PLASMA SPRAYING OF ZIRCONIUM CARBIDE ...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

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 (


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.

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.


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

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

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.


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.

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.


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


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.


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.


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


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.


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.


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

REFERENCES [1] Chraska, P., Hrabovský, M. In: Proceedings of the Int. Thermal Spray Conference

1992. Ed. C.C. Berndt. Orlando : ASM International, 1992 [2] Matejíček, J., Chraska, P., Linke, J.: J. Thermal Spray Technology, vol. 16, 2007, no. 1,

p. 65 [3] Matejíček, J., Neufuss, K., Kolman, D., Chumak, O., Brožek, V. In: Thermal Spray


Connects: Explore its Surfacing Potential, Basel, Switzerland, May 2-4, 2005. DVS, 2005, p. 634

[4] Wang, J., Li, HP., Stevens, R.: Journal of Materials Science, vol. 27, 1992, p. 5397 [5] Rea, KE. et al.: Mat. Sci. Eng., A, vol. 477, 2008, p. 350 [6] O’Dell, JS., McKechnie, TN. In: Thermal Spray Crossing Borders, June 2-4, 2008,

Maastricht, The Netherlands. DVS, 2008, p. 300 [7] Brožek, V., Dufek, V., Neufuss, K. In: Materials Week & Exhibition MATERIALICA,

CD-G5-918, Munich, 2001, October 1-4 [8] Ferro, D., Barinov, SM. et al.: Surface & Coating Technology, vol. 200, 2006, p. 4701 [9] Brožek, V., Ctibor, P., Cheong, DI., Kim, EP. In: Proc. 18th Seminar DMSRE, 2008,

p. 8 [10] Boldyryeva, H., Ctibor, P. et al. In: Book of Abstracts of the 12th Int. Workshop on

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

Documents

PLASMA SPRAYING OF ZIRCONIUM CARBIDE ...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