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Article

Influence of Air Pressure on the Microstructure, Phase Composition, and Tribomechanical Performance of Thin ZrCN Coatings Deposited via HVOF Spraying

by
Sherzod Kurbanbekov
1,
Zhamila Suierkulova
2,
Gaukhar Omashova
2,
Berik Kaldar
1,*,
Alisher Temirbekov
1,
Sardor Kambarbekov
1,
Nurdaulet Shektibayev
1 and
Dilnoza Baltabayeva
1
1
Research Institute of Natural Sciences, Nanotechnology and New Materials, Khoja Akhmet Yassawi International Kazakh-Turkish University, Turkestan 161200, Kazakhstan
2
Department of Physics, Mukhtar Auezov South Kazakhstan Research University, Shymkent 160012, Kazakhstan
*
Author to whom correspondence should be addressed.
Crystals 2025, 15(9), 762; https://doi.org/10.3390/cryst15090762
Submission received: 6 August 2025 / Revised: 19 August 2025 / Accepted: 26 August 2025 / Published: 27 August 2025
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Abstract

The development of thin, wear-resistant coatings is a relevant area in the field of surface engineering, especially given the increasing demand for resource efficiency and reliability of machine elements. In this study, we investigate the structural and phase composition, tribological characteristics, and physical and mechanical properties of zirconium carbonitride (ZrCN) coatings deposited by high-velocity oxygen-fuel spraying (HVOF) on U8G carbon steel substrates. Particular attention is paid to the influence of spraying parameters, in particular air pressure, on the formation of coatings and their performance properties. X-ray phase analysis methods revealed the formation of Zr2CN, ZrC, ZrN, ZrO2, and Fe3O4 phases, with the dominance of the cubic phase ZrN(C) with a lattice parameter of a = 4.6360 Å. Tribological tests have shown that at an air pressure of 0.38 MPa, the minimum friction coefficient is achieved, presumably due to the formation of an amorphous CNx phase with a self-lubricating effect. The wear mechanism is predominantly abrasive in nature; the width of wear tracks is 329–759 μm. The coatings demonstrate a significant increase in microhardness—up to 1512–1857 HV, which is 4–4.5 times higher than the substrate. The results of adhesion tests carried out in accordance with ASTM D4541-22 showed a maximum adhesion strength of 14.56 MPa. The results obtained confirm the high efficiency of thin ZrCN coatings obtained by the HVOF method as a promising solution for protecting metal surfaces subject to intense wear in tribological systems.

1. Introduction

As the cutting industry continually demands enhanced wear resistance of coatings, ongoing research is being conducted to discover new materials that can deliver improved coating performance. Zirconium carbide (ZrC) is a hard crystalline compound with a high melting point (approximately 3530 °C) and significant microhardness. Due to these properties, ZrC is used as a coating material for cutting tools to enhance wear resistance and extend service life [1,2]. In addition, zirconium carbide is employed as an abrasive material for metal polishing [3]. Coatings based on transition metal carbonitrides have attracted considerable attention due to their excellent mechanical strength and corrosion resistance, which make them promising for various applications, including in biomedical devices [4,5]. Among the wide family of transition metal carbonitride-based materials, ZrCN, TiCN, VN, and NbCN coatings have been proven to exhibit superior overall performance. Therefore, the favorable combination of mechanical and corrosion-resistant properties has made ZrCN coatings widely used for coated cemented carbide tools and as high-temperature protective coatings [6,7,8,9,10].
Currently, various spraying techniques are employed to improve the mechanical properties of cutting disks, including Atmospheric Plasma Spraying (APS) [11], Vacuum Plasma Spraying (VPS) [12], Detonation Spraying [13,14], Arc Spraying [15], Flame Spraying [16], plasma electrolyte hardening [17], and High-Velocity Oxy-Fuel (HVOF) spraying [18,19,20]. Among these thermal spraying techniques, HVOF spraying is distinguished by its high flame velocity (~1500 m/s) and flame temperature (~3000 °C) [21,22]. These characteristics allow the powder particles to be accelerated and deposited onto the substrate with high kinetic energy and rapid cooling rates during the spraying process. This results in dense coatings with minimal oxidation. Therefore, the HVOF method is capable of producing coatings with excellent properties [23].
HVOF spraying is a modern thermal spray technology that enables the deposition of hard, dense, and low-porosity coatings due to its higher spraying speeds and temperatures. It can be used for applying coatings to a wide variety of materials [24,25]. The results obtained align with the goals of sustainable development, particularly with the objective “Industry, Innovation, and Infrastructure,” as they contribute to the development and implementation of new wear-resistant coatings that improve the durability and reliability of equipment across various industries.
This work presents a comprehensive investigation into the deposition of ZrCN coatings via the HVOF spraying process, with a particular emphasis on elucidating the role of spraying air pressure in governing microstructural evolution, phase constitution, and functional performance. The study aims to establish robust quantitative correlations between the processing parameters and the resulting mechanical and tribological responses, including microhardness, adhesion strength to U8G steel, friction coefficient, and dry sliding wear mechanisms. The findings offer critical insights into the underlying structure–property relationships, thereby providing a scientific framework for the rational optimization of HVOF spraying conditions to engineer advanced ZrCN coatings capable of withstanding the stringent demands of complex engineering environments.

2. Materials and Methods

Cutting-grade U8G steel (15 × 45 × 3 mm3) was used as the substrate. Prior to spraying, the substrate was sandblasted using white alumina grit, which contributed to improved coating adhesion (Table 1).
Zirconium carbide (ZrC) and zirconium nitride (ZrN) powders were purchased from Hebei Suoyi New Material Technology Co., Ltd. (Handan, China). The zirconium carbonitride (ZrCN) powder was synthesized in-house in accordance with the applicable Technical Specifications (TU). The surface morphology of the powders is shown in Figure 1a. The powders exhibit an irregular polyhedral fragmented morphology, which is known to be less flowable than spherical particles. Particle size analysis revealed sizes ranging from 10 to 100 μm, with the following parameters: d10 = 15 μm, d50 = 45 μm, and d90 = 85 μm. Figure 1b shows the X-ray diffraction (XRD) pattern of the initial ZrCN powder selected for spraying. The presence of a ZrCN peak alongside those of ZrC and ZrN indicates partial retention of the original zirconium carbonitride [26], as well as its decomposition into zirconium nitride. At the same time, the main diffraction lines of this phase correspond in position to the shifted peaks of (ZrC) and (ZrN)-type phases with cubic lattice parameters.
The coatings were deposited using a Termika-3 HVOF spraying system. The experiments were conducted by varying the air pressure from 0.24 to 0.38 MPa while keeping other parameters constant, such as spray distance (350–400 mm), fuel gas pressure (0.17 MPa), and oxygen pressure (0.28 MPa). This approach allowed the production of diverse samples with different structural and mechanical characteristics. The appearance and schematic diagram of the high-velocity HVOF spraying system are presented in our previous work [20]. The deposition parameters for ZrCN coatings are summarized in Table 2.
The phase composition of the coatings was determined using X-ray diffraction (XRD) with an XPERT-PRO Panalytics diffractometer (Philips Corporation, Amsterdam, Netherlands) equipped with a copper (Cu) anode and a Kα radiation source (λ = 1.5406 Å). The measurements were carried out at a temperature of 25 °C, with an operating voltage of 40 kV and a current of 30 mA. The scanning angle ranged from 20.01° to 89.99°, with a step size of 0.02° and a data acquisition time of 2 s per step. The diffraction patterns were analyzed using HighScore Plus software version 5.3a package.
The microstructure and elemental composition of the coatings were examined using a scanning electron microscope (SEM, Tescan Vega 4, Brno, Czech Republic).
Tribological sliding wear tests were conducted using a TRB3 tribometer (Anton Paar, Buchs, Switzerland) in a ball-on-disk configuration under the following conditions: normal load N = 5 N, sliding speed υ = 2 cm/s, ball diameter = 6 mm (material: 100Cr6), sliding distance L = 30 m, and dry friction environment. The wear rate of the samples and counterbody was quantified according to the ASTM G99 standard [27].
Microhardness measurements were performed using the Vickers method in accordance with ASTM E384 on an HLV-1DT microhardness tester. A diamond tetrahedral pyramid indenter with an angle of 136° was used. The notation “HV 0.5” refers to a Vickers hardness test performed under a load of 0.5 kgf (4.9 N) with a dwell time of 10 s. The load was applied to the sample surface, and the diagonals of the indentation traces (d1 and d2) were measured with high accuracy.
One of the important aspects of ensuring coating quality is the adhesion to the substrate. To improve the adhesive bond, the surface of the coating at the “dolly” attachment site was roughened with sandpaper and degreased with ethyl alcohol. Adhesion tests using the pull-off method were conducted in accordance with ASTM D4541-22 at a temperature of 20 ± 5 °C, no earlier than three days after coating deposition. The adhesive was applied following the manufacturer’s instructions. “Epoxy Adhesive 2214” was evenly spread on the dolly surface, which was then pressed onto the coating and held until curing, ensuring proper alignment of the bonded surfaces. Excess adhesive was removed if necessary. Using a cutting tool (ring cutter), the coating was cut down to the metal around the dolly. Cuts were made through the entire coating thickness until the metal substrate was exposed, with a minimum cut width of 1 mm.
To evaluate the adhesion properties of the coatings, tests were performed in accordance with ASTM D4541-22 using a hydraulic adhesion tester, Elcometer 510 (Elcometer Instruments, Manchester, UK). The adhesive strength of the coatings was measured under the following conditions: dwell time 0.50 s, target loading rate 1.00 MPa/s, and dolly diameter of 20 mm.

3. Results

Figure 2 and Figure 3 present the X-ray diffraction (XRD) patterns of the ZrCN-based coatings obtained under different deposition conditions, along with a quantitative comparison of phase composition visualized in the form of a bar chart. Phase analysis revealed the presence of the following components: Zr2CN, ZrC, ZrN, ZrO2, and Fe3O4. The diffraction analysis confirmed the formation of a typical cubic crystal structure of ZrN(C), which is characteristic of many transition metal carbides and nitrides [28]. The Zr2CN lattice was identified as a cubic system with a space group Fm-3m and a lattice parameter of a = 4.6360 Å.
To further evaluate the phase composition of the coatings, quantitative X-ray phase analysis was conducted using the Rietveld refinement method. The results are summarized in Table 3 and visualized in the bar chart (Figure 2) for five samples (Samples a–e) obtained under different spraying conditions. The highest content of the Zr2CN phase was observed in Sample e (27.9%), indicating the most effective stabilization of the original carbonitride phase under these conditions. Sample a also showed a relatively high Zr2CN content (25.4%), whereas Samples b and d exhibited the lowest concentrations (13.2% and 14.5%, respectively).
The maximum content of ZrN was found in Sample e (17.2%), and ZrC was also highest in this sample (16.3%), suggesting partial decomposition of the initial Zr2CN phase and redistribution of its constituents. The oxide phase ZrO2, ranged from 13.5% (Sample e) to 21.9% (Sample b), likely reflecting differences in oxidation levels of zirconium-containing phases due to variations in air pressure and thermal input during spraying. The iron phase (Fe), originating from the steel substrate, constituted a substantial portion of the total phase composition, ranging from 25.1% (Sample e) to 41.2% (Sample b).
Overall, the combined analysis of XRD patterns (Figure 2) and quantitative phase distribution (Figure 3) confirmed that the phase composition of the coatings is highly sensitive to the deposition parameters. Sample e demonstrated the most balanced content of the carbonitride matrix and the lowest concentrations of oxides and iron, indicating formation under near-optimal spraying conditions.
Furthermore, comparison of the diffraction patterns shows that the main diffraction peaks of ZrCN shift toward higher 2θ angles as the air pressure increases from Sample a to Sample e. This shift is interpreted as a result of lattice parameter reduction due to a decrease in nitrogen content in the solid solution ZrC1-xNx and an enrichment of the carbide component. Carbon atoms have a smaller atomic radius compared to nitrogen, which leads to a densification of the crystal lattice and a reduction in interplanar spacing, manifesting as a peak shift to the right. This behavior has been confirmed in previous studies [29,30], where a similar XRD peak shift was observed during the transition from a nitride to a carbide structure in ZrCN systems.
As reported in [29], ZrCN provides excellent resistance to corrosion and abrasive wear and is distinguished by its high hardness, tribological performance, and mechanical strength. The formation of ZrC and ZrN phases is associated with the thermal decomposition of the ZrCN powder. In [31], zirconium carbide is used as an abrasive component in lapping and polishing pastes for ferrous and non-ferrous metal parts. In addition to its considerable hardness, its relatively high thermal conductivity is advantageous, reducing the risk of thermal burns during processing.
Many transition metal nitrides, such as ZrN, crystallize in a sodium chloride-type structure, where first-order combinational scattering is forbidden in a perfect crystal. However, it is known that thermally sprayed coatings often contain vacancies, which distort the structure [32]. As a result, the Raman spectrum consists of broadened bands due to disorder and second-order processes.
According to [33], zirconium nitride possesses a high melting point, excellent electrical and thermal conductivity, high reflectivity in the visible and near-infrared ranges, chemical resistance in acidic and alkaline media, high hardness, wear resistance, and superior mechanical properties. It is known [34] that zirconium oxide mainly exists in various phases: tetragonal ZrO2, monoclinic ZrO2, and cubic ZrO2. Pure tetragonal ZrO2 (t-ZrO2) typically forms in the presence of yttrium oxide (Y2O3) in the coating [35]. HVOF spraying promotes the formation of a mixture of cubic (c-ZrO2) and tetragonal (t-ZrO2) phases, but rapid cooling favors the retention of more cubic phase.
The formation of the ZrO oxide phase is explained by the use of an oxygen-propane mixture as an oxidizing medium during high-velocity oxy-fuel spraying. This resulted in active interaction between ZrCN and oxygen, with partial carbon loss. Excess carbon released during the decomposition of ZrCN diffused into the metallic matrix, promoting the formation of carbide (ZrC) and oxide (ZrO) phases.
In addition, a Fe3O4 phase with a cubic crystal structure was identified at a 2θ angle of 39.36°. The determination of crystallite size is an important parameter in coating analysis, as it influences mechanical properties. The presence of the Fe3O4 phase in the coatings is due to the oxidation of iron occurring during high-temperature spraying using the HVOF method. The source of iron is the steel substrate (the steel grade used in the work), the atoms of which can diffuse into the coating zone during local heating. Additionally, the formation of Fe3O4 is possible due to the inclusion of iron particles separated from the surface of the substrate during mechanical preparation and spraying, with their subsequent oxidation in the presence of flame oxygen. One of the most commonly used methods for estimating crystallite size is the Scherrer equation, which is based on the broadening of X-ray diffraction peaks. In this study, XRD analysis was employed to calculate the average crystallite size of ZrCN-based coatings. According to [25], peak broadening is related to crystallite size effects, with smaller grain sizes resulting in broader peaks. The average crystallite sizes for the coating phases were as follows: Zr2CN ~75.5 nm, ZrC ~56.9 nm, ZrN ~73.8 nm, ZrO2 ~65.8 nm, and Fe2O3 ~48.5 nm.
Figure 4 shows SEM images of the surface morphology of ZrCN coatings obtained by the HVOF method. All coatings exhibit pronounced surface roughness and the presence of deep pores, while no cracks are observed.
The polyhedral and fragmented shape of the powder can influence the HVOF spraying process on several levels. First, it may reduce the powder flowability, potentially causing uneven feeding into the gun and the formation of agglomerates, which can lead to instabilities in the spraying process, particularly at high feed rates. Second, the complex particle geometry can result in non-uniform heating, which in some cases may lead to partial melting. The enlarged specific surface area of the particles also increases their susceptibility to oxidation during spraying. Third, this morphology can affect the coating formation: sharp particle edges may enhance mechanical adhesion but can also contribute to a rougher surface and the possible formation of microcracks. Variations in particle size may further influence the microstructure of the deposits, especially for harder particles.
The short residence time of particles in the HVOF flame and the low thermal conductivity toward the core of the agglomerates mean that larger particles may not reach thermal equilibrium in flight. As a result, their deformability upon impact with the substrate may be limited, which could explain the microstructural inhomogeneity observed in the mixed layer. However, this phenomenon does not occur in the other two layers, where the coating structure is more uniform [36].
Figure 5 illustrates the dependence of the coefficient of friction on the sliding distance. The choice of 100Cr6 is motivated by its status as a standard-bearing material and its broad use in real engineering applications (rolling bearings of motors, pumps, compressors; precision machine-tool spindles; gearbox supports), which ensures relevance and comparability of the tribological data [37,38]. The coating obtained at an air pressure of 0.38 MPa exhibits the longest running-in phase, whereas the coating deposited at 0.28 MPa shows the shortest one. The remaining coatings display more similar running-in durations, indicating that air pressure and powder size influence the tribological performance of the samples.
Typically, wear-resistant coatings are designed to minimize the running-in period and ensure stable performance with low fluctuations. Variations in behavior may result from interfacial delamination between the coatings, as well as from surface preparation procedures, including cleaning or roughening. For accurate analysis of the tribological behavior, the process was divided into three stages: Stage 1—the initial phase up to 10 m (marked in red), Stage 2—the intermediate phase from 10 to 20 m (marked in green), and Stage 3—the final phase from 20 to 30 m (marked in blue).
In Figure 5, for Sample a, during the initial stage, the coefficient of friction increases uniformly from ~0 to ~0.7803 ± 0.012, corresponding to the wear of the top ZrCN coating layer. During this stage, the coating effectively prevents contact between the substrate and the counter-body, ensuring surface adaptation to the applied load. In the intermediate phase, the coefficient of friction stabilizes at ~0.7803–0.7600 ± 0.010 with minimal fluctuations, indicating partial wear of the coating and partial load transfer to the steel substrate (U8G). The coating continues to provide friction stability and substrate protection. In the final stage, the coefficient of friction remains stable (~0.8 ± 0.015) with slight variations, suggesting complete wear of the coating and full load transfer to the substrate.
For Sample b, the coefficient of friction increases uniformly from ~0 to ~0.9338 ± 0.018 in the initial phase, stabilizes at ~0.9338–0.8988 ± 0.014 during the intermediate phase, and remains stable (~0.9 ± 0.016) with minor fluctuations in the final phase. For Sample c, the initial phase shows a gradual increase in the coefficient of friction from ~0 to ~0.8558 ± 0.013, stabilizing at ~0.8558–0.8384 ± 0.011 during the intermediate phase, and remaining steady (~0.8 ± 0.014) with minimal variation in the final phase.
Unlike the other coatings, the ZrCN coating obtained at 0.38 MPa during the running-in phase demonstrates a significantly lower coefficient of friction (~0.2), indicating low friction and moderate wear. The longer running-in period observed for this sample is possibly attributed to the possible presence of an amorphous CNx phase, which is known as a lubricating component and, as detailed in relevant literature, substantially reduces friction. CNx is recognized as a solid lubricant that can significantly influence the friction and wear properties of coatings [39,40,41].
Additionally, SEM images of wear tracks on the coating surfaces are presented in Figure 6. The analysis of the wear zone morphology revealed a clearly defined worn area ranging from 329 to 759 µm in width, exhibiting characteristic parallel grooves indicative of an abrasive wear mechanism. WI spans approximately 0.16–1.90 mm3/(N·m). An increase in the wear track area corresponds to a decrease in wear resistance. Microstructural analysis of the worn surfaces provides direct evidence of differences in the wear mechanisms among Samples a–e.
The wear tracks captured in the images reveal the width of the worn stripe and the nature of the coating damage after the test. For instance, Sample e, deposited at high spraying pressure, which exhibited the best wear resistance, showed narrow wear tracks—significantly narrower than those of the other samples—indicating a smaller volume of material removal. The surface within the wear track for these samples appears relatively smooth, with no large pull-outs, and is partially covered by a thin tribofilm formed from wear products.
The absence of major delamination or cracking suggests that wear primarily occurred through gradual abrasion of the top layer, assisted by the presence of a lubricating film, which is consistent with the low coefficient of friction observed at the later stages of sliding. In contrast, SEM images of samples with higher coefficients of friction (e.g., Samples a and b, deposited at lower pressures) show significantly wider wear tracks. The broad wear scars indicate greater material loss and poorer wear resistance. Within these tracks, localized coating delamination and micro-fractures were observed.
Figure 7 X shows the average microhardness values of ZrCN coatings obtained by the HVOF method at different air flow rates. The microhardness of the coatings increased by 4 to 4.5 times compared to the U8G substrate. The initial microhardness values of the substrate range from 392 to 418 HV, with an average value of 402.5 HV, with a standard deviation of ±11.12 HV, which corresponds to the typical characteristics of hardened U8G steel. The average microhardness of the first coating is 1512 HV, the second sample is 1790 HV, the third coating is 1704 HV, the fourth coating is 1627 HV, and the fifth coating is 1857 HV. The overall average microhardness of the coatings is 1698 HV, with a standard deviation of ±135.48 HV, indicating a high level of uniformity across the tested samples. As mentioned earlier, the increase in hardness is due to the phase composition of the ZrCN powder, particularly the formation of hard phases such as Zr2CN, ZrC, and ZrN.
For adhesion measurement, the dolly was placed in a special adhesion tester device. The tester’s gripping mechanism engaged the dolly, and a normal tensile force was applied by pressing the handle. The applied force was recorded on the device scale.
It should be noted that deviations from the operational conditions of coating application (e.g., climatic factors), insufficient surface preparation, and other technological inconsistencies lead not only to reduced effectiveness and reliability but also to coating failure.
The highest adhesion value was recorded for Sample e—14.56 MPa ± 0.76 at the maximum air pressure of 0.38 MPa, indicating optimal conditions for forming a dense, cohesively strong coating (Table 4). Sample a, deposited at the lowest air pressure of 0.24 MPa, also showed a comparatively high adhesion strength of 9.77 MPa ± 0.63, demonstrating sufficient particle kinetic energy to form a reliable bond despite a less pronounced flow dynamic. Sample d, formed at an intermediate air pressure of MPa, showed an adhesion strength of 7.49 MPa ± 0.54, which is an acceptable level for most engineering applications. However, this result is inferior to Samples a and e, emphasizing the need for precise adjustment of air pressure to achieve maximum coating performance.
Samples c and b, sprayed at air pressures of 0.28 and 0.26 MPa, respectively, demonstrated the lowest adhesion strengths of 5.41 MPa ± 0.44 and 4.24 MPa ± 0.42. Notably, Sample b exhibited the minimum value in the series despite having relatively close parameters to the samples with satisfactory adhesion. This reduction in adhesion strength may be attributed to a combination of factors, including natural microstructural variations in the substrate and coating, local inhomogeneities in the sprayed layer structure, as well as possible differences in the microcharacteristics of the adhesive joint.
The presented SEM images depict the microstructure of a ZrCN coating deposited via HVOF spraying. The coating exhibits a non-dense lamellar structure, which deviates from the typical morphology of thermally sprayed materials. It consists of both fully and partially molten particles that have flattened upon impact with the substrate. Rounded inclusions observed within the matrix likely correspond to incompletely molten particles embedded in the coating. The surface morphology is notably rough, with well-defined splat boundaries. Isolated pores and microcracks are observed, primarily located along inter-splat boundaries, which is characteristic of layered structures formed by rapid solidification (Figure 8).
The interface between the coating and the steel substrate is clearly delineated. The coating thickness varies from 3 to 12.5 μm. For individual measurements, the thickness values were 7.0 ± 3.0 μm (Sample a), 3.0 ± 0.5 μm (Sample b, thin region), 11.7 ± 0.5 μm (Sample c), 12.3 ± 0.2 μm (Sample d), and 12.5 ± 0.5 μm (Sample e). No signs of chemical interaction or the formation of intermetallic phases were detected in the interfacial zone, indicating that adhesion is primarily mechanical. This mechanical bonding is attributed to the high velocity of particle impact and the increased surface roughness of the substrate achieved through sandblasting prior to deposition.
Elemental analysis by EDS confirmed that the coating primarily consists of zirconium (Zr), in accordance with the ZrCN composition. Carbon (C) is uniformly distributed and represents the carbide phase. Although nitrogen (N) is an essential component of ZrCN, its detection is limited due to its low atomic number and weak signal in EDS analysis.
EDS mapping shows a uniform distribution of Zr and C throughout the coating, without evidence of elemental segregation or contamination. A sharp compositional gradient is observed at the coating–substrate interface: the Zr signal remains consistently high throughout the coating and drops abruptly at the interface, while the Fe signal from the steel substrate rises sharply beyond this point. This confirms the absence of diffusion processes or chemical reactions across the interface.
Small, localized amounts of oxygen (O) are detected, likely present as thin oxide films (e.g., ZrO2) along splat boundaries. According to the literature, such oxide layers are a common feature in HVOF-sprayed coatings and result from the oxidation of particles during their flight through the flame. The inevitable exposure of molten particles to atmospheric oxygen during spraying leads to partial surface oxidation. As a result, the deposited coating contains oxide inclusions, particularly surrounding individual splats.
These oxides can influence the coating performance in two significant ways. On one hand, they may act as brittle phases that promote crack initiation and propagation under mechanical loading. On the other hand, the presence of oxide layers at inter-splat boundaries can reduce cohesion between lamellae, which could potentially contribute to lower mechanical integrity of the coating. Although porosity was not measured in this study, such oxides are known from literature to sometimes be associated with increased porosity in thermal spray coatings.

4. Conclusions

This study presents the results of an investigation into the structural-phase and tribo-mechanical properties of ZrCN coatings deposited by the HVOF method onto U8G steel. The main conclusions drawn from the research are as follows:
-
X-ray diffraction (XRD) analysis revealed the presence of several phases, including Zr2CN, ZrC, ZrN, ZrO2, and Fe3O4. The formation of carbide and nitride phases is attributed to the thermal decomposition of ZrCN powder, while the presence of ZrO2 phases is explained by oxidation during the spraying process;
-
Tribological tests showed that the coefficient of friction of the coatings ranged from 0.2 to 0.93, depending on the spraying parameters and air flow rate. The lowest coefficient of friction (~0.2) was recorded for coatings applied at a pressure of 0.38 MPa, which is associated with the formation of an amorphous lubricating CNx phase that significantly reduces friction and wear;
-
Wear track analysis confirmed an abrasive wear mechanism, with wear track widths varying from 329 to 759 µm. The coatings provided effective substrate protection with minimal wear, even under high loads;
-
It was established that the microhardness of the coatings varied between 1512 HV and 1857 HV, depending on the deposition conditions. The initial microhardness of U8G steel is 392.5 HV, indicating a 4–4.5-fold increase after coating. The highest microhardness value (1857 HV) was observed for sample 5, indicating optimal particle densification and minimal coating porosity;
-
Adhesion tests performed by the pull-off method according to ASTM D4541-22 showed that the adhesion strength varied from 4.24 MPa to 14.56 MPa, depending on the air pressure during deposition. The highest value (14.56 MPa) was recorded for the sample deposited at 0.38 MPa, indicating optimal conditions for the formation of a cohesive and well-bonded coating structure.

Author Contributions

Conceptualization, S.K. (Sherzod Kurbanbekov) and B.K.; methodology, Z.S.; software, G.O.; validation, D.B., Z.S. and N.S.; formal analysis, B.K.; investigation, B.K.; resources, S.K. (Sherzod Kurbanbekov); data curation, B.K.; writing—original draft preparation, S.K. (Sardor Kambarbekov); writing—review and editing, N.S.; visualization, G.O. and D.B.; supervision, A.T.; project administration, S.K. (Sardor Kambarbekov); funding acquisition, S.K. (Sherzod Kurbanbekov). All authors have read and agreed to the published version of the manuscript.

Funding

This research is funded by the Committee of Science of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP22787358).

Data Availability Statement

Data are contained within the article.

Acknowledgments

We express our gratitude to the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan. For support of the project “Development of a composite ceramic-metal coating for protection against hydrogenation and high-temperature oxidation of zirconium alloy used in the nuclear industry”.

Conflicts of Interest

The authors declare no conflicts of interest.

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Crystals 15 00762 g001
Figure 1. (a) Morphology of ZrCN powder (b) diffractogram of the initial ZrCN powder.
Figure 1. (a) Morphology of ZrCN powder (b) diffractogram of the initial ZrCN powder.
Crystals 15 00762 g001
Crystals 15 00762 g002
Figure 2. Phase composition (wt.%) of ZrCN coatings deposited by the HVOF method at different air flow rates (based on Rietveld refinement of XRD data).
Figure 2. Phase composition (wt.%) of ZrCN coatings deposited by the HVOF method at different air flow rates (based on Rietveld refinement of XRD data).
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Crystals 15 00762 g003
Figure 3. Diffractograms ZrCN coatings at different air flow rates: 0.24 MPa; 0.26 MPa; 0.28 MPa; 0.3 MPa; 0.38 MPa.
Figure 3. Diffractograms ZrCN coatings at different air flow rates: 0.24 MPa; 0.26 MPa; 0.28 MPa; 0.3 MPa; 0.38 MPa.
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Crystals 15 00762 g004
Figure 4. SEM images of coating surfaces deposited from ZrCN powder at different air flow rates: (a)—0.24 MPa; (b)—0.26 MPa; (c)—0.28 MPa; (d)—0.3 MPa; (e)—0.38 MPa.
Figure 4. SEM images of coating surfaces deposited from ZrCN powder at different air flow rates: (a)—0.24 MPa; (b)—0.26 MPa; (c)—0.28 MPa; (d)—0.3 MPa; (e)—0.38 MPa.
Crystals 15 00762 g004
Crystals 15 00762 g005
Figure 5. Graph of dependence of friction coefficient on friction path.
Figure 5. Graph of dependence of friction coefficient on friction path.
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Figure 6. Microphotographs of wear traces of ZrCN coatings Sample (a–e).
Figure 6. Microphotographs of wear traces of ZrCN coatings Sample (a–e).
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Crystals 15 00762 g007
Figure 7. Average hardness values of the base steel and ZrCN coatings at different air flow rates.
Figure 7. Average hardness values of the base steel and ZrCN coatings at different air flow rates.
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Crystals 15 00762 g008
Figure 8. Mapping the distribution of chemical elements on cross-sections of ZrCN coatings Sample (a–e).
Figure 8. Mapping the distribution of chemical elements on cross-sections of ZrCN coatings Sample (a–e).
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Table 1. Chemical composition of U8G steel.
Table 1. Chemical composition of U8G steel.
CSiMnNiSPCrCu
0.8–0.90.17–0.330.33–0.58max 0.25max 0.028max 0.03max 0.2max 0.25
Table 2. HVOF modes for coating application.
Table 2. HVOF modes for coating application.
No.Sample aSample bSample cSample dSample e
Air, (MPa)0.240.260.280.30.38
Fuel gas, (propane) (MPa)0.170.170.170.170.17
Oxygen, (MPa)0.280.280.280.280.28
Thickness (μm)7.0 ± 0.43.0 ± 0.211.7 ± 0.2512.3 ± 0.2512.5 ± 0.4
deposition efficiency (%)37.7 ± 1.6216.2 ± 0.2863.0 ± 0.3766.2 ± 0.2967.3 ± 0.38
Table 3. Phase composition (wt.%) of ZrCN coatings deposited by the HVOF method at different air flow rates (based on Rietveld refinement of XRD data).
Table 3. Phase composition (wt.%) of ZrCN coatings deposited by the HVOF method at different air flow rates (based on Rietveld refinement of XRD data).
PhaseSample aSample bSample cSample dSample e
Zr2CN25.4%13.2%18.4%14.5%27.9%
ZrN14.8%11.4%12.5%13.2%17.2%
ZrC13.3%12.3%14.8%12.3%16.3%
ZrO215.8%21.9%18.6%20.7%13.5%
Fe3O430.7%41.2%35.7%39.3%25.1%
Table 4. Adhesion strength of ZrCN coatings deposited by the HVOF method at different air flow rates.
Table 4. Adhesion strength of ZrCN coatings deposited by the HVOF method at different air flow rates.
Sample a
(MPa)		Sample b
(MPa)		Sample c
(MPa)		Sample d
(MPa)		Sample e
(MPa)
9.770 ± 0.634.240 ± 0.425.410 ± 0.447.490 ± 0.5414.56 ± 0.76
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Kurbanbekov, S.; Suierkulova, Z.; Omashova, G.; Kaldar, B.; Temirbekov, A.; Kambarbekov, S.; Shektibayev, N.; Baltabayeva, D. Influence of Air Pressure on the Microstructure, Phase Composition, and Tribomechanical Performance of Thin ZrCN Coatings Deposited via HVOF Spraying. Crystals 2025, 15, 762. https://doi.org/10.3390/cryst15090762

AMA Style

Kurbanbekov S, Suierkulova Z, Omashova G, Kaldar B, Temirbekov A, Kambarbekov S, Shektibayev N, Baltabayeva D. Influence of Air Pressure on the Microstructure, Phase Composition, and Tribomechanical Performance of Thin ZrCN Coatings Deposited via HVOF Spraying. Crystals. 2025; 15(9):762. https://doi.org/10.3390/cryst15090762

Chicago/Turabian Style

Kurbanbekov, Sherzod, Zhamila Suierkulova, Gaukhar Omashova, Berik Kaldar, Alisher Temirbekov, Sardor Kambarbekov, Nurdaulet Shektibayev, and Dilnoza Baltabayeva. 2025. "Influence of Air Pressure on the Microstructure, Phase Composition, and Tribomechanical Performance of Thin ZrCN Coatings Deposited via HVOF Spraying" Crystals 15, no. 9: 762. https://doi.org/10.3390/cryst15090762

APA Style

Kurbanbekov, S., Suierkulova, Z., Omashova, G., Kaldar, B., Temirbekov, A., Kambarbekov, S., Shektibayev, N., & Baltabayeva, D. (2025). Influence of Air Pressure on the Microstructure, Phase Composition, and Tribomechanical Performance of Thin ZrCN Coatings Deposited via HVOF Spraying. Crystals, 15(9), 762. https://doi.org/10.3390/cryst15090762

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