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Article

Investigation of the Influence of Powder Fraction on Tribological and Corrosion Characteristics of 86WC-10Co-4Cr Coating Obtained by HVOF Method

by
Bauyrzhan Rakhadilov
1,
Nazerke Muktanova
2,3,*,
Dauir Kakimzhanov
2,3,
Zarina Satbayeva
3,
Leila Kassenova
4 and
Nurtoleu Magazov
2,5
1
Research Center “Surface Engineering and Tribology”, Sarsen Amanzholov East Kazakhstan University, Ust-Kamenogorsk 070000, Kazakhstan
2
International School of Engineering, Daulet Serikbayev East Kazakhstan Technical University, Ust-Kamenogorsk 070002, Kazakhstan
3
PlasmaScience LLP, Ust-Kamenogorsk 070010, Kazakhstan
4
Department of Information Systems and Technologies, Esil University, Astana 020000, Kazakhstan
5
Research Center “Protective and Functional Coatings”, Daulet Serikbayev East Kazakhstan Technical University, Ust-Kamenogorsk 070002, Kazakhstan
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(6), 651; https://doi.org/10.3390/coatings14060651
Submission received: 8 April 2024 / Revised: 8 May 2024 / Accepted: 20 May 2024 / Published: 21 May 2024
(This article belongs to the Special Issue Applications of Ceramic and Cermet Coatings)
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Versions Notes

Abstract

:
Samples using powders of four different fractions, 15–20 μm, 20–30 μm, 30–40 μm and 40–45 μm, were fabricated to investigate the wear resistance, corrosion resistance and tribological properties of the 86WC-10Co-4Cr coating obtained using the HVOF method. The phase composition, microstructure and elemental distribution were analyzed using X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques. The hardness was measured on a Vickers microhardness tester, the friction coefficient and wear rate were investigated using a tribometer, and the corrosion resistance was evaluated on an electrochemical corrosion station. The results showed that the cross-sectional microstructure of the coating is mainly represented by multifaceted WC crystals embedded in the Co-Cr matrix and the presence of lower tungsten carbides, particularly W2C. The 15–20 μm fraction particles were subjected to superheating, contributing to the decarburization process. The 20–30 µm and 30–40 µm sized particles prevented overheating and had a more homogeneous structure. The 40–45 µm powder fractions did not reach sufficient temperature for complete melting, resulting in the formation of pores in the coating layers. The phase composition of the coatings included WC, W2C and CoO phases. According to the results of the study, it was found that the optimal powder fraction for coating the 86WC-10Co-4Cr composition with improved characteristics is the fraction of the 20–30 µm sized particles.

1. Introduction

The priority directions of increasing the competitiveness of products in the oil industry are the introduction of modern technologies to improve the reliability of elements of pipeline valves, reducing the cost of their production and maintenance, extending their service life, and the modernization of units by restoring their performance to a new level [1].
In the Republic of Kazakhstan, large factories engage in the production of pipeline fittings, with many situated in the industrial city of Ust-Kamenogorsk. The largest mining, oil refining, and metallurgical enterprises in the Republic of Kazakhstan and the CIS countries are the main customers of pipeline valves. These valves include various types such as gate valves, wedge valves and return valves.
One of the main technical tasks facing Kazakhstani plants is to increase the reliability and durability of gate valves, which are necessary for the efficient transportation of oil and gas. Gate valves play an important role in the extraction and transportation of raw materials, determining safety conditions and environmental protection. However, domestic plants still use outdated methods of surfacing and coating, which do not provide the necessary tribological and corrosion characteristics for the gate-seat assembly.
In our previous research work, the outdated methods and the current state of the plants of the Republic of Kazakhstan were considered in more detail [2]. Taking into account the problems arising from the use of traditional methods of surface treatment of the gate part of oil pipeline valves, a transition to modern gas-thermal methods is proposed. These methods increase wear resistance and corrosion resistance without deforming the gate surface, which leads to a significant increase in the service life of pipeline valves. Switching to gas-thermal methods will also reduce the environmental burden compared to galvanic chrome plating by reducing the need for a strict industrial ventilation system and electrolyte disposal [3]. This will make the production more environmentally friendly and reduce the cost of complying with environmental regulations. The use of modern gas-thermal methods also contributes to the competitiveness of the products as it provides high quality surface treatment of gate valves [2].
One such technology is the high velocity oxygen fuel (HVOF) process, which is considered a viable alternative to hard chrome plating because of its wear resistance and environmental friendliness. In the HVOF process, one of the commonly used initial materials is WC-Co, or, more generally, a metal–ceramic based on tungsten carbide (WC). By optimizing the HVOF spraying process [4], it is possible to obtain WC coatings characterized by high adhesion strength, excellent compaction, high carbide content, corrosion resistance and exceptional wear resistance [5,6,7,8,9]. WC tungsten carbide-based coatings are widely used in various industries to protect key mechanisms [10], including industrial gas turbines, ball valves and aircraft landing gear shafts [11]. Among the various WC coatings, the most common are WC-Co coatings, where cobalt (Co) is used as the binder component. The presence of cobalt (Co) helps to increase the bonding strength between the WC solid phase and the substrate, which significantly affects the performance of the coating. For example, cobalt (Co) acts as a lubricant [12], forming a thin layer on the contact surface of the coating with the substrate. The introduction of chromium (Cr) metal into the Co binder matrix leads to a marked improvement in the wear and oxidation resistance of the WC-Co coating. This is especially significant considering that the process of applying this coating does not cause the formation of highly toxic hexavalent chromium [13,14,15,16]. However, it should be noted that WC-Co coatings are mainly used in applications requiring high wear resistance, and they are less suitable for use in corrosive environments compared to WC-Co-Cr coatings [17,18]. Therefore, WC-Co-Cr coatings can be applied to pipeline valve parts using the HVOF process as a replacement for conventional hard chrome plating to extend the service life of gate valves [1,19]. WC-Co-Cr coatings can also be deposited using various spraying methods such as air plasma spraying (APS), vacuum plasma spraying (VPS), detonation spraying, arc spraying and flame spraying [20,21,22,23,24,25,26]. Among the above methods, the HVOF process is considered to be the most promising method, successfully providing solutions to many issues related to oxidation, wear resistance, hot corrosion and erosion [27]. Nevertheless, the HVOF process has been widely recognized due to several advantages such as reduced porosity and degradation reactions, the high retention of WC tungsten carbide in the coating structure and improved adhesion between the coating and the substrate. When conducting research, it is necessary to pay attention to the process of the tungsten carbide (WC) decomposition in WC-Co-Cr coating, considering its significant influence on the properties of this coating. Some studies indicate that the degree of WC phase decomposition can be controlled by the spraying process, spraying parameters and fractions of the initial powders. However, among the mentioned processes, the important factors are the initial powder fractions, which must meet the established requirements in the specifications of the coating systems.
Therefore, a considerable number of studies have been carried out using different HVOF technology systems (e.g., JP-5000, DJ 2600, CJS-K4.2, JP-8000, GTV-K2) to investigate the fractions of the initial powder that have a significant influence on the coating characteristics, such as the phase, microstructure and wear resistance. The study conducted by Reza Saharkhiz et al. examined a NiCoCrAlYTa coating deposited using the HVOF method with two different particle fractions (from 5–38 μm to 25–38 μm). In this study, it was observed that increasing the size of the sputtered particles leads to a decrease in the oxide content of the resulting coatings [28]. Rajasekaran et al. [29] investigated in detail the effect of the particle size of the initial powder on the formation of bonded coatings using the HVOF method. They used three kinds of commercial CoNiCrAlY powders as the HVOF starting material with different fractions: fine (5–37 μm), medium (11–62 μm) and coarse (45–75 μm). The results of the study showed that coatings obtained using the coarse fraction had promising oxygen content levels compared to coatings obtained using finer and medium particles. They also highlighted the significance of proper selection of process parameters such as sputtering distance, oxygen fuel ratio, powder flow rate, streamline atmosphere and conditions such as substrate cooling with compressed air, number of sputtering passes and manipulator travel speed to obtain high quality coatings. In [30], a modelling study was used to formulate the control problem of the HVOF process taking into account the influence of the initial powder on the microstructure of the coating, which, in turn, determines its thermal and mechanical properties. The particle size of the initial powder was considered a key parameter affecting the coating quality and served as a basis for controlling the average particle velocity and temperature. From the results obtained, it was concluded that for smaller particles the velocity decreases even more severely, making them less suitable for thermal spraying. At the same time, for larger particles, the velocity and temperature become almost the same once the gas velocity is reached, indicating that the driving forces of motion and heating are too negligible to significantly change their velocity and temperature. In [31,32], it was demonstrated using modelling that WC-Co-Cr particles completely melt in the HVOF sputtering process, provided their fraction is smaller than 15 μm, resulting in the overheating of the material. However, the results of other studies contradict this. During the sputtering process, particles reach different temperatures and velocities depending on their fraction. Fine-grained WC-Co-Cr cermet coating (200–500 nm) showed higher sliding and abrasion resistances compared to conventional cermet coating (2–4 µm) [33].
The aforementioned studies confirm that the maximum values of both velocity and temperature correspond to the average particle size, which emphasises the importance of the particle size of the initial powder as a key parameter determining the quality of the coating. This parameter also serves as a basis for controlling the average particle velocity and temperature. However, the number of studies on the influence of initial powder fractions on the properties of coatings obtained by HVOF method is poorly understood. Most authors in their works mainly considered the influence of powder particle size distribution (including WC) on the microstructure, wear resistance and corrosion properties of thermally sprayed WC-Co-Cr coatings [34,35,36,37,38].
Thus, conducting a study covering a wide range of particle distributions of the initial powder into fractions to assess the influence of each of them on the microstructural, mechanical and tribological properties of HVOF sprayed coatings based on WC-Co-Cr is highly relevant and serves as a basis for this work. Accordingly, the purpose of this study is to investigate the influence of the initial powder fraction on the tribological and corrosion characteristics of 86WC-10Co-4Cr coating obtained using the HVOF method.
These research works were performed on the HVOF unit of Termika-3 system, located on the basis of “PlasmaScience” LLP (Ust-Kamenogorsk, Kazakhstan).

2. Materials and Methods

In the present work, the influence of fractions of initial powders on the microstructure, phase composition, hardness, wear resistance and corrosion resistance of 86WC-10Co-4Cr metal–ceramic coatings was investigated. Spherical powder of tungsten carbide in the cobalt–chromium matrix 86WC-10Co-4Cr produced by the “Polema” company (Tula, Russia) was used to form coatings with the following fractions: 15–20 μm, 20–30 μm, 30–40 μm and 40–50 μm. Samples from high-alloyed, corrosion-resistant steel 30Cr13 were used as a substrate. The chemical composition of the base and 86WC-10Co-4Cr coating materials was presented in the previous study [2].
Prior to the spraying process, the substrate surface was sandblasted at a pressure of 0.6 MPa using electrocorundum. For coating, the technology of high-speed oxygen-fuel spraying was used at the HVOF Termika-3 installation, manufactured by Plasmacenter LLC in St. Petersburg, Russia.
Metal–ceramic coatings 86WC-10Co-4Cr with varying fractions: 15–20 μm, 20–30 μm, 30–40 μm and 40–45 μm were obtained on HVOF Termika-3. The dwell time of all samples during spraying was 10 s. The parameters of the HVOF Termika-3 setup and an example powder code are summarized in Table 1.
The cross-sectional morphology of the coatings was characterized using TESCAN MIRA3 LMH scanning electron microscopy (TESCAN, Brno, Czech Republic). SEM images processed by Science V3 image analysis software on an OLYMPUS BX53M metallographic microscope (Olympus Corporation, Tokyo, Japan) were used to evaluate the porosity of the coatings. An X-ray diffractometer X’PertPRO (Philips Corporation, Amsterdam, the Netherlands) with Cu-Kα -radiation (λ = 0.154 nm), operating at a voltage and current of 40 kV and 30 mA, respectively, was used for X-ray diffractometer analysis of coatings. Measurements were performed over a range of diffraction angles 2θ from 20° to 100°. For the experiments, the step width and exposure time were set to 0.05° and 1 s, respectively, for each step. Material analysis using diffraction (MAUD) full-profile analysis program, version 2.54, was used to analyze the diffraction data. The surface roughness of the coatings was determined using a model 130 profilometer (JSC “Plant PROTON”, Moscow, Russia) by averaging five measurements. The microhardness of the samples was measured on a microhardness meter Metolab 502 (Metolab, Moscow, Russia) on the cross-section of the coatings using the Vickers method. For this purpose, the indenter load m = 100 g and dwell time 10 s were used. Tribological tests on friction and wear were carried out on Tribometer TRB3 (Anton-Paar, Buchs, Switzerland) using the standard method “ball-disk”. A 6.0 mm diameter ball made of 100Cr6 coated steel was used as a counterbody. The load was 10 N and the linear velocity was 3 cm/sec. The wear curvature radius was 2 mm and the friction path was 100 m.
Quantitative volume loss during wear was performed according to the following formulae:
Volume loss of the sample
∆V = S × l
where ΔV—volume loss of the sample, mm3; l—length of the groove mm; S—cross-sectional area of the wear groove, mm2.
The reduced wear I was calculated using the normalization of the test volume loss ΔV by the values of the run N (m) and the applied load P (H):
I = Δ V LP
  • I—reduced wear, mm3/(m·N);
  • L—mileage value, m;
  • P—applied load, N.
Corrosion resistance was investigated on a CS300 potentiostat galvanostat. The coatings were tested with an exposed area of 1 cm2 at room temperature (25 °C) in a 3.5 wt% NaCl solution. A three-electrode cell system was used in the experiment, where the silver chloride electrode served as a reference electrode and the platinum electrode served as an auxiliary electrode. Prior to each polarization experiment, the sample was exposed to the electrolyte for 60 min of immersion until a stable state of open circuit potential (OCP) was established. The corrosion potential and current density were obtained from the polarization curves using the Tafel extrapolation method for the four samples. The potential was scanned between −0.1 and 0.1 V relative to OCP at a scan rate of 0.5 mV/s. The tests were repeated three times for each sample, and the results were analyzed using CS Studio6 software (version 6.3).

3. Results and Discussion

Figure 1 shows plots of experimental and theoretical diffractograms of 86WC-10Co-4Cr metal–ceramic coatings obtained by the HVOF method by varying the powder fractions. Structural data from the ICDD PDF-2 database using the material analysis using diffraction (MAUD) full-profile analysis program (version 2.54) and the Rietveld method were used to construct the theoretical diffractograms. For the CoO phase, data from the inorganic crystal structure database (ICSD) were used. The detailed results are presented in Table 2.
Based on the structural data, a relative quantitative X-ray phase analysis of the coatings was conducted. The Rietveld method was used to refine the structural parameters of the individual phases. For this study, a full-profile refinement of the difference between the calculated intensity of the structural lines of each phase and their experimental values was applied. A high degree of correspondence between the calculated characteristics of the X-ray spectra and the experimental data was achieved (Figure 1). The phases identified in the X-ray spectra were: higher tungsten carbide (WC), lower tungsten carbide (W2C) and weak cobalt oxide (CoO) reflections. In all coatings, hexagonal phase WC reflexes are present and clearly expressed (31.51°, 35.65°, 48.30°, 64.03°, 65.78°, 73.12°, 75.49°, 77.12°, 84.08° and 98.74°). Small differences can be observed with respect to their intensity. For example, in the XRD of the A1 coating, the tungsten carbide WC phase reflections have a higher peak intensity and numerous W2C phase reflections compared to the other coatings. In addition to the reflexes of WC and W2C phases, the weak reflexes of cobalt oxide (CoO) can be detected. The formation of CoO oxide phase is explained by the use of an oxygen–propane mixture, which leads to more active interaction of tungsten carbide with oxygen. The main causes of decarburization during high-velocity oxygen-fuel spraying of 86WC-10Co-4Cr ceramic–metal coatings were discussed in the previously published work [2].
The X-ray diagram shows (Figure 1) that the phase composition of the coating depends on the fractions of the initial powder. The coating obtained from powder with particles of 15–20 μm fraction is more susceptible to decarburization and oxidation. This is due to the fact that part of the tungsten carbide (WC) dissolves in the metal matrix during the spraying process due to overheating. The overheating of small particles is due to the high temperature, which, in turn, leads to a more intense oxidation of the particles in the flight and decomposition of WC into W2C. Thus, it can be summarized that thermally activated phase reactions (such as decarburization reactions or carbide matrix reactions) increase significantly both with decreasing powder fractions. Other researchers have also reached the same conclusion [39,40,41,42,43]. This can be attributed to the higher particle temperature during gas–particle interaction, which arises from the higher surface to volume ratio of smaller particles and strongly increases the risk of overheating. Further, when increasing the fraction of sprayed particles from 20–30 μm (A2) to 30–40 μm (A3), there is a significant decrease in the intensity of the W2C phase, as well as its absence in some angles (62.26° and 34.73°), which, in turn, is characterized by reduced overheating. When further increasing the powder fractions up to 45 μm, the same manifestations as in the first coating (A1) begin to recur in the coatings, which may be due to the redistribution of the powder particles during the spraying process or to the coating structure reverting to a higher surface energy of the particles.
In the relative quantitative phase analysis using the Rietveld method, the lattice parameters were refined separately for each phase (Table 3). It was established with a high degree of reliability that in the coating 86WC-10Co-4Cr, the main compound is tungsten carbide (WC); its share was different for each coating. Table 3 shows the phase composition of the coatings analyzed by the MAUD full profile analysis program (version 2.54). The phase content of each coating and the phase lattice parameters were determined.
According to the analysis of phase composition (Table 3), conducted using the Rietveld method (Figure 1), in the coatings were the identified phases: WC, W2C and CoO. The content of WC phase was 45%, 83%, 82% and 42% in A1, A2, A3 and A4 coatings, respectively, and the content of the W2C phase was 18%, 6%, 9% and 12% for the same coatings. It is observed that the content of the CoO oxide phase in the A1 and A4 coatings is higher than the other A2 and A3 coatings. This indicates that these coatings (A1 and A4) were more exposed to oxidation. The low content of lower tungsten carbide W2C in A2 and A3 coatings indicates a low degree of WC carbide degradation to W2C, it can be hypothesized that increasing the powder fractions to 20–40 μm may result in lower particle surface energy, which may reduce the availability of carbon for decarburization reaction or reduce the number of active surfaces for the process. And so, the A2 and A3 coatings are characterized by reduced superheat and higher WC particle content, lower W2C lower carbide content. These results indicate differences in the melting characteristics of powders with different particle fractions.
Figure 2 shows the cross-sectional morphology of the coatings obtained with varying powder fractions: 15–20 μm (A1), 20–30 μm (A2), 30–40 μm (A3) and 40–45 μm (A4). All coatings adhere tightly to the substrate with no detectable cracks, failures or signs of delamination. For convenience, the analysis was carried out at different magnifications to get more detailed information about the coating structure and to evaluate its characteristics. As can be seen in Figure 2b,d,f,h, a lamellar structure was observed in all coatings. These lamellar structures occur cyclically and are distributed throughout the microstructure. They consist of thin, alternating band-like zones that have different characteristic morphologies. In all coatings, the lamellar structures consist of molten areas of the binder phase, where the WC phases dissolve in the matrix, resulting in the formation of brittle W2C and oxide CoO phases. This effect has already been observed by various researchers [44,45,46,47]. The morphology, shape and size of these lamellar were found to vary with the powder fractions. In Figure 2d,f,h, in all coatings, the inner part of the lamellar structure consists predominantly of coarser and angular, sharp-angled WC phases. In contrast, for the A1 coating, this effect is not observed. This is because A1 coatings generally contain only very fine tungsten carbide (WC) phases.
Microstructural features characterized by lamellar structure are directly related to the deposition process during spraying. During the HVOF spraying process, the starting powders are introduced into a high-temperature flame stream, forming semi-molten droplets. In these droplets, the WC particles remain solid, while the binding phase of Co-Cr becomes molten. Semi-molten droplets spread over the surface of the substrate, and the completely molten binder phase penetrates the gaps between the WC particles, ensuring their connection. As a result of this sequential spraying process, a coating with a lamellar structure is formed.
Consequently, the thicknesses of the coatings were 75 μm, 80 μm, 45 μm and 85 μm for samples A1, A2, A3 and A4, respectively. The porosity of the coatings was evaluated by SEM images using Altami Studio 4.0 image analysis software. All samples of coatings showed relative porosity not exceeding 3.6%. However, A2 and A3 coatings showed the lowest porosity values of 0.8% and 0.2% (Figure 2c,e), while A1 and A4 coatings (Figure 2a,g) exhibited porosity values of 3.6% and 2.7%, respectively. As shown by SEM analysis, the use of different ranges of powder particle fractions during HVOF spraying resulted in the formation of coatings with different structures, thicknesses and porosities. As can be seen from Figure 2a, the coatings have randomly distributed internal porosity (micro- and macropores), which may have resulted from overheating (partially) of the smallest powder particles. The smallest particles (15–20 µm) can heat up and cool down faster compared to larger particles due to their larger surface area. This can lead to inhomogeneous heating and cooling, which contributes to the formation of pores in the coating. Particles of intermediate fractions (20–30 μm and 30–40 μm) were less subjected to overheating compared to the finest particles and had a more uniform structure (Figure 2c,e). This is possibly due to the uniform distribution and melting of the powder during spraying. The coatings obtained using the largest (40–45 μm) of the powder fractions used had a less homogeneous and coarser structure due to the larger particle fraction size (Figure 2g). And, the porosity of the coatings was higher. This is because the relatively large spraying particles do not reach thermal equilibrium in flight as a result of the low thermal conductivity to the interior of the agglomerate. Consequently, only the surface of the particles is melted while the inner core remains in solid state. Due to the incomplete melting of the particles while passing through the HVOF flame, their deformability upon impact with the substrate surface is limited, so pores can easily occur. Scanning electron microscopy analysis of the cross-section of all coatings (Figure 2b,d,f,h) shows that the tungsten carbide (WC) particles are uniformly distributed in the matrix phase (CoCr). The matrix phase (CoCr) is probably represented by the dark gray color, while the lighter particles denote tungsten carbide (WC). The binding element Co is also uniformly distributed among the main WC phase. It is possible that the lower tungsten carbide W2C (light gray) may be dispersed in this cobalt binder as shown in Figure 2b (the same characteristics are repeated in the remaining Figure 2d,f,h). This assumption is consistent with numerous literatures that attribute the formation of W2C around WC to the decarburization process of WC during the HVOF process [48,49,50].
The results of the elemental mapping analysis of the cross-section of the coatings obtained with varying powder fractions (15–20 μm, 20–30 μm, 30–40 μm and 40–45 μm) are presented in Figure 3.
The elemental cross-sectional map of the 86WC-10Co-4Cr coating clearly shows the uniform distribution of W, Co and C elements as the main components of the coating. Moreover, the presence of partial oxidation during HVOF spraying can be judged by the significant presence of element O in the EDS maps. This is due to the use of an oxidizing environment of gas flame products, such as an oxygen–propane mixture, in the high-speed gas flame spraying process. This leads to a more active interaction of the WC phase with oxygen. As a result, partial degradation of the carbon occurs, resulting in the formation of excess carbon. This excess carbon released by the degradation of the WC phase penetrates the metal matrix and forms another carbide phase, W2C. Additionally, it is noted that the coating contains the oxide phase CoO.
Figure 4 demonstrates the elemental point analysis of 86WC-10Co-4Cr coatings, the total map spectrum and the total line-by-line spectrum. All the coatings were subjected to elemental point analysis in three different areas (Figure 3a–d) and the results are presented in Table 4. The point analysis data indicate that the elements W, C and Co were detected at every point, analyzed and were uniformly distributed throughout the coating. It was observed that the oxide content at each point studied in the A3 coating does not exceed the threshold of 0.8 (Table 4). Thus, it was found that the oxide content in coating A3 is lower than the other analyzed coatings (A1, A2, A4), which is also confirmed by the total map spectrum and the total spectrum along the line drawn across the cross-section of the coating.
From a general point of view, the point map analysis revealed the areas with an increased concentration of certain chemical elements in the analyzed coatings (A1, A2, A3, A4),: which were the highest concentrations of tungsten (W) and carbon (C).
Figure 5 demonstrates the distribution of elements along the coating cross-section line. Tungsten is uniformly distributed along the line drawn in all coatings (A1, A2, A3, A4). However, the A1 and A4 coatings show a higher oxide line in areas close to the center along the line (as shown in Figure 5a,d). Thus, no oxides were detected along the line drawn in the structure of coatings A2 and A3. However, in coating A3, compared to coating A2, there is a partial distribution of oxides in the regions where the pores are located closer to the substrate.
Studies of coating roughness showed that changing the fraction of the initial powder from 15–20 μm to 40–45 μm affects the roughness parameter expressed in the average values of Ra. The Ra values were 4 μm, 5.21 μm, 5.4 μm and 5.74 μm for samples A1, A2, A3 and A4, respectively. With increasing fractions of the initial powder, the coating surface acquired a more pronounced character characterized by high roughness.
The study of changes in the microhardness of coatings obtained with different powder fractions revealed significant variations. It was observed that the hardness of the sprayed coating depends on the fractions of the initial powder and pore distribution, as well as on the interaction between the molten particles. In this context, the hardness of the coatings correlates with the WC phase content and the degree of tight interconnection of the lamellar structures. A decrease in the WC phase fraction or the transition of WC to W2C was accompanied by a decrease in hardness. Since WC phases have high intrinsic hardness, their large amount in the coating structure leads to a corresponding high microhardness [51,52,53]. The observations in Figure 2d,f showed good interfacial bonding between the WC phases and the Co matrix, as well as dense bonding of individual plates (lamellar structure) in A2 and A3 coatings. This effect is generally attributed to the complete melting of the Co phase in the HVOF flame, which promotes a good wetting of the WC phase. An earlier study also noted that partial dissolution of WC in the Co matrix can increase the strength between WC and the bonding phase [54]. The smallest particles (15–20 μm) had a greater tendency to overheat, contributing to the decarburization process, while the coarse powder fractions (40–45 μm) did not reach sufficient temperature for complete melting, resulting in the formation of pores in the coating layers, as shown in Figure 2g. Consequently, increasing porosity and increasing powder fractions are accompanied by a decrease in hardness. Thus, successive increases in the initial powder size are accompanied by different changes in A1, A2, A3 and A4 coatings, presented as 572 ± 16 HV0.1, 780 ± 15 HV0.1, 735 ± 11 HV0.1 and 559 ± 13 HV0.1, respectively.
Figure 6a shows the dependence of the friction coefficient on the friction path, and Figure 6b shows the dependence of the wear volume on the powder fractions. It is established that at variation of fractions of initial powder, the average friction coefficient of coatings has the following values: µ = 0.455 for sample A1, µ = 0.488 for sample A2, µ = 0.502 for sample A3 and µ = 0.516 for sample A4. It was determined that the wear volume of the 86WC-10Co-4Cr metal–ceramic coating decreased with increasing WC carbide phase content in the coatings. The maximum wear resistance was characterized for A2 coating (0.0891 mm3). However, the difference in the wear volume between the A2 and A3 coatings was insignificant. For the other coatings, the wear volume was v = 0.09586 mm3 for sample A3, v = 0.10542 mm3 for sample A1 and v = 0.15954 mm3 for sample A4 (Figure 6a,b).
Figure 7 shows the wear marks on the pavement surface under a 10 N load. The edges of the wear marks are marked with dashed lines. No significant changes in the depth of the wear marks were observed when a 10 N load was applied. Instead, a slight increase in the width of the traces from 523 µm to 556 µm for samples A1 and A4 was observed. The increase in wear mark area indicates a decrease in wear resistance.
The corrosion resistance of 86WC-10Co-4Cr coatings obtained with four kinds of powder fractions was evaluated by testing with an exposed area of 1 cm2 at room temperature (25 °C) in 3.5 wt% NaCl solution. Three polarization tests were performed for each sample. The potentiodynamic polarization curves of the four kinds of 86WC-10Co-4Cr coatings deposited by HVOF method are shown in Figure 8. To obtain average values of electrochemical parameters, the Tafel region of both cathodic and anodic branches was extrapolated, and the point of intersection of these two lines was used to determine the corrosion current density (Icorr) and the corrosion potential (Ecorr). The detailed results are presented in Table 5.
In Table 5, βa and βc are the slopes of the anodic and cathodic branches, respectively. According to Table 5, the corrosion current densities for coatings A2 and A3 were similar and showed lower corrosion current densities compared to the other coatings (A1 and A4), indicating their improved corrosion resistance. Additionally, coating A1 also showed itself to be relatively corrosion resistant. In contrast, coating A4 exhibited the highest corrosion current density among all the coatings investigated, indicating its low corrosion resistance.
The corrosion resistance can also be attributed to various changes that occur in the powders during the spraying process. For example, fine powder fractions (up to 20 µm) are easily susceptible to overheating, which can lead to the decomposition of WC to W2C. The W2C phase has a higher chemical activity than WC (tungsten carbide), making it more susceptible to corrosion in various environments. Consequently, the fraction of retained WC in the coating will be low, resulting in lower corrosion resistance. In contrast, the medium-sized powder fractions (20 to 40 μm) do not suffer from significant overheating, which contributes to the high retention of the WC phase in the material. As a result, the coating has a higher corrosion resistance. However, if the powder fractions are too large (up to 45 μm), a porous material structure may occur during spraying due to insufficient heating. The high porosity of the material can increase its surface area, which, in turn, increases the probability of corrosive media penetration and therefore leads to a deterioration of its corrosion resistance.

4. Conclusions

The following conclusions can be drawn from the study:
(1)
It was found that varying the fractions of the initial powder formed coatings with lamellar structure, where in coatings obtained from powder with particles of fractions 20–30 µm and 30–40 µm, the structure was more homogeneous;
(2)
It was determined that the relative porosity of all coating samples did not exceed 3.6%. It was observed that the minimum values of porosity (0.8% and 0.2%) were observed in coatings obtained from powder with particle fractions of 20–30 μm and 30–40 μm, respectively;
(3)
It was found that with increasing fractions of the initial powder, the thickness of the metal–ceramic coating changed by jumps. The thickness of coatings varied from 45 μm to 85 μm.
(4)
According to the relative quantitative X-ray diffraction analysis, it was found that in coatings obtained from powder with particle fractions of 20–30 μm and 30–40 μm, the proportion of CoO and W2C phases have the lowest values;
(5)
On the basis of investigations, it was found that the maximum microhardness (780 HV0.1) was characteristic of the coating obtained from powder with particles of 20–30 μm fraction, which is due to an increase in the content of WC carbide phase. The high conservation of WC phase led to an increase in the corrosion resistance of coatings obtained from powder with particles of 20–30 µm and 30–40 µm fractions, which was also explained by the low corrosion current density;
(6)
Coatings obtained from powder with particles of 20–30 µm fraction were characterized by maximum wear resistance (wear volume 0.0891 mm3), while the coating obtained from powder with particles of 40–45 µm fraction showed minimum wear resistance (v = 0.15954 mm3);
(7)
It was determined that with increasing fractions of initial powder the surface of coatings acquires more pronounced roughness.

Author Contributions

B.R. and N.M. (Nazerke Muktanova) formal analysis; supervision; writing—review and editing; D.K. and Z.S. investigation; methodology; L.K. and N.M. (Nurtoleu Magazov) resources; data curation. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the Committee of Science of the Ministry of Science and Higher Education of the Republic of Kazakhstan (grant No. AP14870977).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data were contained within the article.

Conflicts of Interest

Author Nazerke Muktanova, Dauir Kakimzhanov, and Zarina Satbayeva were employed by the company PlasmaScience LLP. 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.

References

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Coatings 14 00651 g001aCoatings 14 00651 g001b
Figure 1. Diffraction pattern of 86WC-10Co-4Cr metal–ceramic coatings when varying the Rietveld powder fractions using the MAUD program: (a) 15–20 μm (A1); (b) 20–30 μm (A2); (c) 30–40 μm (A3) and (d) 40–45 μm (A4). (line on round mark (O)—experimental diffractogram; red line (−−−)—calculated, blue line (−−−)—difference between experimental and calculated diffractograms, vertical line (│)—Bragg position).
Figure 1. Diffraction pattern of 86WC-10Co-4Cr metal–ceramic coatings when varying the Rietveld powder fractions using the MAUD program: (a) 15–20 μm (A1); (b) 20–30 μm (A2); (c) 30–40 μm (A3) and (d) 40–45 μm (A4). (line on round mark (O)—experimental diffractogram; red line (−−−)—calculated, blue line (−−−)—difference between experimental and calculated diffractograms, vertical line (│)—Bragg position).
Coatings 14 00651 g001aCoatings 14 00651 g001b
Coatings 14 00651 g002aCoatings 14 00651 g002b
Figure 2. SEM images of cross-sectional morphology of 86WC-10Co-4Cr metal–ceramic coatings obtained with varying powder fractions: (a,b) 15–20 μm (A1); (c,d) 20–30 μm (A2); (e,f) 30–40 μm (A3); (g,h) 40–45 μm (A4).
Figure 2. SEM images of cross-sectional morphology of 86WC-10Co-4Cr metal–ceramic coatings obtained with varying powder fractions: (a,b) 15–20 μm (A1); (c,d) 20–30 μm (A2); (e,f) 30–40 μm (A3); (g,h) 40–45 μm (A4).
Coatings 14 00651 g002aCoatings 14 00651 g002b
Coatings 14 00651 g003aCoatings 14 00651 g003b
Figure 3. Maps of elemental distribution of sprayed elements in the analyzed 86WC-10Co-4Cr coating area. (a) 15–20 µm; (b) 20–30 µm; (c) 30–40 µm; (d) 40–45 µm.
Figure 3. Maps of elemental distribution of sprayed elements in the analyzed 86WC-10Co-4Cr coating area. (a) 15–20 µm; (b) 20–30 µm; (c) 30–40 µm; (d) 40–45 µm.
Coatings 14 00651 g003aCoatings 14 00651 g003b
Coatings 14 00651 g004aCoatings 14 00651 g004bCoatings 14 00651 g004c
Figure 4. Elemental spot analysis of 86WC-10Co-4Cr coatings. (a) 15–20 μm (A1); (b) 20–30 μm (A2); (c) 30–40 μm (A3); (d) 40–45 μm (A4).
Figure 4. Elemental spot analysis of 86WC-10Co-4Cr coatings. (a) 15–20 μm (A1); (b) 20–30 μm (A2); (c) 30–40 μm (A3); (d) 40–45 μm (A4).
Coatings 14 00651 g004aCoatings 14 00651 g004bCoatings 14 00651 g004c
Coatings 14 00651 g005
Figure 5. Distribution of elements along the cross-sectional line of 86WC-10Co-4Cr metal–ceramic coating. (a) 15–20 µm (A1); (b) 20–30 µm (A2); (c) 30–40 µm (A3); (d) 40–45 µm (A4).
Figure 5. Distribution of elements along the cross-sectional line of 86WC-10Co-4Cr metal–ceramic coating. (a) 15–20 µm (A1); (b) 20–30 µm (A2); (c) 30–40 µm (A3); (d) 40–45 µm (A4).
Coatings 14 00651 g005
Coatings 14 00651 g006
Figure 6. Graph of dependence of friction coefficient on friction path (a) and dependence of wear volume on initial powder fractions (b).
Figure 6. Graph of dependence of friction coefficient on friction path (a) and dependence of wear volume on initial powder fractions (b).
Coatings 14 00651 g006
Coatings 14 00651 g007
Figure 7. Microphotographs of wear traces of 86WC-10Co-4Cr coatings. (a) 15–20 µm (A1); (b) 20–30 µm (A2); (c) 30–40 µm (A3); (d) 40–45 µm (A4).
Figure 7. Microphotographs of wear traces of 86WC-10Co-4Cr coatings. (a) 15–20 µm (A1); (b) 20–30 µm (A2); (c) 30–40 µm (A3); (d) 40–45 µm (A4).
Coatings 14 00651 g007
Coatings 14 00651 g008
Figure 8. Potentiodynamic polarization curves of 86WC-10Co-4Cr coatings.
Figure 8. Potentiodynamic polarization curves of 86WC-10Co-4Cr coatings.
Coatings 14 00651 g008
Table 1. Spraying regimes for 86WC-10Co-4Cr—coatings.
Table 1. Spraying regimes for 86WC-10Co-4Cr—coatings.
Powder Code ExampleA1A2A3A4
Fraction size, µm15–2020–3030–4540–45
Parameter modesOptimal values
Propane pressure2.9 bar
Oxygen pressure5 bar
Compressed air pressure3.2 bar
Table 2. Results of the database.
Table 2. Results of the database.
PhaseCrystal LatticeCardboardSpatial Group
WCHexagonal00-025-1047P-6m2
W2CHexagonal00-035-0776P-3m1
CoOCubicICSD 245320Fm3m
Table 3. Results of X-ray phase analysis.
Table 3. Results of X-ray phase analysis.
SamplePhasesPhase Content, Wt.%Lattice Parameters, (Å)
A1 CoatingWC45a = 2.9029; c = 2.8335
W2C18a = 2.9772; c = 4.7051
CoO37a = 4.2502
A2 CoatingWC83a = 2.9007; c = 2.8289
W2C6a = 2.9736; c = 4.6100.
CoO6a = 4.2345
A3 CoatingWC82a = 2.9033; c = 2.8268
W2C9a = 2.9717; c = 4.6708
CoO8a = 4.2723
A4 CoatingWC42a = 2.9315; c = 2.8605
W2C10a = 2.9697; c = 4.7161
CoO48a = 4.3255
Table 4. Results of point analysis noted in Figure 4.
Table 4. Results of point analysis noted in Figure 4.
Coatings A1 (wt%)WCFeOCoCrCoatings A2 (wt%)WCFeOCoCr
Spectrum 176.113.6 0.98.50.9Spectrum 173.416.3 0.67.32.4
Spectrum 267.313.8 0.89.09.1Spectrum 238.412.6 0.538.99.6
Spectrum 380.512.5 0.95.20.8Spectrum 372.014.1 0.79.93.4
Spectrum 4 9.280.30.9 9.7Spectrum 4 11.677.71.0 9.6
Spectrum 5 8.681.00.8 9.5Spectrum 5 12.377.00.7 9.9
Coatings A3 (wt%)WCFeOCoCrCoatings A4 (wt%)WCFeOCoCr
Spectrum 182.514.0 0.71.50.4Spectrum 168.913.3 0.714.13.0
Spectrum 274.212.3 0.49.22.8Spectrum 281.112.7 0.85.3
Spectrum 382.113.1 0.82.91.1Spectrum 374.413.0 6.23.31.5
Spectrum 4 8.979.10.5 11.5Spectrum 4 9.078.10.7 12.2
Spectrum 5 9.981.20.8 8.2Spectrum 5 10.078.70.610.7
Table 5. Results of corrosion tests.
Table 5. Results of corrosion tests.
Coatings15–20 μm (A1)20–30 μm (A2)30–40 μm (A3)40–45 μm (A4)
Ecorr (mV)−457−405−367−429
Icorr (A/cm2)2.01 × 10−51.88 × 10−51.90 × 10−53.6 × 10−5
βc (mV)23518117778
βa (mV)133129112122
rcorr (mm/y)0.240.220.2220.42
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Rakhadilov, B.; Muktanova, N.; Kakimzhanov, D.; Satbayeva, Z.; Kassenova, L.; Magazov, N. Investigation of the Influence of Powder Fraction on Tribological and Corrosion Characteristics of 86WC-10Co-4Cr Coating Obtained by HVOF Method. Coatings 2024, 14, 651. https://doi.org/10.3390/coatings14060651

AMA Style

Rakhadilov B, Muktanova N, Kakimzhanov D, Satbayeva Z, Kassenova L, Magazov N. Investigation of the Influence of Powder Fraction on Tribological and Corrosion Characteristics of 86WC-10Co-4Cr Coating Obtained by HVOF Method. Coatings. 2024; 14(6):651. https://doi.org/10.3390/coatings14060651

Chicago/Turabian Style

Rakhadilov, Bauyrzhan, Nazerke Muktanova, Dauir Kakimzhanov, Zarina Satbayeva, Leila Kassenova, and Nurtoleu Magazov. 2024. "Investigation of the Influence of Powder Fraction on Tribological and Corrosion Characteristics of 86WC-10Co-4Cr Coating Obtained by HVOF Method" Coatings 14, no. 6: 651. https://doi.org/10.3390/coatings14060651

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