Tungsten fibers have high tensile strength but a poor oxidation resistance at elevated temperatures. Using this first characteristic and to prevent oxidation of tungsten coated composite materials in which the primary requirement: reinforcement against destruction or deformation, was studied on tungsten fibers and tungsten wires which were coated by applying the metal and ceramic powders via plasma spraying device in plasma generator WSP®. Deposition took place in an atmosphere of Ar + 7 % H2, sufficient to reduce the oxidized trace amounts of tungsten

J. Kolisko

M. Vokac

P. Pokorny

T. F. Kubatik

V. Brozek

English

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79METALURGIJA 56 (2017) 1-2, 79-82V. BROZEK, M. VOKAC, J. KOLISKO, P. POKORNY, T. F. KUBATIKINCORPORATION OF TUNGSTEN METAL FIBERS IN A METAL AND CERAMIC MATRIXReceived – Primljeno: 2016-06-21Accepted – Prihvaćeno: 2016-10-15Original Scientific Paper – Izvorni znanstveni radV. Brožek, T. F. Kubatík, Institute of Plasma Physics CAS v.v.i. Prague, Czech RepublicM. Vokáč, J. Kolísko, P. Pokorný, Czech Technical University - Klokner Institute, Prague.Tungsten fibers have high tensile strength but a poor oxidation resistance at elevated temperatures. Using this first characteristic and to prevent oxidation of tungsten coated composite materials in which the primary requirement: reinforcement against destruction or deformation, was studied on tungsten fibers and tungsten wires which were coated by applying the metal and ceramic powders via plasma spraying device in plasma generator WSP®. Deposi-tion took place in an atmosphere of Ar + 7 % H2, sufficient to reduce the oxidized trace amounts of tungsten. Key words: tungsten wires, tungsten fibers, plasma spraying, metallic coatings, ceramic coatingsINTRODUCTIONResearch on materials to be used under extreme con-ditions, i.e. at high temperatures and pressures, has a large number of variants. While ultra - high vacuum or ultra - high pressures can certainly be considered ex-treme conditions, with temperature it is easier. At very low temperatures, all substances are in a solid state, where the design parameters are more readily achieva-ble. Conversely, temperature has no upper limit and its increase leads phase changes to the liquid state, the gase-ous state, and finally plasma. Today there is no problem to achieve in the plasma state, temperatures of hundreds of thousands of degrees Kelvin. The problem, however, lies with constructional materials for devices where with ultra - high temperatures must be used. The current limit of all elements in this direction is represented by tung-sten with a melting point (mp) of 3 420 °C and for alloys by tantalum carbide and hafnium with melting points of approximately 3 800 °C. These data highlight the fact that above a temperature of 4 000 °C, there is nothing on Earth in a solid state that could be used to construct de-vices for use in extreme temperatures.At present, the latest approach to using ultra - high temperatures is for the generation of plasma, which is necessary to begin nuclear fusion in Tokamak - type de-vices with a view to obtaining an infinite source of en-ergy solutions [1, 2]. This relies on the development of tungsten – based materials, e.g. cermets which alter or improve the thermal conductivity, corrosion resistance, resistance to neutron radiation, etc. e.g. in ITER project [3 - 5]. However, these also reduce the limit for high – temperature usability. Such materials, however, can be used in many other technical sectors where it is neces-sary to increase mechanical strength at high tempera-ISSN 0543-5846METABK 56(1-2) 79-82 (2017)UDC – UDK 669.27.621 – 426:667.6:621.793:666.7.002.4=111tures and, at the same time, protect metal parts from oxidation and potential chemical corrosion.Current suitable methods for processing materials with the highest melting points are thermal  spraying, with plasma spraying having the highest high - temper-ature tolerance [6], or hot stamping or sintering, again with the highest limit variant being Spark Plasma Sin-tering (SPS) or also called Field Assisted Sintering Technique (FAST).This work is focused on the use of the unique proper-ties of tungsten filaments and wires which are character-ized by high tensile strength up to 3,4 GPa, the highest density and smallest thermal expansion coefficient of all metals ( 4,3 ÷ 4,6 . 10-6 K-1) to the reinforcement of metal-lic and ceramic materials with other utility properties. The incorporation of tungsten in materials with lower melting points increases their mechanical reinforcement against all kinds of stress, particularly at high tempera-tures. Coatings on tungsten wires and fibers also prevent oxidation, which becomes critical from 600 °C.EXPERIMENTALThe experiments used a tungsten substrate in the form of sheets of 1 mm thickness, a Metallwerk Plansee product, and tungsten wire of 0,8 mm diameter and tungsten filaments of a 0,02 mm diameter, both prod-ucts of Osram Bruntál Ltd. Chemical composition of the substrates is shown in Table 1.These tungsten substrates or mandrels were also used for further experiments:a) without changing the original morphologies and chem. composition,b) after mechanical surface treatment – abrasive blast-ing with corundum F 240 to Ra 7 μm – 8 μm  as measured on the Mitutoyo roughness tester SJ 210.c)  after chemical treatment consisting of SiC and WSi2 coatings. Bundles of several tens of fibers with longitu-dinal orientation were between electrical contacts of 80V. BROZEK et al.: INCORPORATION OF TUNGSTEN METAL FIBERS IN A METAL AND CERAMIC MATRIX METALURGIJA 56 (2017) 1-2, 79-82direct current (14 V; 20 A – 40 A) incandescent in an evacuated quartz apparatus, to which were fed a meth-yltrichlorsilane vapour entrained with Ar carrier gas from the container, see Figure 1. Based on the deposi-tion time, rigid rods, about 1 mm thick, covered with silicon carbide, which is used as another type of tung-sten substrates, particularly to improve the adhesion of other coatings by increasing the surface roughness with a permanent chemical, rather than a mechanical, link-age, were created. A typical cross section of resulting formation is shown in Figure 1(b).Tungsten wires of 0,8 mm diameter with a protec-tive surface layer of WSi2 (blended with small quantities of phase W5Si3), were formed by reacting the surface of a W - wire heated to about 1 400 °C in powdered silicon. The uniformity of the coating thickness was largely ensured by moving the tungsten wire between the sliding electrical contacts and hexagonal boron nitride drawing die Figure 2. Cross - section of this type of coated tungsten substrate is shown in Figure 3.Subsequent geometrical modification of substrates: bundles of individual tungsten fibers or wires and coat-ed tungsten wires were assembled into parallel molds longitudinally, in the shape of a flat lattice or helically wound on an inert ceramic boron nitride or graphite substrate - mandrel.Blank tungsten substrates were used to determine deposition conditions during plasma deposition, i. e. to determine the optimal feeding distance and scattering an-gle between the neutral and oxidative zones of the plasma jet. A record of the spray footprint is shown in Figure 4. For deposition of tungsten powder with grain size classes of 40 μm - 63 μm, a WSP® - H500 plasma gen-erator was used, working with the parameters 450 V, 320 A, 144 kW power. Successful tungsten deposition using this device is described in a number of previous works by UFP [7, 8]. The obtained materials were ana-lyzed metallographically and SEM microscopically, ad-hesion of tungsten to metal materials and the ceramic matrix was measured according to EN ISO 4624.RESULTS AND DISCUSSIONFrom the geometry of the spray pattern of tungsten on the flat substrate (see Figure 4) it is evident that the neutral plasma portion with molten tungsten particles has a diameter of 50 mm with a deposition distance of 300 mm, i. e., 0,038 sr. A spatial angle over 0,080 sr al-ways leads to oxidation, which, due to the high turbu-lence of the plasma cannot be prevented even with very Table 1 Chemical composition of tungsten substrates / wt. %W plate W fi ber W wireW 99,5 99,92 96,67Na -- -- 0,29K 0,02 -- 0,35Si -- -- 0,36Fe 0,31 -- 0,51Cu -- -- 0,10Zn -- 0,07 0,04Cr 0,03 -- -- a) b)Fi gure 1  (a) Scheme of SiC coatings preparation on a tungsten fibers and (b) the cross-section of the sheaf tungsten fibers in SiC matrixFigure 2   Scheme of WSi2 coatings preparation on a tungsten wiresFigure 3 Cross-section of tungsten wire with WSi2 coatingFigure 4  Evaluation scheme of footprint deposition tungsten by thermal spraying81V. BROZEK et al.: INCORPORATION OF TUNGSTEN METAL FIBERS IN A METAL AND CERAMIC MATRIXMETALURGIJA 56 (2017) 1-2, 79-82intense shrouding [9]. That is why the deposition of tungsten on any substrate was carried out in the graphite apparatus, see Figure 5. This enables longitudinal and rotational movement of the substrate while maintaining the substrate with deposited tungsten in a slightly re-ducing atmosphere of Ar + 7 % H2. Under these condi-tions there is no oxidation of the tungsten or its melting and the formation of splats. In this apparatus it is pos-sible to coat tungsten fibers with other metal and ce-ramic materials, e. g. titanium, titanium nitride or ce-ramic aluminum oxides.The first experimental variant in the preparation of the molded part was deposition of tungsten on tungsten wires with a diameter of 0,8 mm, in order to connect them and create a tube with a freely shaped profile, in this case a non-circular shape. The bundle of tungsten wires were inserted into the graphite sleeve. Figure 6 (a), (b) show the initial stage of deposition of tungsten molten splats and detail of the surface tungsten rein-forcement wires.Increasing thickness of the tungsten coating balanc-es out waviness in the coating. Other preform variants of the tungsten substrate for subsequent plasma deposi-tion were carried out so that, on a temporary substrate made of graphite or mechanically, readily - removable materials (e. g. hexagonal boron nitride ceramics), the tungsten wire or a tungsten filament could be once or Figure 5  Scheme of plasma spraying on movable or stationary tungsten substrates in a controlled atmosphere a) b)Figure 6  (a) Deposition of tungsten on a complex armor wires and (b) the detail of the connection of individual wires continuously rebound with the simultaneous deposition of the feed material, see scheme on Figure 7.Variations of continuous deposition of titanium, re-active deposition of TiN or oxide ceramics, for.example corundum on W – fibers and wires or bars, see Figures 8 – 12, were already successfully tested us-ing the temporary graphite mandrel [10].CONCLUSIONS  In terms of the process of thermal spraying, it is not critical if sprayed on the flat or shaped W -substrate, parallel or lattice sorting of fibers and wires or winding it around the helix. Such modified tungsten wires fulfill a reinforcement function for the substrate under me-chanical stress across the system. Because auxiliary construction materials usually do not withstand tem-peratures greater than 2 000 °C during atmospheric plasma spraying APS, they must be cooled or the plas-ma spraying process interrupted. In this case, an inter-layer between which there is a strain due to temperature Figure 7  Scheme deposition on the moving parts of tungsten substratesFigure 8 Examples of W-coated parts on graphiteFigure 9   Details of metal bars in the corundum matrix prepared by plasma spraying82V. BROZEK et al.: INCORPORATION OF TUNGSTEN METAL FIBERS IN A METAL AND CERAMIC MATRIX METALURGIJA 56 (2017) 1-2, 79-82changes or to oxidation, since hydrogen of protective mixture of Ar + H2 has a reducing activity from about 900 °C, is formed in the coating. The disadvantage of a smooth surface on fiber and wires is low adhesion with ceramic coatings. This problem is solved by roughening the surface via chemical reactions in the preparation stage of the fiber. In the case of a metal matrix, the ad-hesion value cannot be quantified, as it is always at the level of strength of the implied metal to metal diffusion bonding, thus higher (over 60 MPa) than the adhesive strength specified in EN ISO 4624 or ASTM C633 [11].AcknowledgmentThis paper presents research carried out at the Klokner Institute within the framework of Czech Sci-ence Foundation, project 13 – 15175S.REFERENCES[1] M. Rieth, S. L. Dudarev, S. M. Gonzales de Vicente, J. Aktaa, S. Antusch, D. J. E. Amstrong, M. Balden, N. Ba-luc, M. F. Barthe, W. W.Basuki, M. Battabyal, H. Boldyrye-va, J. Brinkman, M. Celino, Recent progress in Research on tungsten materials for nuclear fusion applications in Europe. Journal of Nuclear Materials 432 (2013), 482-500.[2] S. Wurster, N. Baluc, M. Battabyal, T. Crosby, C. Garcia Rosalez, A. Hasegawa, J. 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Journal of Thermal Spray Technology 22 (2013), 2-3, 221-232.Note:  The responsible translator for the English language is K. Štětková, CTU – Klokner Institute, Prague, Czech RepublicFigure 10  Detail of the building of tungsten bars  into a titanium matrix Figure 11  (a) Gradual coating of W-wires d = 0,8 mm with corundum ceramics, poor adhesion in the original, (b) improve  adhesion to the WSi2 surface Figure 12  Example of practical application protective heating of W-coils for steam apparatus 

Incorporation of tungsten metal fibers in a metal and ceramic matrix

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