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Powder Metallurgy Progress, Vol.9 (2009), No 1 49
PLASMA SPRAYING OF ZIRCONIUM CARBIDE - HAFNIUM CARBIDE -
TUNGSTEN CERMETS
V. Brožek, P. Ctibor, D. I. Cheong, S.-H. Yang
Abstract Preparation of coatings and self-standing parts from
materials with the highest melting points is allowed only by a
limited number of techniques. Plasma torch WSP® with exit nozzle
plasma temperature about 30 000 K, operating at 150 kW, enables
treatment of such refractories with extreme melting points in an
amount of tens of kilograms per hour [1]. A mixture of the W powder
was prepared with 10 vol.% to 20 vol.% of ZrC or HfC. This
feedstock having spheroidal character and micrometric size was fed
into the plasma of the water stabilized plasma torch (WSP®) by
means of inert gas carrying. Coatings thickened up to 2 mm were
sprayed on various substrates, namely graphite. Self-standing
bodies were obtained by substrate removal. Pure tungsten and pure
zirconium carbide were sprayed at similar conditions. Various
manners of coating improvement by shrouding and sample controlled
cooling were tested. XRD, XRF, mercury porosimetry, dilatometry and
various microscopic structural techniques were used for the
coatings characterization. Resulting coatings are hard and can
serve as a surface protection of graphite substrates with various
shapes and grain orientations. Keywords: plasma spraying, tungsten
cermets, zirconium carbide, hafnium carbide, water stabilized
plasma
INTRODUCTION Tungsten, tungsten alloys and tungsten-based
cermets with high-density, high-
strength, low coefficient of thermal expansion, excellent
corrosion resistance and mechanical properties have found
applications in aerospace, nuclear and military equipment,
electronics, the chemical industry and many other applications. One
of most promising technologies for these refractory materials
fabrication is plasma spraying.
Major trends in plasma spraying of tungsten-based composites are
summarized in [2]. Plasma sprayed tungsten and tungsten-copper
coatings are being developed for potential application as plasma
facing materials for fusion reactors e.g. ITER [3].
Hafnium-based materials, on the other hand, are traditionally
regarded as a class of valuable materials in nuclear industries, as
they have an exceptionally high neutron cross-section absorption
coefficient (>150 x 1024 cm2/atom for thermal neutrons). Their
high neutron absorption coefficients, for example, make them
attractive as a control rod material in water-cooled reactors,
Zirconium-based materials, in contrast, have an extremely low
specific neutron cross-section absorption coefficient (
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Powder Metallurgy Progress, Vol.9 (2009), No 1 50
Pure W free-standing plasma-sprayed tungsten plates were
produced using an atmospheric plasma spray with porosities from 4
to 6%. During plasma spraying, tungsten oxide (WO3) was formed by
the rapid solidification of droplets. An increase in the elastic
modulus and hardness was caused by defect structures, high density,
and the presence of reinforcing carbide strengthening throughout
grain boundaries.
In order to improve the mechanical properties of W, tungsten
matrix composites containing up to 40 vol. % of ZrC particles were
prepared by vacuum hot-pressing at 2000°C, 20 MPa for 1h [5]. As
ZrC content increases from 0 to 40 vol. %, the density of ZrC/W
composites decreases from 18.31 to 12.69 g/cm3. Vickers hardness
increases from 3.4 GPa to 11.2 GPa and elastic modulus increases
from 313 GPa to 388 GPa. The strengthening mechanism is a load
transfer and the toughening mechanism is crack deflection. A
special method called Pressureless Reversible Infiltration of
Molten Alloys by the Displacive Compensation of Porosity [5] was
successfully applied for performing a solid mixture of ZrC and
W.
W (1.3 µm) plus 1.3 wt.% HfC nano-particles were vacuum plasma
sprayed and nano-sized HfC was found along the grain boundary in
coatings sprayed from spray-dried powder. W-Re and W-Re-HfC alloys
can be produced using VPS forming techniques. Above a 0.5% HfC
addition to VPS W-Re alloys can degrade tensile properties and
increase ductile to brittle transition temperature primarily due to
a reduction in density [6].
The goal of our study is to find applicable spray conditions for
producing deposits of pure W, pure ZrC and W-ZrC cermets. The spray
equipment used was a high throughput water-stabilized plasma gun
WSP®500. This Czech type of plasma gun works on the Gerdien arc
principle. In the orifice of its nozzle the temperature reaches
30.000 K. This device is suitable primarily for the preparation of
layers, coatings or free standing parts from oxidic precursors with
a high melting point. Moreover, the chemical composition of water
plasma has an unbalanced redox character. This fact enables under
certain conditions a spray of metallic powders, which are extremely
susceptible to unwanted oxidation e.g. Ti or W.
EXPERIMENTAL
Powder characteristics Tungsten was obtained as a commercial
product (Osram Sylvania, now GPT
Bruntal Czech Republic) with a nominal size from 32 to 63 µm
(see Fig.1). To limit the oxidation of the surface during plasma
spraying of W powder, part of the feedstock was spheroidized in
advance – see Fig.2 – by the process described in [7]. ZrC powder,
also a commercial product (Atl. Equip. Eng., NJ, USA) – size <
50 µm, was used in the spraying of sole ZrC (Fig.3) and for
preparing mechanical mixture with W. Morphology of the HfC powder
(Sigma Aldrich) with grain size 5 µm is shown on Fig.4. The
agglomerated powder containing 20 vol.% of ZrC (8.02 wt.%) in
W-matrix was prepared as a laboratory product – (IUCF - Chungnam
National University, Daejeon, South Korea). The size of this powder
was from 32 to 63 µm, see Fig.5. This composite powder was prepared
via sintering of the spray dried composite powders after mixing
with ball mill of W and ZrC. The size distribution of all used
feedstock powders is displayed in Fig.6 and their thermogravimetric
analysis in air is shown in Fig.7.
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Powder Metallurgy Progress, Vol.9 (2009), No 1 51 Tab.1.
Chemical analysis of starting powders and its principal
impurities.
powder origin wt. % O Na Al Ca Fe Mo structure W GTP Bruntál W
99.87 0.07 - - - - - Im3m a=316
pm ZrC AEE ZrC
99.80 - - 0.10 0.01 - 0.03 Fm3m a=469
pm HfC Aldrich HfC
99.88 - . 0.11 - - - Fm3m a=446
pm Spray-drying
W80ZrC20
IUCF CNU Korea
W 91.2; ZrC 8.7
0.04 - - 0.02 -
Fig.1. Starting W powder.
Fig.2. Spheroidized free flying particles
(FFP) W powder. Fig.3. Starting ZrC powder.
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Powder Metallurgy Progress, Vol.9 (2009), No 1 52
Fig.4. Starting HfC powder. Fig.5. Starting W80ZrC20 powder.
8W80ZrC20
Fig.6. Size distribution of starting powders.
Fig.7. Thermogravimetric analysis of starting powders in
air.
Plasma spray deposition Spraying was carried out by a
water-stabilized plasma WSP®500 gun at IPP,
Prague, Czech Republic (see Fig.8). Powder was fed into the
plasma jet by two injectors and forced in by Ar gas with a flow
rate 3.2 slpm. Substrates were preheated to 180 – 250°C in all
cases. The temperature was monitored during spraying to not exceed
200°C
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Powder Metallurgy Progress, Vol.9 (2009), No 1 53 corresponding
to the activation energy of carbon self-diffusion that is reported
as 473 kJ/mol for ZrC [8].
All metallic substrates were prior to spraying grit blasted by
the common way whereas also the graphite substrate surfaces were
adapted from the original Ra 1.1±0.1 µm and Ry max 14.5±2.0 to a
roughness typical for a sand blasted surface. Substrates were
enveloped by a steam of protective and cooling gas (pure Ar as well
as Ar+7%H2 were tested) oriented in an opposite direction towards
the plasma jet of WSP®. Individual steps of the process and also
input and output parameters were observed according the scheme
shown in Fig.9.
Fig.8. Scheme of WSP spraying: 1 – cathode (consumable graphite
rod), 2 – cooling water
for cathode, 3 – water inlet for stabilizing channel, 4 – water
outlet, 5 – powder feeding tubes, 6 – anode shift, 7 – anode
rotation, 8 – water vortex, 9 – electric arc, 10 - coating, 11
– substrate, FD – feeding distance, SD – spray distance.
startingpowder
WSPtorch
Chemical analysis
XRD phase analysis
granulometry
Geometric trace analysisStructure, SEM, Hg-porozime
density measurementPhoto of arrangement
photo
Powder injectionFeedstock distance FD
Spray distance SD
A
B
ATJ or other substrat
Spatial angle of free flying parts sr
FD
C
A
B
C
Fig.9. a - Scheme of plasma spraying of W-based powders on
metallic or graphite
substrates. Between A and B the slit was used for stream of the
protective gas, see the photo 9b – detail.
Characterization techniques Powder size distribution was
determined by the laser scattering device, Analysette
22 (Fritsch, Germany), in water solution of sodium phosphate. A
scanning electron microscope Camscan 4DV (Camscan, UK), and light
microscope Neophot 32 equipped by
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Powder Metallurgy Progress, Vol.9 (2009), No 1 54 a CCD camera
were used for structural investigation. X-ray diffractometer D 500
(Siemens AG, Germany) with filtered Cu radiation was used for phase
analysis. Angle 2 theta from 10 to 90° was recorded with step
0.02°. Depth sensing indentation (DSI) at 20 mN was used for
“nano-hardness” and local elastic modulus determination (Shimadzu
Murasakino).
The dilatometric tests included thermal expansion measurements
on samples 8 to 10 mm long, at a 5°Cmin−1 rate up to 1550°C in Ar
atmosphere. The measurements were performed on a Setsys 16/18
contact dilatometer (Setaram, France) with a vertical measurement
chamber that enables measurements up to 1750°C in a controlled
atmosphere. The thermal expansion coefficients were calculated from
various temperature intervals.
Raman spectra were collected using the LabRAM system (Jobin
Yvon, France) model LabRAM HR, equipped with a 532 nm line laser
for excitation of the studied materials. Raman spectroscopy was
used predominantly for identification of ZrN and ZrO2 at the grain
boundaries of W-ZrC. Oxidation and nitridation of the original ZrC
took place namely at the scanning of the plasma torch over large
area coatings. Hg-porosimetry was measured by AutoPore IV 9500
V1.06 tester (Micromeritics, USA) up to 400 MPa.
RESULTS AND DISCUSSION
Plasma spraying of studied powders Plasma spraying of all
powders (W, ZrC, W+10ZrC, W+20ZrC, W+10HfC) was
carried out according to the scheme (Fig.9) server as an
auxiliary mean of looking for conditions given. The goal of our
spray experiments was to reach maximal coating density and lowest
porosity. The spraying is done, anyway, in an ambient atmosphere;
the huge and powerful WSP system does not allow to make it
air-tight and the only means of prevention of undesired reactions
with air is shrouding.
Samples extracted from the coating directly in the centerline of
the plasma plume (i.e. plasma jet plus hot droplets inside) - zone
1 in the Fig.9 - are further labeled “A“, samples from annulus in
the medium distance zone - i.e. zone 2 in the Fig.9 - are labeled
“B“, and samples from the outer annulus - i.e. zone 3 in the Fig.9
- are labeled “C“. Coatings in the zone “C” are oxidized in all
cases because of the transport of ambient air into the plasma edges
due to the turbulent plasma flow.
TGA analyses of W, ZrC and HfC feedstock powders, is reported in
Fig.7. It could be seen that above 600°C spontaneous oxidation took
place. Further in our paper we do not show results of oxidized
outer zone samples “C“; we are only summarizing that this zone –
corresponding to spatial angle over 10° - is not suitable for
plasma spraying of dense coatings.
Table 2 shows spatial angle values for various feeding distances
FD and spray distances SD. There it is visible that a main role
plays the feedstock size distribution. The spatial angles were
measured from the footprints of the plasma plume – see Figs.10 to
12 – and also from video recordings of the spray process.
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Powder Metallurgy Progress, Vol.9 (2009), No 1 55
Fig.10. Footprint of plasma plume. Fig.11. Footprint of plasma
plume with
use of two parallel input tubes.
Fig.12. Footprint of plasma plume with two serial input
tubes.
Porosity determined by mercury intrusion for various tungsten
coating zones (A, B, C) and for the central zone of W80ZrC20
coating is demonstrated on scheme, Figs.9. The porosity of W80ZrC20
has wider size distribution and moreover a certain quantity of
pores with radii from 2 to 10 µm is present, which is not the case
of W coatings from zones A and B. Monocomponent material forms a
compact coating easier than a cermet. Footprints of the plasma
plume, Figs.10 to 12, were created by short-time spraying with a
static torch. The arrangement of the feeding tubes influences the
footprint character. The use of two tubes with the same FD
corresponds to Fig.11, whereas the use of two tubes in different FD
arranged in one line downstream of the torch nozzle leads to the
footprint shape shown in Fig.12. Results of porosity analyses of
individual compositions sprayed by WSP® are summarized in Table 3
and results of mercure intrusion posity measurement are documented
in the Figs.13-16. The character of porosity could be observed on
cross sections – Figs.17, 19 and 21. Diffraction pattern of the
sample A-W; contact part with graphite substrate with presence of
interlayer W2C (SD 200 mm, FD 65 mm) is given in Fig.18. The same
conditions were used for spraying ZrC. Here we are reporting spray
parameters SD 200 mm, FD 25 to 65 mm.
Figure 20 shows an XRF spectrum of the surface of ZrC
coating.
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Powder Metallurgy Progress, Vol.9 (2009), No 1 56 Tab.2. Zones
of plasma plume (in sr).
spraying distance (mm) SD 200 SD 200 SD 200 SD 220 SD 220 SD 220
SD 300 feeding distance (mm) FD 25 FD 40 FD 65 FD 25 FD 40 FD 65 FD
25
light density powder ZrC >20 µm neutral zone (A) no 0.006
0.007 0.011 0.006* no red-ox zone (B) 0.006 0.016 0.015 0.016
0.014* 0-0.097** catastrofic oxid. zone (C) 0.029 0.029 0.026
>0.024
high density powder W 32-63 µm neutral zone (A) 0.0035 0.008
0.016 0.004 0.006 red-ox zone (B) 0.007 0.018 0.023 0.008 0.045
catastrofic oxid. zone (C) 0.026 0.027 0.058 0.049 0.072
high density powder W 63-120 µm neutral zone (A) 0.018 0.033
0.012 red-ox zone (B) 0.044 0.044 0.064
W80ZrC20 powder 32-63 µm neutral zone (A) 0.050 0.045 0.037
red-ox zone (B) 0.090 0.065 0.055
*unmelted particles; ** all particles oxidized, velocity of
carrier gas Ar > 16 m/s;
elipsoidal – footprint angle measured for larger elipse axis
Tab.3. Results of porosity measurements of sprayed samples.
Hg-porosity Zones A, B, C of various coatings
A - W B - W C - W A - ZrC A -
W90ZrC10 A -
W80ZrC20 Total Intrusion Volume ml/g 0.0086 0.0091 0.0096 0.034
0.086 0.0149
Total Pore Area m²/g 0.040 0.041 0.070 0.063 0.049 0.079 Average
Pore Radius (2V/A) µm 0.427 0.4490 0.2731 1.083 0.349 0.377
Bulk Density at 0.1000 MPa g/ml 15.931 15.6294 14.5431 5.367
15.420 12.657
Skeletal Density g/ml 18.365 18.0090 16.6692 6.257 17.670
15.392
Fig.13. Pore distribution in sample A – W. Fig.14. Pore
distribution in sample B – W.
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Powder Metallurgy Progress, Vol.9 (2009), No 1 57
Fig.15. Pore distribution in sample C – W. Fig.16. Pore
distribution in sample A –
W80ZrC20.
Fig.17. Microstructure of sample A-W. Fig.18. Diffraction
pattern of the sample A-W; contact part with graphite substrate
with presence
of interlayer W2C (SD 200 mm, FD 65 mm).
Fig.19. Cross-section of the ZrC coating – sample A-ZrC.
Fig.20. XRF spectrum of the surface of ZrC coating.
C
Zr
Zr
Zr
Zr
keV0
1000
2000
3000
4000
5000
6000
7000
8000
0 5 10 15 20
Elt Line Int K Kr W% C Ka 1.0 0.0127 0.0117 9.66 Zr La 588.2
0.9873 0.9072 90.34
1.0000 0.9189 100.00
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Powder Metallurgy Progress, Vol.9 (2009), No 1 58
With the comparison of Raman spectra at frequences186, 224 a 473
cm-1, typical for ZrN, and 103, 152 a 635 cm-1, typical for ZrN,
the content of homogeneously distributed nitride content can be
estimated as about 5% while the step was 10 µm. Near pores and
cracks the intensity with maxima of about 300 cm-1 is enhanced,
which is a feature typical for ZrO2.
Simultaneous plasma spraying of W and ZrC powders Owing to the
difference in densities of W (19.1 g/cm3) and (6.73 g/cm3) it
was
difficult to concentrate the footprint of the plasma plume
within a spherical angle lower than 0.024 sr. This was the reason
for using two separate powder feeders with serial configuration
along the plume axis, while FD - W was 65 mm and FD - ZrC was 115
mm. Feeding velocities were adjusted in the ratio from 1:1 to 10:1.
The desired ratio of W to ZrC content in the coating was not
approached by this way. The microstructure and other parameters of
the product W90ZrC10 are reported in Figs.21, 22 and Table 3. This
experiment confirmed that for obtaining a homogeneous product the
separate feeding is not advantageous. Therefore we have continued
the experiments with agglomerated powder W80ZrC20.
Fig.21. Cross-section of the W90ZrC10 coating.
Fig.22. XRD pattern of W90ZrC10 with W2C interlayer.
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Powder Metallurgy Progress, Vol.9 (2009), No 1 59
Plasma spraying of W and ZrC agglomerated powders Tungsten and
nanometric powder of ZrC were mixed with an organic binder and
processed in spray-drying chamber and subsequently heated for
hardening purposes. During this operation a small fraction of
W6C2.54 was created in the final powder. For plasma spraying
spheroidal particles with size distribution 32-63 µm have been
selected. The arrangement consisted of two serial feeding tubes
with parameters SD 220 mm, FD 65 mm, angle to plasma axis 45°. By
chemical as well as XRD analyses the corresponding coating
composition was W80ZrC20 (vol.%). Results are shown in Tab.3 and
Figs.23 and 24. The use of the mixed protective gas led to a
remarkable improvement of the coating microstructure. Argon with a
small addition of hydrogen is used in this way because of
reactivity of hydrogen with ambient air, which is predominant in
reactions with sprayed material. This protection effect causes an
important decrease of tungsten oxide at grain boundaries, see
Fig.24.
Fig.23. Cross-section of the W80ZrC20
coating from the blended powder protected by Ar.
Fig.24. Cross-section of the W80ZrC20 coating from the blended
powder protected
by Ar-H2.
Plasma spraying of powder mixture W and HfC Since only
ultra-fine HfC was available on the market, we decided to mix
it
together with W powder in the powder feeder (Mark XV, Powder
Fluid Dynamics, USA). Preliminary experiments showed bad behavior
of the powder in plasma if these two markedly different powders
were fed by two separate injectors. The homogenized mixture (90/10)
was elevated in the powder feeder before captivation by the feeding
gas into hoses leading to the plasma gun. However, we expected
problems with the coating homogeneity because of very different
sizes and densities of both components. Figure 25 brings the XRD
pattern of plasma sprayed coating surface that was in contact with
graphite. Similarly as in the case of the sample A-W, see Fig.17,
also here the first layer being in contact with graphite substrate
reacted with them, but here the interlayer was built from WC and
not from W2C. This effect could be ascertained to higher
temperature at the spraying from shorter spray distance.
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Powder Metallurgy Progress, Vol.9 (2009), No 1 60
Fig.25. XRD-pattern of W90HfC10 coating on ATJ.
Fig.26. W90HfC10 coating cross section. Fig.27. W90HfC10 coating
cross section –
detail.
Fig.28. Spot analysis of the W and Hf distribution.
[wt.%] [at.%] [wt.%] [at.%] ObjNr Hf Lα Hf Lα W Lα WLα
1 97. 91 97. 97 2. 09 2. 03 2 98. 68 98. 71 1. 32 1. 29 3 94. 19
94. 35 5. 81 5. 65 4 98. 22 98. 27 1. 78 1. 73 5 97. 57 97. 64 2.
43 2. 36 6 97. 89 97. 95 2. 11 2. 05 7 94. 16 94. 32 5. 84 5. 68 8
0. 52 0. 54 99. 48 99. 46 9 0. 68 0. 70 99. 32 99. 30
10 0. 51 0. 53 99. 49 99. 47 11 0. 63 0. 65 99. 37 99. 35
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Powder Metallurgy Progress, Vol.9 (2009), No 1 61
Fig.29. W90HfC10 sintered in BELT apparatus.
The coating contains deformed splats of HfC which are not only
joined together with the W matrix but also exhibit signs of
remelting – Fig.26. Also we can conclude that the HfC powder - a
substance with the absolutely highest melting point (approx.
3800°C) - could be successfully molten via WSP® process. The
microstructure of the resulting coatings W-HfC is depicted in
Fig.26. The same microstructure is in larger detail in Fig.27 and
result of corresponding spot analysis in Fig.28. To have a
comparison with sintered sample W-HfC 10 vol.%, produced by BELT
technique, the cross section of it is also displayed - Fig.29.
Temperature 2000°C and pressure 5.5 GPa was used for this sintered
sample [9]. Porosity of the W-HfC is, as expectable, higher than in
the case of sintered samples, and is present exclusively inside HfC
particles, see Fig.27 .
Plasma spraying on cylindrical graphite substrates and formation
of free-standing parts
A cylindrical configuration is complicated from the viewpoint of
spraying. Figures 30 and 31 bring a scheme of spray setup enabling
the production of the A-zone coatings covering a cylinder longer
than plasma plume width. This setup offers perfect quality of the
boundary between the coating and graphite substrate, see Figs.32
and 33. A mechanical anchoring of our coating is the only adhesion
mechanism on graphite substrates. This anchoring is a result of
graphite coarsening before spraying – smoother graphite substrates
were used (and useful only) for free standing parts production by
release of the coating at cooling. Free standing parts are
advantageous for e.g. dilatometric measurement. With cylindrical
geometry the problems with adhesion were avoided completely and
coatings up to 2 mm in thickness were formed, see Fig.33.
Free-standing parts (FSP) can be produced by a release of the W
coating due to the CTE mismatch between them and the substrate, see
Fig.34. Here the gap between the coating and substrate corresponds
to very weak bonding before complete detachment when the coating is
cooled down to room temperature. This approach however could cause
cracking in the coating. In Fig.34 one large crack is indicated by
the white arrow.
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Powder Metallurgy Progress, Vol.9 (2009), No 1 62
Fig.30. Scheme of plasma spraying of cermet coatings on
cylindric graphite substrates.
Fig.31. Protection tube.
Fig.32. Section of the coating on cylindrical
ATJ graphite substrate. Fig.33. Examples of coatings on
cylindrical
graphite diam. 20 mm.
Fig.34. Side view of the W coating (top) on a steel
substrate.
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Powder Metallurgy Progress, Vol.9 (2009), No 1 63
CONCLUSIONS The experiments showed that powders of W, ZrC and
HfC can be processed by
plasma spraying with WSP equipment and successfully transformed
into coatings, namely on graphite substrates of various shapes.
Substrates must be placed in graphite boxes continuously filled
with protective and cooling gas. The velocity of such a gas (e.g.
Ar+7%H2) must be higher than 6 m·s-1, measured in front of the
opening oriented towards the plasma plume. This arrangement
prevents reactions of sprayed substances with oxygen or hydrogen
from the ambient air. The products are slightly porous, which is
inherent for atmospheric plasma spray technique. Porosity could be
minimized by a decrease of spray distance and maintaining the
spatial angle of the impinging droplets below 0.024 sr.
At the spraying of pure W, its contamination with nitrogen is
avoided easily; oxidation caused by oxygen entrapped from ambient
air into the turbulent plasma jet takes place to a certain degree:
1-3% WO3 is the consequence of that. The oxidation could be further
minimized by an increase of the hydrogen in the protective gas. In
opposition, the spraying of pure ZrC is accompanied inherently by a
certain nitridation. The amount of ZrN in the ZrC coating does not
exceed 5%, as detected with Raman micro-spectroscopy.
Plasma spraying of a mixture of W and HfC led to coatings with
pores present predominantly inside larger particles of HfC, which
in effect is probably due to incomplete melting of such particles.
Precise description of physical parameters of mentioned coatings
will be subject of other paper, here we are only briefly reporting
typical values. – Microhardness of the W matrix is 5.7 – 7.8 GPa,
in proximity of HfC grains is it about 9.7 GPa and if the indents
are placed within HfC grains, the value is about 13.2 GPa. Thermal
expansion in the interval 500 –1550°C is 6.18x10-6 for pure W,
about 6.12x10-6 for W90ZrC10 and 6.77x10-6 for W90HfC10 samples.
Elastic modulus determined by the indentation technique of Shimadzu
Murasakino in homogeneous areas was in the case of W 150 to 180
GPa, in the case of W-ZrC grains proximity, there was measured
microhardness 9.23 GPa and a resulting E-modulus of about 129-135
GPa. Tungsten coating tested by four-point bending exhibited an
E-modulus lower than (or in best cases equal to) 80 GPa.
The major disadvantage of the WSP arranged as reported in our
paper is the possibility of continuous covering of only small
areas, and high consumption of protective gases, whose factors
shift the economical effectiveness to a less practical situation
than for the conventional WSP® process. On the other hand, the
advantage is a possibility given by WSP® to process up to 100 kg of
metallic powder per hour which also represents the here described
configuration of higher quantity of coated material per time unit
compared to other spray techniques. Due to extremely high
temperatures it is necessary to keep in mind the probability of
creation of interlayers between substrate and coating as a
consequence of chemical reactions. In our experiments described in
[10] these interlayers enhanced the compactness of the
coating-substrate system.
Acknowledgment Acknowledgment for financial support to IUCF
Chungnam National University,
Project 2008-0342-2. Presented at the 17th Plansee Seminar, May
2009
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Plasma- Facing Materials and Components for Fusion Applications,
Juelich, Germany, 2009, p. 54
PLASMA SPRAYING OF ZIRCONIUM CARBIDE - HAFNIUM CARBIDE -
TUNGSTEN CERMETS V. Brožek, P. Ctibor, D. I. Cheong, S.-H.
Yang Abstract Keywords: plasma spraying, tungsten cermets,
zirconium carbide, hafnium carbide, water stabilized
plasmaINTRODUCTION EXPERIMENTALPowder characteristics Tab.1.
Chemical analysis of starting powders and its principal
impurities.Fig.1. Starting W powder.Fig.2. Spheroidized free flying
particles (FFP) W powder.Fig.3. Starting ZrC powder.Fig.4. Starting
HfC powder.Fig.5. Starting W80ZrC20 powder.Fig.6. Size distribution
of starting powders.Fig.7. Thermogravimetric analysis of starting
powders in air.
Plasma spray depositionFig.8. Scheme of WSP spraying: 1 –
cathode (consumable graphite rod), 2 – cooling water for cathode, 3
– water inlet for stabilizing channel, 4 – water outlet, 5 – powder
feeding tubes, 6 – anode shift, 7 – anode rotation, 8 – water
vortex, 9 – electric arc, 10 - coating, 11 – substrate, FD –
feeding distance, SD – spray distance.Fig.9. a - Scheme of plasma
spraying of W-based powders on metallic or graphite substrates.
Between A and B the slit was used for stream of the protective gas,
see the photo 9b – detail.
Characterization techniques RESULTS AND DISCUSSIONPlasma
spraying of studied powdersFig.10. Footprint of plasma
plume.Fig.11. Footprint of plasma plume with use of two parallel
input tubes.Fig.12. Footprint of plasma plume with two serial input
tubes.Tab.2. Zones of plasma plume (in sr).Tab.3. Results of
porosity measurements of sprayed samples.Fig.13. Pore distribution
in sample A – W.Fig.14. Pore distribution in sample B – W.Fig.15.
Pore distribution in sample C – W.Fig.16. Pore distribution in
sample A – W80ZrC20.Fig.17. Microstructure of sample A-W.Fig.18.
Diffraction pattern of the sample A-W; contact part with graphite
substrate with presence of interlayer W2C (SD 200 mm, FD
65 mm).Fig.19. Cross-section of the ZrC coating – sample
A-ZrC. Fig.20. XRF spectrum of the surface of ZrC coating.
Simultaneous plasma spraying of W and ZrC powdersFig.21.
Cross-section of the W90ZrC10 coating.Fig.22. XRD pattern of
W90ZrC10 with W2C interlayer.
Plasma spraying of W and ZrC agglomerated powdersFig.23.
Cross-section of the W80ZrC20 coating from the blended powder
protected by Ar.Fig.24. Cross-section of the W80ZrC20 coating from
the blended powder protected by Ar-H2.
Plasma spraying of powder mixture W and HfCFig.25. XRD-pattern
of W90HfC10 coating on ATJ.Fig.26. W90HfC10 coating cross
section.Fig.27. W90HfC10 coating cross section – detail.Fig.28.
Spot analysis of the W and Hf distribution.Fig.29. W90HfC10
sintered in BELT apparatus.
Plasma spraying on cylindrical graphite substrates and formation
of free-standing partsFig.30. Scheme of plasma spraying of cermet
coatings on cylindric graphite substrates.Fig.31. Protection
tube.Fig.32. Section of the coating on cylindrical ATJ graphite
substrate.Fig.33. Examples of coatings on cylindrical graphite
diam. 20 mm.Fig.34. Side view of the W coating (top) on a steel
substrate.
CONCLUSIONSAcknowledgmentREFERENCES