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38SYNTHESIS OF METAL-LIKE REFRACTORY COMPOUNDS AND ULTRA-HIGH-TEMPERATURE MATERIALS IN MOLTEN SALTS
S. A. Kuznetsov*", V. S. Dolmatov", A. R. Dubrovskiy", and Yu. V. Stulov"
aTananaev Institute of Chemistry of the Federal Research Centre "Kola Science Centre of
the Russian Academy of Sciences", Apatity, 184209 Russia *e-mail: kuznet@chemy.kolasc.net.ru
DOI: 10.24411/9999-0014A-2019-10078
Extreme conditions of articles exploitation cause utilization of expensive refractory metals and their metal-like refractory compounds ensuring their adequate performance. The problem can be approached by depositing coatings on the surface of units and mechanisms, i.e. creating composite materials. In this case, the substrate material provides the strength and electric characteristics while the coatings realize the necessary surface properties.
Nowadays coatings of metal-like refractory compounds are used in aircraft, rocket space vehicle and mostly produced by CVD, PVD, SHS methods, plasma and detonation spraying. At the same time, using molten salts for deposition coatings is highly attractive. Molten salts provide the production of coatings by electrochemical synthesis, precise surface alloying, by employing the reaction of disproportionation and currentless transfer.
Electrochemical methods have the following advantages, as compared with other possible techniques of production of metal-like refractory compounds:
(i) smooth coatings can be deposited even at surfaces of complicated geometry;
(ii) single crystals, polycrystalline coatings and even ultra-disperse powders can be deposited depending on the parameters of the process;
(iii) the composition of the deposit can be precisely controlled through process parameters;
(iv) relatively low temperature of synthesis (down to 600oC), cheap and simple technological background and the cheap raw materials required make the process attractive from the economic point of view.
Borides. Process of electrochemical synthesis HfB2 from the NaCl-KCl-NaF (5 wt %)-K2HfF6 (5-15 wt %)-KBF4 (5-10 wt %) molten system was studied in detail [1]. It was shown that the wave of electrochemical synthesis HfB2 has a more positive potential than discharge potentials of boron and hafnium. Therefore, galvanostatic electrolysis is a convenient regime for deposition of hafnium diboride. Electrochemical synthesis HfB2 was performed at the cathodic current density of 5-50 mA cm-2 and temperature of 700-850OC. SEM micrograph of cross-section HfB2 on a molybdenum substrate is presented in Fig. 1. As can be seen from Fig. 1 the coating had a column structure and microhardness determined on the cross-section was found 32+2 GPa [1].
Molybdenum can be used as a basic reactor material. However, molybdenum corrodes in contact with aggressive and water vapors at elevated temperatures. Therefore, low temperature oxidative stability of molybdenum substrates can be improved by siliciding in molten salts to produce a MoSi2 layer followed by boronizing of the MoSi2 phase.
Boronizing treatments were conducted in molten salts under inert gas atmosphere in the temperature range of 700-1000OC for 3-7 h. The oxidation resistance of the films was investigated by the weight change in an air-water (2.3 vol %) mixture at a temperature of 500OC for a period up to 700 h.
In non-current boronizing the MoSi2 phase was transformed to the Mo5Si3 phase, which became a major (content 10-30 wt %) phase after boronizing step performed in the range of 750-900OC and the matrix phase (> 60 wt %o) above 950OC. The MoSi2 phase was always present
XV International Symposium on Self-Propagating High-Temperature Synthesis
in the films in the temperature range of 800-950oC after electrochemical synthesis. The main boride phases were MoB2 and Mo2B5. The formation of the Mo5(BSi)3 phase was only observed in electrochemical synthesis at 760OC. Pure M0B4 was formed in the bulk MoSi2 phase after electrochemical boronizing in the range of 760-840OC with a maximum content of ca.15 wt % at 800OC. The presence of small amounts (12-15 wt. %) of the MoB4 phase formed during boronizing of MoSi2/Mo films greatly improved the oxidation resistance of the molybdenum substrates. The weight gain observed was 6.5-10-4 mg/cm2h. To provide the maximum protection, the duration of the boronizing step was adjusted to boronize an external MoSi2 layer without boronizing the molybdenum substrate. No pest disintegration of the molybdenum substrate was observed in an air-water mixture at 500OC after 700 h.
Fig. 1. Morphology (a) and cross-section of HfB2 coating (b) on a molybdenum substrate obtained from the NaCl-KCl-NaF (5 wt %)-K2HfF6 (5 wt %)-KBF4 (5 wt %) melt; cathodic current density, 0.03 A cm-2; time of electrolysis 3 h; temperature 850oC.
Carbides. Synthesis of NbC and TaC coatings on the surface of carbon steels was carried out using currentless transfer from the molten salt NaCl-KCl-K2MeF7(30 wt %)-Me, where Me = Nb, Ta. Temperature of synthesis was 800-850OC with time of the process from 6 up 24 h. The mechanism of formation, for example, for niobium carbide by currentless transfer can be described by the following reactions:
4Me(V) + Nb ^ 5Me(IV) 5Me(IV)+ C(steel surface) ^ MeC + 4Me(V) Me + C ^ MeC
(1) (2) (3)
This process occurs due to the formation of lower oxidation states of niobium complexes, which formed owing to interaction of salt with own metal. The complexes of lower oxidation states are diffused to the steel substrate and disproportionate on its surface. The disproportionation (2) is accompanied by the formation of NbC and niobium complexes with higher oxidation state of +5. Nb(V) complexes again interact with niobium (1), the process forms cycle. The driving force of reaction (2) is energy of NbC formation.
The chromium carbide Cr7C3 coatings were also synthesized by the currentless transfer method. As the melt, an equimolar NaCl-KCl mixture was used with the addition of 10 wt % CrCl3 and the excess of metallic chromium. The melt was held for 2 h before the synthesis. The synthesis was realized at the temperature 850OC with time of the process 8 h.
The corrosion resistance of NbC, TaC, and &7C3 carbides coatings was studied in concentrated HCl, H2SO4, and H3PO4 acids at a 25OC during 48 h. The rate of corrosion was evaluated by the mass loss of the samples. The corrosion tests allowed to establish following rows of the chemical stability of samples with carbide coatings in HCl: &7C3 > NbC > TaC; in H2SO4: TaC > Cr7C3 > NbC; in H3PO4: &7C3 > TaC > NbC. Of course, the quality of obtained
coatings affects the results to a certain degree. The wear resistance was evaluated also by the sample mass loss with the accuracy of 0.1 mg. The mass loss of a sample from hardened St.45 was 35.2 mg cm-2 and that of a coated with a &7C3 - 4.7 mg cm-2, NbC - 2.9 mg cm-2 and TaC - 2.2 mg cm-2 [2].
Due to the high wear and corrosion resistance Cr7C3, NbC, and TaC coatings on carbon steels can be applied in aggressive medium with the abrasive wear. Deposition of NbC coatings on parts of oil pumps increased their lifetime in several times. Tests carried out by LLC "Ecotech" showed that the coatings of &7C3 or TaC on the knifes for cutting rubber, made of St. 3, can improve their wear resistance and increase a tool lifetime in 2 (for CnC3)-2.5 (for TaC) times.
Mo2C coatings were obtained on Mo substrate at the temperature 850oC by electrochemical synthesis in an equimolar NaCl-KCl mixture containing 0.92 wt % of Li2CO3, 5 wt % of Na2MoO4 at stoichiometric ratio Mo/C = 2:1 and with a cathodic current density 5-10 mA cm-2. The Mo2C/Mo composition was tested as a catalyst for the water-gas shift reaction. The steady-state reaction rates for the Mo2C/Mo composition were higher than those for the bulk Mo2C and commercial Cu/ZnO/Al2O3 catalysts over in the temperature range explored [3]. The catalytic activity remained constant during 5000 h on-stream. The coatings were also stable during the thermal cycling, while the activity of commercial catalysts tends to decrease with time. A novel microstructured reactor/heat-exchanger containing eight sections with a cross-section of 10 mm x 10 mm and a length of 100 mm has been designed and constructed. Each section of the reactor contains flat, goffered Mo plates and Mo wires with a diameter of 250 |m and a length of 100 mm coated with a catalytic Mo2C layer.
Silicides. Galvanostatic electrolysis of the melt NaCl-KCl-NaF (10 wt %)-K2HfF6 (5 wt %)-K2SiF6 (1.0 wt %) in contact with Si (anode) at temperature of 750OC and the cathodic current density of 0.30 A cm-2 led to the formation of hafnium silicide HffSi2.
Electrochemical synthesis of tantalum silicides was performed in the NaCl-KCl-NaF (10 wt %)-K2TaF7 (3 wt %)-K2SiF6 (2 wt %) melt at temperature of 750OC. Tantalum silicides coatings on a silver substrate were synthesized by the galvanostatic electrolysis. The cathodic current density was varied from 30 up to 50 mA cm-2. Parameters for electrodeposition of the Ta5Si3 and TaSi2 coatings on a silver substrate were found.
Ultra-high-temperature Nb-Hf coatings. Articles based on graphite are widely used presently in aerospace techniques because of the unique properties of graphite. However, a drawback of graphite-based compositions is a low heat resistance in oxidizing atmosphere. It was shown that Nb-Hf coatings of a + P-composition with a planar growing front and thickness of 20-40 |m can be obtained from the NaCl-KCl-NaF (5 wt %)-K2NbF7 (1 wt %)-K2HfF6 (10 wt %) electrolyte [1]. It was found that niobium stabilized the low temperature monoclinic modification of HfO2, which was formed during oxidation. The temperature of exploitation graphite articles can be increased up to 2100°C if to use niobium-hafnium protective coatings.
1. S.A. Kuznetsov. Electrodeposition of hafnium and hafnium-based coatings in molten salts, Chem. Pap., 2012, vol. 66, no 5, pp. 513-518.
2. Yu.V Stulov, V.S. Dolmatov, A.R. Dubrovskiy, S.A. Kuznetsov, Electrochemical Methods for Obtaining Thin Films of the Refractory Metal Carbides in Molten salts, Int. J. Electrochem. Sci., 2017, vol. 12, no. 6, pp. 5174-5184.
3. A.R. Dubrovskiy, O.V. Makarova, S.A. Kuznetsov, Effect of the Molybdenum Substrate Shape on Mo2C Coating Electrodeposition, Coatings, 2018, vol. 8, no. 12, 442.
