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Article

Chemical Vapor Deposition of Tantalum Carbide in the TaBr5–CCl4–Cd System

by
Tibor Krenicky
1,*,
Oleg Y. Goncharov
2,
Jiri Kuchar
3,
Irina V. Sapegina
2,
Jan Kudlacek
3,
Ravil R. Faizullin
4,
Alexander I. Korshunov
2 and
Daniel Cerny
3
1
Department of Technical Systems Design and Monitoring, Faculty of Manufacturing Technologies with a seat in Presov, Technical University of Kosice, Bayerova 1, 080 01 Presov, Slovakia
2
FSBIS Udmurt Federal Research Center, Ural Branch of the Russian Academy of Sciences, T. Baramzinoy Str. 34, 426067 Izhevsk, Russia
3
Department of Manufacturing Technology, Faculty of Mechanical Engineering, Czech Technical University in Prague, Technicka 4, 16000 Prague, Czech Republic
4
Scientific Production Association Protective Coatings, LLC, Lenina Pr. 142-116, Moscow Region, 142100 Podolsk, Russia
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(5), 547; https://doi.org/10.3390/coatings14050547
Submission received: 11 March 2024 / Revised: 22 April 2024 / Accepted: 23 April 2024 / Published: 27 April 2024
(This article belongs to the Section Surface Characterization, Deposition and Modification)
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Versions Notes

Abstract

:
The tantalum carbide coatings were deposited on substrates made of 12Kh18N10T steel, ZhC6 alloy, and molybdenum by reduction of TaBr5 and CCl4 vapors with cadmium vapors at temperatures of 950–1000 K. The average deposition rate of coatings on molybdenum was ~5 μm/h, on ZhC6 alloy was ~6 μm/h, and on 12Kh18N10T steel was ~8 μm/h. The coatings were formed as columnar grains on the substrate surface and as a diffuse layer in the substrate material. The surface layers contained mainly tantalum monocarbide TaCy (y = 0.72–0.86) and a small fraction of tantalum. The coatings on the surface of ZhC6 alloy and 12Kh18N10T steel flaked off with increasing thickness, which was due to different thermal expansion of the coating and substrate, as well as concentration inhomogeneity and phase transitions in the substrate material during coating deposition and during the heating and cooling processes.

1. Introduction

Various non-stoichiometric carbides can be formed in the Ta-C system [1,2,3], among which Ta2C, TaC, and zeta-phase ζ-Ta4C3 [4,5] are the most studied. Monocarbide TaC may contain from 37 to 50 at.% carbon (at a temperature of 3749 K), β-Ta2C may contain from 26 to 35 at.% carbon at a temperature of 3047 K, and zeta-phase ζ-Ta4C3 may contain from 38 to 39 at.% carbon.
Tantalum monocarbide TaC has a high melting point of 4270 K, is resistant to thermal shocks, corrosion, chemicals [6], oxidation, and ablation [7], and therefore can be used with various high-temperature materials. For example, ceramic composites TaC/C [8] and TaC/SiC [9] are used for high-temperature heavy-duty applications. TaC coatings are used as diffuse barrier layers [10,11]. TaC–C–Al composites containing 10–20 at.% Ta and 6–7 at.% Al are innovative high-temperature wear-resistant coatings with excellent antifriction properties [12,13].
Ta2C and Ta4C3 carbides are frequently formed as by-products in the preparation of tantalum monocarbide. For example, some carbide phases are formed in coatings where there are concentration gradients of elements (carbon and tantalum). The melting point of Ta2C carbide is ~3600 K; it is more resistant to oxidation [14] and has higher ductility [15] than TaC. The presence of zeta-phase ζ-Ta4C3 in the mixture of tantalum carbide increases the fracture toughness and improves the mechanical strength of the material [16,17].
Tantalum carbide coatings may be obtained by various methods [18,19,20,21,22,23,24,25,26,27]. For example, the coating is deposited on the substrate in the form of solutions of metal-containing complex compounds with polymers [18,19], which are then dried and pyrolyzed (at T = 623–873 K). After that, carbothermal synthesis is performed (at T = 873–1473 K and a pressure not more than 10−4 Pa). The disadvantages of this method are non-uniform distribution of the gel over a surface and shrinkage of the coating during drying and annealing, which results in non-uniform thickness and density of the coatings. In addition, expensive organometallic chemicals are required to produce coatings. To reduce the cost of tantalum carbide coatings production, TaC powder was sintered with organic binders at temperatures above 2300 K, which made it possible to form dense carbide layers (up to 200 μm) [20,21]. However, this method of coating is not applicable for parts with complex shapes, as well as substrates with a melting point of 2300 K and below.
Tantalum carbide coatings were deposited on parts with complex shapes by organometallic compounds pyrolysis at 623–1573 K [22,23]. In addition, tantalum carbides were prepared by plasma atomic layer deposition (PEALD) from organometallic compounds at temperatures around 673 K [24]. The high cost and polymerization ability of organometallic compounds limit their application because, for example, polymerization products can block evaporators and gas pipelines.
Usually, a mixture of reagents containing tantalum source, carbon source, and hydrogen is used for chemical vapor deposition of tantalum carbides. For example, tantalum carbide coatings were deposited from a mixture of TaCl5, CH4, H2, and He on a substrate heated up to 1493–1653 K [25] according to the following reactions:
TaCl5 + CH4 + 1/2 H2 = TaC + 5 HCl,
2 TaCl5 + CH4 + 3 H2 = Ta2C + 10 HCl.
The temperature of carbide deposition can be reduced to about 1323 ÷ 1423 K by reducing the pressure in the reaction chamber (to ~104 Pa) [26]. Replacement of methane CH4 by ethane C2H6 results in an increase in the rate of carbide deposition. Thus, tantalum carbides were deposited from a mixture of TaCl5, C2H6, and H2 at a temperature of 1373 ÷ 1573 K and pressure of ~104 Pa [19,27].
There are some constraints for deposition of tantalum carbide from a TaCl5–C3H6–H2 (or TaCl5–CH4–H2) mixture. The melting temperature of substrate should not be lower than the coating deposition temperature. An equipment used for deposition should have excellent corrosion resistance to chlorine and hydrogen chloride, as well as resistance to hydrogen brittleness. In addition, hydrogen and hydrocarbons should be used with extreme caution due to their flammability and explosion hazard.
The heat of tantalum bromide TaBr5 formation (−143 kcal/mol) is lower than that of chloride TaCl5 (−205 kcal/mol) [28]. Therefore, to reduce the deposition temperature in Reactions (1) and (2), it is reasonable to use TaBr5 (instead of TaCl5), which is volatile at relatively low temperatures (vapor pressure is ~10 torr at 473 K). In addition, the corrosion of metals in bromine compounds is less than in chlorine compounds. Instead of hydrocarbons in Reactions (1) and (2), it is convenient to use carbon tetrachloride CCl4, which is a volatile fire-safe liquid that easily dissociates upon heating. Hydrogen in Reactions (1) and (2) can be replaced by vapors of reducing metals such as zinc [29], calcium [30], cadmium [31,32], magnesium, sodium, and potassium. Cadmium is a convenient reducing agent because it is volatile at relatively low temperatures (saturated vapor pressure is 1333 Pa at 763 K) [33]). Cadmium bromides and chlorides are also easily volatile at temperatures above 850 K, and therefore, their co-deposition with carbides is not possible. Thus, the TaBr5–CCl4–Cd system could be promising for the deposition of tantalum carbides according to the following reactions:
TaBr5 + CCl4 + 4.5 Cd = TaC + 2 CdCl2 + 5/2 CdBr2,
2 TaBr5 + CCl4 + 10 Cd = Ta2C + 5 CdCl2 + 5 CdBr2.
The optimal parameters of the deposition of tantalum carbides in the TaBr5–CCl4–Cd–He system were evaluated in the temperature range T = 673 ÷ 1673 K, pressure P = 1.01 × 105 Pa, and the ratio of elements in the reaction mixture Ta/(Ta + C) = 0.01 ÷ 0.909; Cd/(Cd + hal) = 0 ÷ 0.952 (hal = Cl + Br) [34]. It was found that pure tantalum carbides could be deposited at temperatures above T = 840 K, while lower temperatures of cadmium chloride impurities will be deposited. Monocarbide TaCy (y = 0.815 ÷ 1) can be formed at T = 840 ÷ 1100 K, and at 973 K, the maximum fraction of tantalum conversion to monocarbide ~0.6 can be achieved at a slight excess of tantalum in the reaction mixture Ta/C = 1.35 and 0.6 < Cd/hal < 9. Carbide Ta2−zC (z = 0.085 ÷ 0.199) can be formed at T = 960 ÷ 1070 K, and at 973 K, the maximum fraction of tantalum conversion to Ta2−zC is ~0.6 and can be achieved at ratios of Ta/C = 3 and 0.5 < Cd/hal < 9. The maximum fraction of tantalum conversion to carbides of ~0.9 can be achieved with two-phase compositions TaC + C, which is achieved at the following ratios: 0.1 < Ta/C < 1 and 0.6 < Cd/hal < 9.
The present work investigates the processes of chemical vapor deposition of tantalum carbides by reduction of tantalum bromide (and CCl4) with cadmium vapor on substrates made of 12X18H10T steel, ZhC6 superalloy and molybdenum. The coatings were deposited on equipment that we developed specifically to enable metal vapor reduction processes [31,32].

2. Materials and Methods

2.1. Coating Deposition Equipment

Tantalum carbide coatings in the TaBr5–CCl4–Cd–He system were deposited in a CVD reactor of our own design [35], with separate delivery of reagents to the reaction zone using inert carrier gas streams (Figure 1). The carrier gas was saturated with reagent vapors in the lower and upper evaporator chambers [36]. The lower evaporator was used to supply cadmium vapor at temperatures above 723 K. The upper evaporator was used to supply tantalum bromide vapor at temperatures above 423 K. The content of reagents in the carrier gas flows was set by the temperature of the evaporators. The content of CCl4 vapor was set using a temperature-controlled evaporator [37].
All reagents were delivered into the reaction chamber through separate channels, mixed in the vicinity above the sample and reacted, resulting in the precipitation of tantalum carbide. To heat samples and reagents, resistive heating was used (reagent heaters are not shown in Figure 1).

2.2. Sample Preparation and Coating Conditions

The coatings were deposited on flat polished samples (substrates) with a diameter of 12 mm, made of molybdenum (99.7 wt.%), 12Kh18N10T steel (Fe67Cr18Ni11Mn2Ti0.8Si0.8Cu0.23C0.12S0.02P0.03 wt.%), nickel alloy ZhC6 (Ni59Cr11Co9W9Al6Ti2Mo2Nb1 (C, Si, Mn, Zr, B, Y) 1 wt.%). In addition, samples made of 12Kh18N10T steel and Mo with a previously deposited tantalum coating were used [31]. The samples were installed in a special cassette [38] inside the reactor and pushed out from the holder tube as required. The distance from the nozzle cut-off to the sample was set at no more than 30 mm. Increasing the distance from the nozzle to the sample increases the possibility of homogeneous reactions in the gas phase with the formation of powders.
The concentration of reagent vapor in the evaporators, ci (mol/m3), was calculated by the Clapeyron–Mendeleev equation:
ci = pi/RT,
where pi, Pa, is the partial vapor pressure at the evaporator temperature T, K.
The amount of reagent delivered to the reaction chamber, Ci (mol/h), was determined by the equation:
Ci = ci·Gi,
where Gi, m3/h, is the gas flow rate in the evaporator.
The conditions of coatings deposition at deposition temperature of TD = 950–1000 K are given in Table 1. The ratios of reagents for TaC and Ta2C deposition were chosen according to calculations [34] so that the cadmium content in the reaction mixture was minimized.

2.3. Methods of Coating Analysis

To investigate the crystalline structure of coatings, X-ray diffractometers DRON-6 and DRON-3 were used (Bourevestnik, JSC, Saint Petersburg, Russia). X-ray diffraction patterns were made in monochromatized Cu Kα and Co Kα radiations; angular range 20–100 deg.; scan step 0.1 deg.; counting time per data point 10 s. X-ray focusing was made using the Bragg–Brentano method.
Layer-by-layer analysis of elements content and coating thickness measurements were performed using Glow Discharge Optical Emission Spectrometer (GDOES) GDA 650 HR (Spectruma Analytik GmbH, Hof, Germany), with detection limits from 1 ppm to 50 ppm and a depth resolution of about 50 nm.
The composition and chemical structure of the surface layers of the coating were analyzed by X-ray photoelectron spectroscopy (XPS) using SPECS spectrometer with excitation of the photoelectron spectrum by Mg Kα (E = 1253.6 eV). The Casa XPS software ver 2.3.26 (Casa Software Ltd., Teignmouth, UK) was used to process the spectral data. The relative error of element concentration determination is ±3%. XPS analysis was performed at depths of ~5 nm and ~25 nm with argon etching at a rate of ~1 nm/min. The chemical state of elements was analyzed by XPS spectra of core electron levels with reference data [39] and reference spectra. The 4f XPS spectrum of metallic tantalum was obtained and used as a reference spectrum.
To investigate the microstructure of the coating surface, we used scanning electron microscope (SEM), a Thermo Fisher Scientific Quattro S with an energy dispersive spectroscopy system based on EDAX spectrometer Octane Elect Plus EDS System (Thermo Fisher Scientific, Brno, Czech Republic).

3. Results and Discussion

3.1. Features of Tantalum Carbide Deposition at Early Stages (Up to 30 min)

Tantalum carbide coatings were deposited on the surface of all prepared substrates. At a deposition time of 30 min, the thickness of coatings reached from 1 to 2 μm. The tantalum carbide coatings on the molybdenum samples had a homogeneous microstructure and inherited scratches that were formed by polishing the substrate (Figure 2). The intensity of the X-ray diffraction peaks from the tantalum carbide coating (Figure 3) are much smaller than the peaks from the molybdenum substrate. This is because the tantalum carbide coatings on molybdenum were thinner than coatings on other types of substrates. In addition, in the spectra of characteristic radiations of elements for the area in Figure 2, the most intense lines corresponded to the characteristic radiation of molybdenum, and the lines of coating elements (tantalum and carbon) were much less intense.
In the C1s XPS spectrum of molybdenum coatings, there were peaks with binding energies of 283 and 284.8 eV (Figure 4a). The peak with a binding energy of 283 eV corresponds to carbon–metal compounds in carbide-like compounds, and the second peak with a binding energy of 284.8 eV corresponds to C–C bonds in graphite-like structures [40]. For the spectrum of Ta4f7/2 at a coating depth of 25 nm, the maximum peak is at 23 eV, whereas for the tantalum reference sample, this value is 21.8 eV (Figure 4b). This difference indicates tantalum carbide formation [39,41]. Analysis of the C1s and Ta4f7/2 spectra shows that tantalum carbide formed at a depth of 25 nm has non-stoichiometric composition TaC1−x with x ≤ 0.17. Simultaneously, a very small fraction of carbon has C–C bonds.
Analysis of the XPS spectra also revealed that at a depth of ~25 nm the tantalum/carbon ratio is ~43/45, and at a depth of ~5 nm the tantalum/carbon ratio is ~26/53 (Table 2). A small increase in the carbon content in the top layers of the coating up to ~5 nm is due to the features of the experiment. Toward the end of the experiment, the flow CCl4 vapor to the reaction chamber was stopped last, which resulted in a small excess decomposition of CCl4 vapor on the coating surface. Analysis of XPS spectra also showed that the content of oxygen impurities decreased significantly from the surface to the depth of the coatings (Table 2). That is, the oxygen impurities in the surface layers of the coatings were introduced mainly in the examination of the coatings and much less during deposition.
The coatings deposited for 30 min on ZhC6 substrates were thicker than those on molybdenum. Also, no polishing marks were observed on the coatings (Figure 5), and the intensity of the X-ray peaks of the coating material was higher than that of the substrate material (Figure 6). On the energy dispersive spectra, the peaks of Ta and C were also more intense than those of the substrate elements. Randomly placed conglomerates of crystallites ranging in size from 0.2 to 1.5 μm were observed on the surface of the coatings (Figure 5).
The analysis of XPS spectra of C1s and Ta4f7/2 coatings from ZhC6 alloy shows that at a depth of 5 nm, carbon forms mainly C–C bonds, and C–Ta bonds are much less. The number of C–C bonds decreases with depth. The tantalum/carbon ratio at a depth of ~5 nm is ~1/3.3, and at a depth of 25 nm it is ~1/1.3 (Table 2). At a 25 nm depth, the carbide composition corresponds to that of TaC1−x with x ≤ 0.17, as for the coating on molybdenum. The increase in the number of C–C bonds in the top layers of the coating (up to 5 nm) is due to the fact that the CCl4 flux is switched off last at the end of deposition, as for coatings on molybdenum substrate. In addition, the content of oxygen impurity in the upper layers of the coating (up to 5 nm) increases due to the adsorption of oxygen on the surface during the coating analysis process.
Tantalum carbide coatings deposited on 12X18N10T steel for 30 min were thicker than coatings deposited on molybdenum, but slightly thinner than coatings deposited on ZhC6 alloy. The surface of the coatings showed no traces of substrate polishing (Figure 7), and the intensity of the TaC X-ray diffraction peaks was comparable to that of the substrate material (Figure 8). The coating surface is a homogeneous layer of conglomerates of crystallites, with a diameter of ~0.5 µm each (Figure 7), located normal to the surface.
Analysis of the C1s XPS spectra of the coatings on 12Kh18N10T showed that there are more C–C bonds than C–Ta bonds in the layers at depths of 5 and 25 nm (Figure 8). The number of C–C bonds decreases, and the number of C–Ta bonds increases with depth. For tantalum carbide TaC1−x, the stoichiometry was determined from the spectrum of Ta4f7/2 and was x ≤ 0.17 at a depth of 25 nm, which coincides with the stoichiometry of coatings deposited on molybdenum and ZhC6. The tantalum/carbon ratio is ~1/4 at a depth of ~5 nm, and ~3/2 at a depth of 25 nm (Table 2). In addition, oxygen impurities up to 8 at.% and chromium impurities up to 2 at.% were detected in the surface layers of the coatings, which appeared there during deposition and analysis of the coatings (Table 2).

3.2. Deposition of Tantalum Carbide Coatings over 30 min

The coatings deposited on molybdenum for 1 h had a thickness of about 4 μm (Figure 9) and uniform fine-crystalline structure. Substrate polishing marks are noticed on the coatings, and the X-ray diffraction spectra contain peaks from both tantalum monocarbide and the substrate material (Figure 9). The ratio of the intensities of the (002) and (111) TaC X-ray peaks is ~2.3 (standard one is ~0.6), which indicates the formation of texture in the coating.
The analysis of XPS C1s and Ta4f7/2 spectra demonstrated that the Ta/C ratio in the coating at a depth of 5 nm is higher than in a similar layer of the coating that was deposited on molybdenum for 30 min (Figure 10). This means that the Ta/C ratio on the coating surface increases with deposition time. Already at a depth of 25 nm, the Ta/C ratio is close to 1, as in the coating deposited on molybdenum for 30 min. Despite this, at a depth of 25 nm, tantalum carbide is non-stoichiometric TaC1−x with x ≤ 0.17 since part of the carbon is in the form of C–C bonds. The impurity content of the coating decreases with depth, hence most of the impurities are adsorbed after deposition.
When the deposition time was increased up to two hours, columnar crystals growing perpendicularly to the substrate were observed on the coating surfaces (Figure 11). This behavior is typical for CVD coatings at relatively low temperatures [42]. The crystals are equiaxed within the axes parallel to the deposition surface and are spheroidal in the growth direction. This normal crystallite growth was observed on all substrates used, with coating deposition times longer than 30 min. The growth of columnar crystals is driven by geometric selection [43]. The randomly oriented small grains are gradually dissolved by the coalescence and grain coarsening, and the grains grow preferentially in a direction perpendicular to the surface of the coating, resulting in the formation of columnar crystals with the same orientation.
For the molybdenum coatings, analysis of the XPS spectra showed that near the surface and at a depth of 25 nm, the Ta/C ratio is approximately 1, and the tantalum carbide has a composition of TaC1−x with x ≤ 0.17. The layer-by-layer analysis of the elemental composition (Figure 12) demonstrates that the total thickness of the carbide layer is about 10 μm, and for TaC1−x, the average non-stoichiometry is x ~ 0.5. In addition, there is a diffuse layer with tantalum and carbon in a molybdenum matrix at a depth of 6 to 10 μm between the coating and the substrate.
The ZhC6 alloy coatings deposited for 1 h were also formed as columnar crystals oriented perpendicularly to the substrate, with a spheroidal top in the growth direction (Figure 13). The thickness of these coatings was about 4 μm, there were no marks of substrate polishing on the coatings, and both tantalum monocarbide peaks and less intense peaks corresponding to the substrate were present in the X-ray spectrum (Figure 14).
The analysis of XPS spectra of the coatings on ZhC6 alloy at depths of 5 and 25 nm demonstrates the same patterns as for the coatings deposited on Mo. The tantalum/carbon ratio increases slightly with increasing depth (Table 2), and at a depth of 25 nm, the tantalum carbide has the composition TaC1−x with x ≤ 0.17. In the upper layers of the coating, we observed up to 6 at.% of nickel, which apparently diffused from the substrate.
When the deposition coating time on the ZhC6 alloy was increased to 4 h, flaking of the upper coating layers was observed while the lower coating layers remained intact (Figure 15).
X-ray diffraction analysis of the coatings demonstrates presence of tantalum monocarbide (Figure 16). The coating is relatively thick, so only weak reflections from the substrate are visible in the X-ray diffraction pattern. The thickness of the coatings was about 13 μm, of which 8 μm is the carbide layer and 5 μm is the diffuse layer (Figure 15). Layer-by-layer analysis showed that near the surface, the coating composition was close to stoichiometric tantalum monocarbide (Figure 17). The tantalum content in the carbide decreased with depth down to 5 μm. Between 5 and 8 μm, we observed a slight increase in tantalum content of the coating, but the carbide composition was still non-stoichiometric.
The coatings deposited on 12Kh18N10T steel within one hour (Figure 18) are similar to those on ZhC6 alloy (Figure 13). The thickness of the coatings is about 5 μm, and the coatings consist of two phases of TaC and Ta2C (Figure 19). The substrate had a dual-phase structure composed of γFe and αFe (Figure 19).
When the deposition time was prolonged by up to two hours, the coatings cracked (Figure 20), similar to the coatings deposited on ZhC6 for 4 h (Figure 15). The thickness of the coatings was ~17 μm, of which ~8 μm was in the carbide layer and ~9 μm in the diffuse layer (Figure 21). The composition of TaC in the carbide layer was non-stoichiometric even at the surface. The coatings contained iron impurities up to ~5% in the carbide layer.
As a rule, flaking and damage of the coating is the result of internal stresses. One of the reasons for internal stress formation is the different thermal expansion of the substrate and coating layers. Therefore, the coefficients of thermal expansion (CTE) for substrates and coatings (at temperatures of 295 to 1000 K) were compared in order to estimate the possibility of coating failure.
The coefficient of thermal expansion of TaCy increases with increasing “y” and is equal to 6.04, 6.37, 6.96 × 10−6 K−1 for TaC0.789, TaC0.912, and TaC0.997, respectively [44]. The CTE of TaC films is 6.7–8.2 × 106 K−1 [45]. The CTE of Ta2C films is 7.2 × 10−6 K−1 [46]. The coefficient of thermal expansion of molybdenum is 5.8 × 10−6 K−1 [47], which is close to the coefficient of thermal expansion of tantalum carbides. That is why relatively thick layers of tantalum carbide deposited on molybdenum did not flake off. The coefficient of thermal expansion of ZhC6 alloy is 15 × 10−6 K−1 [48], which is approximately twice that of tantalum carbides. Therefore, internal stresses increase with the increase in the thickness of ZhC6 coating, and this results in flaking of coatings thicker than 12 μm after four hours of deposition (Figure 15). The coefficient of thermal expansion of 12Kh18N10T steel (an analogue of ANSI321) is even higher: CTE = 18.6 × 10−6 K−1 [49,50]. In addition, the αFe/γFe phase ratio in 12Kh18N10T steel varies with the temperatures of coating deposition and subsequent cooling [51]. Thus, the significant difference in the CTEs between the 12Kh18N10T substrate and a coating, as well as α-γ phase transformations in the steels, results in the formation of cracks in coatings thicker than 12 μm within two hours after the start of deposition (Figure 20) rather than four hours, as with the ZhC6 alloy substrate.
During the coating process, there is an induction period when nucleation of deposition products begins to form on surface defects [43,52]. The induction period is characterized by a low deposition rate. As the nucleation layer forms, the observed deposition rate increases in response to the growth of the solid phase interfaces. The growing coating slowly covers the nucleation centers on the substrate, resulting in a decrease in the observed deposition rate. Another reason for deposition deceleration is the decrease in phase interface area, caused by the competitive growth of grains that are oriented perpendicularly to the substrate surface.
At the temperature of carbide deposition, molybdenum’s surface has relatively few point defects [53]. The low concentration of surface defects results in a low initial rate of coating formation and a relatively long induction period. Therefore, the initial deposition rate of tantalum carbide on molybdenum was lower than the rate of deposition on ZhC6 alloy and 12Kh18N10T steel. For ZhC6 alloy, the main defects are dislocations at γ′/γ phase interface and produced carbide particles of MeC type where (Me = Ti, Mo, W, etc.) [54]. The formation of defects on austenitic steels, such as 12Kh18N10T, has been studied [55,56]. Segregation of solid γ-solution occurs in steels, and at temperatures above T = 673 K, concentration inhomogeneities with high chromium content and α-phase particles are formed. In the range between 723 K and 948 K, a reverse α-γ transformation takes place that results in the formation of reticular dislocation structure in austenite and a slight decrease in steel volume. In addition, at temperatures of 673–973 K, there is a precipitation of carbide and intermetallic phase. The existence of different surface defects in 12Kh18N10T steel seems to cause an increase in coating thickening and the formation of a diffuse zone in the substrate, as well as iron diffusion in the coating.
Tantalum monocarbide TaCy composition may vary from y = 0.6 to y = 1 with an unchanged cubic crystalline structure [3]. To estimate the value of “y”, one may use tantalum monocarbide lattice constant a as a function of “y” [57]:
a(y, 0) = a0 + a1y + a2y2,
where a0 = 4.1630 Å, a1 = 0.4675 Å, and a2 = −0.1750 Å.
Lattice constants and “y” evaluation results for deposited tantalum carbides obtained by Equation (7) are summarized in Table 3. The “y” value stays within a range of 0.72–0.86, which is close to the 0.83 value obtained by XPS methods at a depth of 25 nm for the same coatings.
As per GDOES data, the average Ta/C ratio of coatings on various substrates is about 2/1 (Table 3), except for coatings on steel, for which the Ta/C ratio is 2/0.6. It means that the carbon content of the coating is closer to the Ta/C ratio in Ta2C rather than to Ta/C ~0.6–1 in tantalum monocarbide [3]. As X-ray diffraction of a coating fails to determine if tantalum is present in any phase, it is likely that a part of it presents in the amorphous state.
It should be noted that the tantalum carbide deposition technique applied by us is similar to those techniques in which reaction mixtures based on TaCl5–CH4–H2–Ar or TaCl5–C3H6–H2–Ar were used [26,28,42]. All techniques produce a dense tantalum carbide coating with a columnar structure having a thickness of 10 μm within two hours at a deposition rate of ~5 μm/h. But only in our technique were such coatings deposited at temperatures of 950–1000 K and atmospheric pressure, whereas in [26,28,42], temperatures from 1323 to 1573 K and pressure reduction P ~104 Pa were required for carbide deposition.

4. Conclusions

We deposited tantalum carbides on 12Kh18N10T steel, ZhC6 alloy, and molybdenum through the reduction of TaBr5 and CCl4 vapors with cadmium vapors. The deposition was performed at 950–1000 K, within 30–240 min, in the flow reactor, with reagents fed in a carrier gas (helium) stream through the split channels, at a rate from 0.5 × 10−3 to 5 × 10−3 mol of a reagent per hour for each channel, with the reagent concentrations optimal for the formation of a mixture of TaC and Ta2C carbide.
The coatings were deposited on the surface in the form of columnar grains, and additionally, a diffuse zone was formed in the substrate material. After two hours of deposition, the coating thickness on molybdenum was about 10 μm (4 μm diffuse layer), on ZhC6 alloy was 13 μm (5 μm diffuse layer), and on 12Kh18N10T steel was 17 μm (9 μm diffuse layer). The average rate of deposition was about 5 μm/h for molybdenum, 6 μm/h for ZhC6 alloy, and 8 μm/h for 12Kh18N10T steel. It seems that the difference in average rates of deposition is due to the different induction period time, which is dependent on substrate defect types and concentration.
X-ray diffraction revealed that the coatings on all substrates consisted mainly of TaC0.72–0.86 (with an admixture of Ta2C on steel). All coatings contained some part of tantalum, which was not determined by X-ray diffraction. According to XPS and GDOES data, near the coating surface the composition of tantalum carbide was close to stoichiometric, and at a depth of 25 nm, the composition of tantalum carbide was TaC0.83.
The coatings on the molybdenum were deposited as dense layers. The coatings on the ZhC6 alloy partially flaked off after 4 h of deposition. The coatings on the 12Kh18N10T steel partially flaked off after 2 h of deposition. The cause of coating flaking was the difference between CTEs of the coating and the substrate, as well as in concentration inhomogeneities and phase transitions in the substrate material in the course of deposition during heating and cooling.

Author Contributions

Conceptualization, T.K., O.Y.G., J.K. (Jiri Kuchar) and R.R.F.; methodology, O.Y.G., J.K. (Jan Kudlacek) and R.R.F.; software, I.V.S., A.I.K. and D.C.; validation, T.K., J.K. (Jiri Kuchar) and R.R.F.; formal analysis, I.V.S., J.K. (Jan Kudlacek) and A.I.K.; investigation, O.Y.G. and I.V.S.; resources, T.K., O.Y.G., J.K. (Jiri Kuchar) and A.I.K.; data curation, T.K. and A.I.K.; writing—original draft preparation, O.Y.G. and R.R.F.; writing—review and editing, T.K., O.Y.G., J.K. (Jiri Kuchar) and A.I.K.; visualization, I.V.S., J.K. (Jan Kudlacek) and R.R.F.; supervision, T.K., O.Y.G., J.K. (Jiri Kuchar) and A.I.K.; project administration, T.K., O.Y.G., I.V.S. and J.K. (Jan Kudlacek); funding acquisition, T.K., O.Y.G., J.K. (Jiri Kuchar) and D.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Slovak Ministry of Education, Science, Research and Sport within projects VEGA 1/0509/23 and KEGA 052TUKE-4/2024. The research was also supported by the project SGS22/156/OHK2/3T/12 “Influence of surface treatments on the quality of production technologies” and by the Russian Federation Ministry of Education and Science within projects No. RFMEFI 58214X0005, FUUE-2024-0007, and FUUE-2024-0010. The studies were performed using equipment of core shared research facilities of the “Center of physical and physical–chemical methods of analysis, investigations of properties and characteristics surface, nanostructures, materials and samples” of UdmFRC UrB RAS.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy reasons.

Conflicts of Interest

Ravil R. Faizullin was employed by the company Scientific Production Association Protective Coatings, LLC. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; nor in the decision to publish the results.

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Coatings 14 00547 g001
Figure 1. Reagent supply to the CVD reactor.
Figure 1. Reagent supply to the CVD reactor.
Coatings 14 00547 g001
Coatings 14 00547 g002
Figure 2. SEM image of the tantalum carbide coating on molybdenum substrate (30 min) with more detailed view on the right.
Figure 2. SEM image of the tantalum carbide coating on molybdenum substrate (30 min) with more detailed view on the right.
Coatings 14 00547 g002
Coatings 14 00547 g003
Figure 3. X-ray diffraction pattern of the coating on molybdenum substrate (30 min).
Figure 3. X-ray diffraction pattern of the coating on molybdenum substrate (30 min).
Coatings 14 00547 g003
Coatings 14 00547 g004
Figure 4. XPS spectra of (a) C1s and (b) Ta4f for coatings at a depth of 25 nm.
Figure 4. XPS spectra of (a) C1s and (b) Ta4f for coatings at a depth of 25 nm.
Coatings 14 00547 g004
Coatings 14 00547 g005
Figure 5. SEM image of the tantalum carbide coating on ZhC6 substrate (30 min) with more detailed view on the right.
Figure 5. SEM image of the tantalum carbide coating on ZhC6 substrate (30 min) with more detailed view on the right.
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Coatings 14 00547 g006
Figure 6. X-ray diffraction analysis of the coating on ZhC6 substrate (30 min).
Figure 6. X-ray diffraction analysis of the coating on ZhC6 substrate (30 min).
Coatings 14 00547 g006
Coatings 14 00547 g007
Figure 7. SEM image of the tantalum carbide coating on 12Kh18N10T substrate (30 min) with more detailed view on the right.
Figure 7. SEM image of the tantalum carbide coating on 12Kh18N10T substrate (30 min) with more detailed view on the right.
Coatings 14 00547 g007
Coatings 14 00547 g008
Figure 8. X-ray diffraction analysis of the coating on 12Kh18N10T substrate (30 min).
Figure 8. X-ray diffraction analysis of the coating on 12Kh18N10T substrate (30 min).
Coatings 14 00547 g008
Coatings 14 00547 g009
Figure 9. SEM image of the tantalum carbide coating on molybdenum substrate (60 min) with more detailed view on the right.
Figure 9. SEM image of the tantalum carbide coating on molybdenum substrate (60 min) with more detailed view on the right.
Coatings 14 00547 g009
Coatings 14 00547 g010
Figure 10. X-ray diffraction analysis of the carbide coating on the molybdenum substrate (60 min).
Figure 10. X-ray diffraction analysis of the carbide coating on the molybdenum substrate (60 min).
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Coatings 14 00547 g011
Figure 11. SEM image of the tantalum carbide coating on molybdenum substrate (120 min) with more detailed view on the right.
Figure 11. SEM image of the tantalum carbide coating on molybdenum substrate (120 min) with more detailed view on the right.
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Coatings 14 00547 g012
Figure 12. Layer-by-layer analysis of the element content of the coating on molybdenum (120 min).
Figure 12. Layer-by-layer analysis of the element content of the coating on molybdenum (120 min).
Coatings 14 00547 g012
Coatings 14 00547 g013
Figure 13. SEM image of the tantalum carbide coating on ZhC6 substrate (60 min) with more detailed view on the right.
Figure 13. SEM image of the tantalum carbide coating on ZhC6 substrate (60 min) with more detailed view on the right.
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Coatings 14 00547 g014
Figure 14. X-ray diffraction analysis of the carbide coating on the ZhC6 substrate (60 min).
Figure 14. X-ray diffraction analysis of the carbide coating on the ZhC6 substrate (60 min).
Coatings 14 00547 g014
Coatings 14 00547 g015
Figure 15. SEM image of the tantalum carbide coating on ZhC6 substrate (4 h) with more detailed view on the right.
Figure 15. SEM image of the tantalum carbide coating on ZhC6 substrate (4 h) with more detailed view on the right.
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Coatings 14 00547 g016
Figure 16. X-ray diffraction analysis of the carbide coating on ZhC6 substrate (240 min).
Figure 16. X-ray diffraction analysis of the carbide coating on ZhC6 substrate (240 min).
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Coatings 14 00547 g017
Figure 17. Layer-by-layer analysis of the element content of the coating on the ZhC6 (120 min).
Figure 17. Layer-by-layer analysis of the element content of the coating on the ZhC6 (120 min).
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Coatings 14 00547 g018
Figure 18. SEM image of the tantalum carbide coating on 12Kh18N10T steel (60 min) with more detailed view on the right.
Figure 18. SEM image of the tantalum carbide coating on 12Kh18N10T steel (60 min) with more detailed view on the right.
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Coatings 14 00547 g019
Figure 19. X-ray diffraction analysis of the carbide coating on the 12Kh18N10T substrate (60 min).
Figure 19. X-ray diffraction analysis of the carbide coating on the 12Kh18N10T substrate (60 min).
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Coatings 14 00547 g020
Figure 20. SEM image of the tantalum carbide coating on 12Kh18N10T steel (120 min).
Figure 20. SEM image of the tantalum carbide coating on 12Kh18N10T steel (120 min).
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Coatings 14 00547 g021
Figure 21. Layer-by-layer analysis of the element content of the coating on the 12Kh18N10T steel (120 min).
Figure 21. Layer-by-layer analysis of the element content of the coating on the 12Kh18N10T steel (120 min).
Coatings 14 00547 g021
Table 1. Conditions for the deposition of tantalum carbides.
Table 1. Conditions for the deposition of tantalum carbides.
Deposition Parameters 1Reagents
TaBr5CdCCl4
pi, Pa533–9331066–16001466–1866
Gi, ml/s2–53–80.3–1
Ci × 10−3, mol/h1.7–2.52.4–4.60.6–3.1
t, h0.5–4
Ta/Cd/C1/(1 ÷ 2)/(0.5 ÷ 1.2)
1 Deposition parameters: pi, Gi, Ci are the parameters of Equations (1) and (2); t is the deposition time; Ta/Cd/C is the molar ratio of elements in the reaction mixture.
Table 2. Percentage element composition of the coating deposited on different substrates at depths of 5 and 25 nm (in parentheses).
Table 2. Percentage element composition of the coating deposited on different substrates at depths of 5 and 25 nm (in parentheses).
Sample/Deposition Time (min)Ta, at.%C, at.%Cr, at.%Fe, at.%Ni, at.%O, at.%
Mo/3026 (43)53 (45) 21 (12)
12Kh18N10T/3018 (36)74 (54)0 (2) 8 (8)
ZhC6/3018 (35)61 (45) 2 (4) 19 (16)
Mo/6039 (44)47 (48)3 (4)3 (1) 7 (3)
ZhC6/6043 (51)44 (39) 5 (6)8 (3)
ZhC6/24030 (46)57 (46)2 (2)1 (1)1 (1)8 (4)
Table 3. Lattice constants a and y values for tantalum carbide.
Table 3. Lattice constants a and y values for tantalum carbide.
Substratea, ÅNon-Stoichiometry DegreeTa/C GDA Estimation
12Kh18N10T/304.408TaC0.721/0.33
Mo/304.414TaC0.741/0.5
Mo/604.409TaC0.721/0.52
ZhC6/304.438TaC0.871/0.52
ZhC6/604.414TaC0.741/0.54
ZhC6/2404.435TaC0.861/0.55
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Krenicky, T.; Goncharov, O.Y.; Kuchar, J.; Sapegina, I.V.; Kudlacek, J.; Faizullin, R.R.; Korshunov, A.I.; Cerny, D. Chemical Vapor Deposition of Tantalum Carbide in the TaBr5–CCl4–Cd System. Coatings 2024, 14, 547. https://doi.org/10.3390/coatings14050547

AMA Style

Krenicky T, Goncharov OY, Kuchar J, Sapegina IV, Kudlacek J, Faizullin RR, Korshunov AI, Cerny D. Chemical Vapor Deposition of Tantalum Carbide in the TaBr5–CCl4–Cd System. Coatings. 2024; 14(5):547. https://doi.org/10.3390/coatings14050547

Chicago/Turabian Style

Krenicky, Tibor, Oleg Y. Goncharov, Jiri Kuchar, Irina V. Sapegina, Jan Kudlacek, Ravil R. Faizullin, Alexander I. Korshunov, and Daniel Cerny. 2024. "Chemical Vapor Deposition of Tantalum Carbide in the TaBr5–CCl4–Cd System" Coatings 14, no. 5: 547. https://doi.org/10.3390/coatings14050547

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