditions: Exp is experiment
number, feed rate is amount of WC+C in g/min, carrier
gas in slpm, Cblack in wt%, and quench gas in slpm.
Exp.
feed
rate
Carrier gas
Ar
CH4
Cblack
quench
gas
1
5.0
9.6
0
-
70
2
5.6
8.4
1.2
-
70
3
6.0
7.2
2.4
-
70
4
6.6
4.8
4.8
-
70
5
1.3
9.6
0
3.9
70
6
1.3
9.6
0
3.8
70
7
1.1
9.6
0
11.1
70
8
1.9
9.6
0
0.5
70
9*
2.2
9.6
0
6.2
70
10
4.0
9.6
0
0
2
11**
6.6
9.6
0
0
70
* Exp. 9 metallic W was used instead of WC.
** Exp. 11 used N2 as quench gas instead of Ar.
Microscopical pictures were made using a Scanning
Electron Microscope (Zeiss, DSM 962) and
Transmission Electron Microscope (Philips CM 30).
These pictures were analysed by an automatic
image analysis system for powders called
Powdershape (IST Ltd., Switzerland). Besides an
algorithm for analysis of isolated particles, to
determine shape and particle size distribution, a
new developed fractal algorithm was used for
determination of the
size distribution of
agglomerates [6].
3 RESULTS
The as-received precursor consists of WC with
traces of WC1-x. Fig. 3 compares the as received
precursor with Exp. 4.
X
intensity, arbitrary units [-]
RF
13.56 MHz
Exp.4
as received
X
X
#
O
30
O
40
O
#
#
O
50
60
2Θ (Cu radiation) [-]
X
X
70
Figure 3: XRD spectra of the raw material and experiment
4 (4.8 slpm CH4). The phases are marked with the
following symbols: X = WC, # = WC1-x, and O = W2C.
Table 2. Phase compositions in wt% as determined by
XRD. Cdeficit means the amount of carbon necessary to
obtain stoichiometric WC. dBET is the diameter in nm
calculated from the specific surface.
Exp
WC
WC1-x
W2C
W
Cdeficit
dBET
1
27
25
16
32
46
84
2
37
41
10
12
27
122
3
61
29
6
5
13
50
4
90
7
3
0
3
32
5
6
33
46
15
46
19
6
6
37
41
16
46
-
7
5
39
38
18
47
-
8
6
25
48
21
51
23
9
1
40
39
20
50
14
10
4
14
64
18
54
-
11
11
19
40
30
55
54
Intensity arbitrary units [-]
Fig. 5 and Tab. 2 demonstrate the influence of
quenching on the several tungsten carbide phases.
X
Exp. 10
Exp. 11
Exp. 1
+
O
X
#
O
32
36
O
#
40
2 Θ (Cu radiation) [-]
Figure 5: XRD spectra of experiments in which different
quench gases and amounts are used (X = WC, # = WC1-x,
O = W2C, and + = W).
Fig. 4 compares the XRD spectra from exp. 1-3. A
trend to more wt% WC with increasing methane is
clearly visible.
X
Only traces of WC were found in the XRD spectrum.
About 20wt% of the metallic W remained unreacted
after processing.
Intensity, arbitrary units [-]
The experimental results are summarised in Tab. 2.
Fig. 6 shows a TEM picture of ICP elaborated WC
nanoparticles. Powdershape calculated the median
of the primary particle size distribution to be about
14-27 nm, which is in good agreement with visual
measurements of the TEM picture.
Exp.1
Exp.2
Exp.3
X
#
O+
X
#
O
O
+
O
15
20
2 Θ (Mo radiation) [-]
25
Figure 4: XRD spectra of experiments using different
amounts of methane (X = WC, # = WC1-x, O = W2C, and +
= W).
The addition of carbon black to WC in a range from
0.5-11wt% did not result in an improvement of the
WC stoichiometry compared to Exp. 1. But the
phase distribution changed drastically. The amount
of WC remained around 5wt%, WC1-x in the order of
30-40wt%, W2C 40-50wt% and W 15-20wt%.
The plasma reaction of metallic tungsten with
carbon black (Exp 9) led to a partial carburisation.
Figure 6: TEM picture of synthesised WC nanoparticles.
4 DISCUSSION
The experiments showed the influence of
compensation with methane and carbon black for
the WC decarburisation as it occurs due to the high
temperature in the plasma.
The amount of stoichiometric WC phase increases
with the methane flow, but a complete carburisation
44
is not achieved. Moreover, the methane flow rates
are very high in comparison to the theoretically
required amount of carbon (the weight loss of a
sample without the addition of any carbon, amounts
approximately 3 wt% C).
The addition of carbon black to the WC inlet powder
led not to pure WC, but mainly to sub-stoichiometric
phases. About 70-80wt% of the products consist out
of WC1-x and W2C, 15-20wt% W and only 5wt% WC.
The carbon weight loss is comparable to samples
without the addition of methane.
The route to synthesise WC from W and carbon
black was also investigated (exp. 9). This approach
resulted in only traces of WC. Around 80wt%
consisted out of WC1-x and W2C. 20wt% W did not
react with carbon.
Excessive amounts of carbon species fed into the
plasma lead to a high supersaturation of the gas
phase. Therefore, homogeneous nucleation is
promoted leading to soot. Moreover, carbon is
reduced by hydrogen to form stable gaseous C-H
species. This carbon will therefore not be available
for carburisation. OES measurements will be carried
out to see if a correlation between H2 and C-H exists
and the content of free carbon in the synthesised
nanoparticles will be determined.
The poor carburisation effect of carbon black
compared to methane on the carburisation of WC is
probably also due to the mixing process of both inlet
powders. The ball milling was not sufficient enough
to get carbon mechanically alloyed with WC. The
grains may then follow different trajectories within
the plasma due to the different momentum of
tungsten carbide and carbon black. Moreover, within
the plasma, thermophoretic effects may influence
the respective trajectories. These assumptions are
correlated with optical observations of the plasma
which has a green colour (C2 species) on the
outside and some soot observed on the view-ports
when carbon black or methane is used. There is no
blue colour on the outside of the plasma originating
from excited W metal. Only part of the methane and
carbon black entrained in the plasma with the WC is
available for the carburisation. Finally it is also
energetically easier to fragmentise methane than to
evaporate carbon black.
The optimisation of the precursor injection is
therefore of prime importance and will be further
investigated.
Quenching of the hot gaseous phase influences not
only the size of the elaborated nanoparticles but
also the resulting powder chemistry. Different
quench
gases
result
in
different
phase
compositions. The amount of W in samples
quenched with N2 is larger than in cases where Ar
was used. This could be due to the fact that N2 is
not inert and/or the different heat transfer behaviour
compared to Ar. If N2 is chemically active, for
example by forming NH and CN, this would be
detected by OES. Next investigations will deal the
injection position and flow rate of the quenching gas.
According to the XRD measurements the addition of
methane proved to be an efficient way to
counterbalance the decarburisation of WC during
plasma processing. Further experiments are
required to find out if a complete in-line carburisation
can be achieved.
5 ACKNOWLEDGEMENTS
This work was supported by the Commission for
Technology and Innovation (CTI) TOP NANO 21
(project 5978.2). Ceratizit, Luxembourg is kindly
acknowledged for providing the tungsten carbide
precursor and the Particle Technology Laboratory,
ETH Zurich for their support in characterisation of
some samples.
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