Review of Physical and Mechanical Properties, Morphology, and Phase Structure in Cr₃C₂-NiCr Composite Coatings Sprayed by HVOF Method

Abstract

This review paper presents a detailed analysis of the influence of high-velocity oxygen–fuel (HVOF) spraying parameters on the microstructure formation and performance characteristics of Cr₃C₂-NiCr coatings. Key HVOF parameters, including the spray distance, oxygen-to-fuel ratio, powder feed rate, and spraying temperature, are examined in relation to their impact on coating properties. Structural parameters such as density, porosity, adhesive strength, and microhardness, which determine the mechanical behavior of the coating, are analyzed. Special attention is paid to wear resistance mechanisms, adhesion to the substrate, and resistance to fatigue failure. Additionally, the thermal stability of the coatings, their coefficient of thermal expansion, and oxidation resistance are investigated. This study also evaluates the morphology and phase composition of the coatings under different HVOF spraying conditions. An overview of modern diagnostic techniques, such as electron microscopy and spectroscopy, is provided. Compared to traditional surface treatment methods, HVOF spraying offers superior coating density, higher adhesion strength, and enhanced wear and corrosion resistance, making it an effective solution for extending the service life of components. Based on the findings, this paper highlights promising applications of Cr₃C₂-NiCr coatings in the aviation, power engineering, and mechanical engineering industries, where high wear resistance and thermal stability are crucial.

1. Introduction

As is known, modern achievements in the field of surface engineering attract considerable attention from researchers, due to a wide range of advantages, the functional characteristics of substrates, the optimization of microstructural parameters, and increased resistance to destructive environmental impacts [1,2]. One of the priority areas in this field is the development of composite materials and coatings with unique physical, chemical, and mechanical properties. Of particular interest are materials that can maintain strength and stability when exposed to extreme temperatures, significant mechanical loads, aggressive environments, and radiation [3].

A key direction for improving the performance properties of materials is the formation of wear- and corrosion-resistant coatings using thermal spraying technologies. Among the most widely used in industry are HVOF spraying, oxygen–fuel wire spraying, plasma-arc powder spraying, electric arc spraying, D-gun spraying, and cold spray technology [4,5]. These methods enable the formation of high-density, wear-resistant coatings with strong adhesion, effectively protecting surfaces from abrasive wear. Thermal spraying ensures the robust adhesion of the coating to the substrate due to the heating and accelerated transfer of particles in the gas flow. This technology is in demand in various industries, including aviation, energy, metallurgy, and mechanical engineering, as it significantly extends the service life of structural elements [6,7].

D-gun spraying is a highly effective deposition method used to create coatings with high density, low porosity, and excellent wear resistance. This method involves the detonation of a fuel gas mixture to accelerate powder particles to supersonic speeds, ensuring strong adhesion to the substrate. Studies have shown that Fe-based amorphous coatings deposited via D-gun spraying exhibit a nearly complete amorphous structure, high surface hardness (822 ± 10 HV_0.1), and exceptional wear resistance under dry sliding conditions [8]. The detonation gun spraying method has been successfully employed to apply WC-Co and Cr₃C₂-NiCr coatings with rare-earth additives, significantly improving their wear resistance. This method allows precise control over process parameters, enabling the formation of coatings with tailored microstructures and mechanical properties. Additionally, oxide coatings, such as aluminum oxide, can be deposited using D-gun spraying to obtain coatings with increased α-phase content, further enhancing their mechanical and thermal stability [9,10].

Cold spray coating is another promising technique that allows the deposition of coatings without significant heat input, thereby preventing oxidation and phase transformations in the feedstock material. This method utilizes the high-velocity impact of powder particles onto a substrate to form coatings with minimal porosity (<1%) and high bond strength [11,12]. The absence of a heat-affected zone makes cold spraying particularly suitable for structural and corrosion-resistant repairs of critical components where traditional welding is not feasible. Various materials, including metals, ceramics, composites, and even polymers, can be deposited using cold spray technology [13]. Recent research highlights the potential of cold spraying in additive manufacturing, where it is used for the sustainable fabrication, restoration, and mass production of engineering components. The integration of cold spraying with additive manufacturing (cold spray additive manufacturing, CSAM) enables the fabrication of 3D components with superior mechanical and structural properties, paving the way for future advancements in the manufacturing industry [14,15].

The selection of materials and processes for coatings depends on clear criteria specific to each application area. In the shipbuilding, aviation, and energy sectors, corrosion, erosion, and wear are key degradation factors. Wear-resistant coatings, particularly those applied via thermal spraying, must exhibit high density (porosity < 1%), hardness, strong adhesion, and resistance to both corrosion and mechanical loads. The high kinetic energy of particles during deposition ensures the formation of dense coatings with a thickness exceeding 1.5 mm, with controlled cooling conditions that further enhance performance properties [16].

In the energy and nuclear industries, materials are exposed to extreme conditions, including radiation, high temperatures, and hydrogen environments, which induce hydrogen embrittlement and high-temperature oxidation. Hydrogen embrittlement leads to the formation of brittle hydride phases, increasing the risk of cracking, while oxidation degrades protective oxide films, reducing material durability [17]. Therefore, the development of protective coatings for alloys in the energy sector is crucial to enhance their longevity and resistance to aggressive environments. These coatings must prevent hydrogen permeability and oxidation while maintaining mechanical integrity [18].

A promising approach involves the creation of composite metal–ceramic coatings that act as a barrier against corrosion and structural degradation [19]. Such coatings form a stable protective layer, improving oxidation resistance and minimizing hydrogen permeation, thereby increasing material longevity under high-temperature and radiation conditions [20].

In high-temperature applications, alloys form protective oxide layers that reduce corrosion in coolants. For example, zirconium-based alloys (zircaloids) are widely used in nuclear engineering due to their low-thermal-neutron-absorption cross-section, excellent corrosion resistance, and stable mechanical properties under irradiation conditions [21]. However, these alloys become brittle and undergo exothermic oxidation above 1204 °C, which can lead to the release of significant amounts of hydrogen and subsequent explosion hazards [22]. Key factors contributing to their degradation include radiation-induced swelling, oxidation, and hydrogenation [23,24].

To mitigate these effects, protective coatings such as metal–ceramic and chromium-based coatings are being developed to provide enhanced resistance to corrosion, hydrogen permeability, and wear [25]. The application of chromium coatings with a refractory sublayer has demonstrated significant improvements in the performance of fuel element cladding, reducing oxidation, hydrogen absorption, and high-temperature wear [26].

On the other hand, non-oxide ceramics such as Group IV transition metal carbides and nitrides possess a unique combination of ionic, covalent, and metallic bonds, granting them exceptional hardness (~25 GPa), extremely high melting points (~3000 K), and good thermal (10 W m⁻¹ K⁻¹) and electrical conductivity (200 × 104 Ω⁻¹ m⁻¹) [27]. Due to these properties, zirconium carbide and zirconium nitride are increasingly being considered for applications in modern nuclear reactors. They are being investigated as fuel cladding materials, inert matrix fuel (IMF), or protective coatings for advanced fuel particles in gas-cooled fast reactors (GFRs) [28].

Among composite coatings, tungsten- or chromium carbide-based coatings with a ductile metal matrix (Co/Ni alloyed with Cr) are the most popular [29,30,31,32,33,34,35]. Compared to WC-based coatings, Cr₃C₂-based compositions, particularly Cr₃C₂-NiCr, demonstrate superior resistance to erosion, corrosion, and wear at elevated temperatures up to 800–900 °C [36,37,38,39,40,41].

Thus, modern surface engineering research focuses on the development of wear- and corrosion-resistant coatings to enhance material performance in extreme environments, including the aviation, energy, and nuclear industries. Special attention is given to metal–ceramic and composite coatings, such as Cr₃C₂-NiCr, which provide protection against oxidation, hydrogen permeability, and wear at high temperatures. The HVOF method remains a leading technology for forming dense, well-adhered coatings, significantly extending the service life and operational safety of critical structural components.

2. Reliable Coating Methods

Modern trends in materials science and nanotechnology have led to the development of new methods for surface treatment and improvement in material properties. Various methods are used to remove coatings, among which physical and chemical methods are widely used. Physical methods include HVOF spraying, plasma spraying, and magnetron spraying, which allow the formation of highly adhesive, dense coatings [42,43,44]. Magnetron spraying is used to obtain particularly thin, uniform, and high-quality coatings [45]. Among the chemical methods, chemical vapor deposition (CVD) and electrochemical deposition (electrolysis) are widely used, which are effective in forming uniform layers on complex-shaped parts [46]. In addition, sol–gel and laser coating technologies are also used to obtain highly wear-resistant, anti-corrosion coatings [47]. In the 1960s, the main deposition method for such coatings was the detonation gun [48]. With the development of plasma spraying technology, atmospheric plasma spraying later became an industrially significant deposition method [49,50].

Table 1. Comparison of various deposition methods.

№	Deposition Method	Deposition Temperature (°C)	Coating Thickness (µm)
1	D-Gun Spraying	1000–1200	100–500
2	Cold Spraying	<400	50–1000
3	Plasma Spraying	10,000–15,000 (plasma temp.)	50–500
4	HVOF	1000–3000	50–300
5	HVAF (High-Velocity Air–Fuel)	900–1600	50–400
6	PVD	100–500	0.1–5
7	CVD	500–1100	1–10
8	Thermal Spray (General)	500–3000	50–500
9	Multi-Arc Ion Plating (MAIP)	400–600	1–10
10	Micro-Arc Oxidation (MDO)	200–1000	10–100
11	Laser Cladding	800–2500	100–2000
12	Sol–Gel Coating	Room temperature—800	0.1–10
13	Magnetron Spraying	Room temperature—500	0.1–5

provides a comparative analysis of different surface deposition methods, highlighting key parameters such as the deposition temperature, coating thickness, and main characteristics. The methods include physical, chemical, and thermal spraying techniques, each offering unique advantages in terms of adhesion, porosity, process efficiency, and material versatility. Understanding these differences helps in selecting the most suitable method for specific industrial applications, such as in the aerospace, automotive, and energy sectors.

Studies by Ulianitsky V.Y. et al. [51] have shown that the optimum indentation distance for the detonation spraying (DS) of Cr₃C₂-NiCr is 150–250 mm, providing dense coatings with a hardness ~1100 HV300 and wear resistance ~3 mm³/1000 r. The coatings are characterized by compression stresses and adhesion to the substrate ≥ 150 MPa. Increasing the NiCr content from 10 to 25 wt% reduces the wear resistance by 1.5 times and the compressive stress level by 3 times. The composition of Cr₃C₂-20NiCr is optimal for high mechanical loads, which is important for parts with complex shapes. A schematic of the detonation spraying experiment is shown in Figure 1. The barrel had a length of 1400 mm and a diameter of 20 mm. In the aforementioned work, two-component acetylene–propane fuel was chosen for the deposition of Cr₃C₂-20NiCr coatings.

CrAlN and CrAlSiN coatings applied by MAIP on Zircaloy effectively protect the substrate from oxidation at 1000 °C (up to 2 h) and slow down the process at 1200 °C. The formation of mixed oxide layers reduces the diffusion of elements, and nitrogen bubbles have been found inside them. CrAlSiN has better oxidation resistance but is susceptible to cracking [52]. Lin. R. et al. used Cr12MoV cast steels with dimensions of 20 × 20 × 4.5 mm³ as substrates. The substrates were ultrasonically cleaned in ethanol and acetone for 30 min using an ultrasonic cleaner (2CRD 200, Novatec, San Martino di Lupari Padova, Italy). CrN, CrAlN, and TiAlN coatings were deposited using a MAIP deposition system (S800 Pro, ICS Technologies, Grottammare, Italy); a schematic diagram of the MAIP process is shown in Figure 2 [53].

The method of micro-arc oxidation (MDO) is promising for the protection of zirconium alloys from corrosion and hydrogen embrittlement, but has not yet been sufficiently studied [54]. One paper presents the results of a study of the corrosion resistance of an E-110 alloy with MDO coating, proves its advantages over ion–plasma methods, and considers the mechanisms of plasma electrolysis. The obtained data are relevant for applications in nuclear power units [55]. MDO treatment is usually carried out for 5–180 min at current densities from 500 to 2000 A⸱m⁻² and voltages up to 1000 V [56]. Figure 3 shows a schematic representation of the MDO unit [57].

Chromium carbides are widely used as coatings to protect steel structures in high-tech applications such as energy and aerospace due to their resistance to corrosion and wear at high temperatures. In the work of Rubino Felice et al., low-power plasma equipment (CPS) was used to apply Cr₃C₂ coatings on carbon steel. The optimization of spraying parameters using ANOVA analysis resulted in dense coatings with a hardness of about 600 HV, a thickness of 600 μm, and an adhesion strength of about 14 MPa. The partial dissolution of Cr₃C₂ during melting and re-solidification caused the formation of weaker carbide phases and porosity, leading to variations in coating hardness [58].

Modern research shows that CrN coatings applied by magnetron spraying, as well as the high-intensity ion beam (HIPIB) treatment of zirconium alloy surfaces, improve coating adhesion and increase resistance to radiation exposure [59]. The development and optimization of protective layer application technology is underway in leading research centers in Russia, the USA, France, China, South Korea, the Czech Republic, and Ukraine. Of particular interest are chromium coatings, which demonstrate a 5-fold lower loss of mass due to wear, a 20-fold reduction in the corrosion rate at 1200 °C, and a significant reduction in hydrogen absorption during high-temperature tests [60,61,62].

Plasma spraying equipment consists of a power source, a control unit for regulating gas, cooling water, a powder feed system, and a plasma gun, as well as other auxiliary equipment. Thermal spraying as an accessible and economical coating preparation method has been widely reported [63]. Cr₃C₂-NiCr coatings produced by thermal spraying are actively used for wear reduction at temperatures between 500 and 900 °C. During the application process, compositional changes occur in the coating material, and at an elevated temperature, the metastable structure begins to transition to an equilibrium state. The aim of one study was to determine the effect of heat treatment temperature on the mechanisms and rate of coating formation in the range of 500–900 °C for 30 days. Compositional development was analyzed using X-ray diffraction, while microstructure formation mechanisms and the carbide grain growth rate were evaluated through image analysis. It was found that the rapid transformation of metastable phases to an equilibrium composition occurred within 1–5 days at 500 °C and in 1 day at higher temperatures. The steady-state composition was consistent with the initial powder [64].

Compared to air plasma spraying (APS) and suspension plasma spraying (SPS), HVOF produces coatings with lower porosity and less oxidation due to the higher jet velocity and lower flame temperature [65]. HVOF spraying is preferred over thermal spraying for conventional coatings because of its relatively lower cost than vacuum plasma spraying. Cermet coatings based on ceramic carbides embedded in a metallic matrix phase have been used to protect steel rollers and have been considered as a potential replacement for chrome coatings used in aircraft landing gear and petrochemical and marine applications [66]. HVOF-derived Cr₃C₂-NiCr coatings have been widely studied and used to resist oxidation, corrosion, and abrasion at high temperatures, even up to 850 °C [67,68,69]. In the field of corrosion-resistant coatings, Cr₃C₂-NiCr coatings have been shown to reduce fretting damage to components in high-temperature gas-cooled reactors [70,71,72].

Currently, HVOF spraying and, more recently, HVAF spraying have become the most commonly used spraying processes for Cr₃C₂-NiCr coatings [73]. As mentioned earlier, the dissolution of Cr₃C₂ in molten Ni already starts at 1255 °C [74]. Thus, the reduced heat input to the feedstock granules during spraying, which is a characteristic feature of HVAF spraying, results in coatings with impeded carbide dissolution [75,76].

In one study, the substrate material used was SSAB (Svenskt Stål AB (English: Swedish Steel)) Domex^® 350LA (Stockholm, Sweden), (low-alloy steel, SSAB AB) in the form of Ø25 mm round coupons. Prior to coating, all substrates were sandblasted using aluminum oxide powder (220 grit) until a surface roughness of approximately 3 μm (Ra) was achieved. The surface roughness was measured using an MITUTOYO SURFTEST-301 profilometer (Mituyoto, Takatsu-ku, Kawasaki, Japan). The coatings were applied using an HVAF spray system (Uniquecoat, Richmond, VA, USA) with a novel proprietary two-component arrangement as shown in Figure 4 [77].

In connection with the above, one way to improve corrosion resistance and high-temperature oxidation resistance, as well as to improve the physical and mechanical properties of the material, is the application of composite metal–ceramic coatings, which have high homogeneity, low porosity, high adhesion strength and corrosion resistance, and low hydrogen permeability. Gas thermal spraying methods such as air plasma spraying and suspension plasma spraying, HVOF, etc., are widely used for metal–ceramic coatings. Gas thermal methods are available and economical methods of coating preparation. Among them, HVOF is the most promising.

Rakhadilov B. et al. investigated the HVOF process (in Figure 5) with the modeling of the gas flow and behavior of WC-Co-Cr particles of different fractions and showed that the maximum temperature of the gas flow reaches 2700 °C, after which it decreases, and the pressure in the combustion chamber exceeds 400,000 Pa, stabilizing at the nozzle outlet at 100,000 Pa. The gas velocity increases to 1300–1400 m/s and decreases to 400 m/s at a distance of 150 mm. It was determined that particles of a 21–35 μm fraction have the most stable velocity and temperature parameters, contributing to the formation of high-quality coating, while small particles (up to 20 μm) lose energy faster, which negatively affects the coating structure [78].

The samples were sprayed using a high-velocity oxygen fuel (HVOF) unit with a geometry corresponding to the modeling results, as shown in Figure 5. The figure illustrates the following components of the torch: 1—torch body, 2—combustion chamber, 3—cooling system, 4—mixer body, 5—nozzle (atomizer), 6—adapter, and 7—powder feed tube [78].

In a study by Hong S. et al., a Cr₃C₂-NiCr coating was obtained by the HVOF method and pro-analyzed for resistance to cavitation–silt erosion (CSE) at different sludge concentrations. The coating revealed Cr₃C₂, Cr₇C₃, Cr₂O₃, and (Cr, Ni) phases, with low porosity and high microhardness. The crystallization temperature of the amorphous phase was about 559 °C. The mass loss of the coating after 20 h of erosion at a concentration of 40 kg⸱m⁻³ was 1.15 and 1.23 times more compared to at 20 kg⸱m⁻³ and 0 kg⸱m⁻³, respectively. With increasing sludge concentration, the CSE rate increases due to the larger interaction area of sand particles with the pavement. Lips, craters, micro-cuts, and cracks are observed on the eroded surface, indicating a composite damage mechanism consisting of ductile and brittle modes [79]. In the nuclear industry, Cr₃C₂-NiCr coating has been demonstrated to reduce fretting damage to components in high-temperature gas-cooled reactors [80,81]. Yang et al. demonstrated the high oxidation resistance of the HVOF coating of Cr₃C₂-NiCr at 1200 °C in steam for 1 h due to the formation of a dense chromium scale [82].

Thermally sprayed carbides based on Cr₃C₂ carbides are widely used as wear-resistant protective coatings at high temperatures and in corrosive environments. Especially in heavy industrial sectors such as the metalworking, power generation, and oil and gas industries, these coatings are used in various applications [83,84]. As a consequence, Cr₃C₂-NiCr is currently the second-most used hard alloy for thermal spraying. The metallurgy of the Cr₃C₂-NiCr system and the properties of the coatings are discussed here.

Rakhadilov B. et al. found that varying the fractional composition of the initial powder significantly affects the morphology and properties of the formed coatings. The optimal homogeneity of the lamellar structure and minimum porosity (0.8% and 0.2%), as well as the lowest content of CoO and W₂C phases, were recorded in coatings obtained from powders with fractions of 20–30 μm and 30–40 μm. The thickness of the coatings varies between 45 and 85 μm, changing discontinuously with increasing particle sizes. The maximum microhardness (780 HV_0.1), due to the high concentration of the WC carbide phase, is recorded for the coating with 20–30 μm particles, which correlates with increased corrosion resistance due to reduced corrosion current density. This coating also exhibits the highest wear resistance (0.0891 mm³), whereas the coating with 40–45 μm powder exhibits the lowest wear resistance (0.15954 mm³). As the powder fraction increases, there is an increase in the surface roughness of the coatings, which may have an effect on their tribological characteristics [85].

Several studies have been published comparing the abrasive behavior of different carbide coatings [86,87,88,89]. The vast majority of these studies have been conducted using the ASTM G65 abrasion test rig, where the wear regime corresponds to low-stress three-component abrasion. Cr₃C₂-NiCr coatings sprayed by HVOF have superior low-stress-abrasion resistance compared to self-fluxing Ni-based alloys, hard chromium coatings, and hardened steels; however, they are still significantly inferior to WC-Co hard alloys and (Ti,Mo)(C,N)-NiCo cermets [90].

Thus, it can be said that HVOF is superior to other spraying methods due to its high particle velocity, providing a dense, wear-resistant coating with low porosity and minimal oxidation. Unlike plasma and arc spraying, HVOF reduces the risk of thermal damage to the substrate and provides better adhesion. Compared to gas thermal spraying, it forms denser coatings with high hardness. These characteristics make HVOF the optimal choice for wear and corrosion protection in aviation, power generation, and mechanical engineering.

3. Study of Structure and Phase Composition

At high temperatures (700–850 °C), microstructural changes in Cr₃C₂-NiCr coatings deposited by HVOF and HVAF methods influence their erosion resistance. B.Q. Wang et al. [91] reported that coatings sprayed at 300 °C exhibited lower erosion rates than those deposited at room temperature, attributing this to improved ductility. Similarly, S. Matthews et al. [92] found that the enhanced ductility of the NiCr binder significantly increased erosion resistance at 800 °C. However, prolonged exposure to high temperatures led to Ostwald ripening, forming a network of large carbides that restricted the binder’s ductile response. As a result, HVOF-sprayed coatings demonstrated superior erosion resistance at 800 °C compared to those heat-treated at 900 °C for extended periods (2 or 30 days) [92]. These findings highlight that the high-temperature-erosion resistance of Cr₃C₂-NiCr coatings is primarily determined by binder phase behavior and carbide coarsening dynamics.

Additionally, Cr₃C₂-25%NiCr coatings deposited via HVOF on stainless steel exhibited low porosity, high microhardness, and strong adhesion. The powder feed rate was a key parameter influencing coating performance, with an optimal rate of 33.5 g/min yielding the best results. Wear resistance tests at 500 °C confirmed that coatings with a dense structure and sufficient fracture toughness demonstrated superior durability [93].

Table 2. Comparison of XRD analysis of coatings obtained by HVOF, HVAF, and supersonic atmospheric plasma spraying technology (SAPS) deposition methods.

No.	Authors	Method	Type of Coating	Phases	Structure	Lattice Period, Å	Ref.
1	Xie M. et al.	HVOF	Cr3C2-NiCr	Cr3C2, Cr7C3, NiCr	-	-	[93]
2	Poirier D. et al.	HVOF	Cr3C2-NiCr (7102)	Cr23C6, Cr7C3, Cr3C2, Cr, Ni	-	-	[94]
			Cr3C2-NiCr (7305)	Cr23C6, Cr7C3, Cr3C2, Cr, Ni
3	Sahraoui T. et al.	HVOF	Cr3C2-25NiCr	Cr3C2, Cr3Ni2, Cr	-	-	[95]
4	Bolelli G. et al.	HVOF	Cr3C2-25NiCr	Cr3C2, Cr2O3, γ-Ni	f.c.c.	-	[96]
		HVAF		Cr3C2, Cr2O3, Cr7C3, γ-Ni
5	Zhou W. et al.	HVOF	Cr3C2-NiCr	Cr3C2, Cr7C3, NiCr	-	-	[97]
			Cr3C2-WC-NiCoCrMo	WC, Cr3C2, Cr7C3, Niss amorphous
6	Zhou Z. et al.	HVAF	Cr3C2-NiCr	Cr3C2, Cr7C3, Ni, Cr	-	-	[98]
7	Selvam Kevin et al.	HVOF	Cr3C2-NiCr	Cr3C2, NiCr, Cr7C3, Cr23C6, Ni, Cr	-		[99]
8	Rakhadilov B. et al.	HVOF	86WC-10Co-4Cr	(A1) WC	-	a = 2.9005 c = 2.8330	[100]
				W2C		a = 2.9614 c = 4.6884
				CoO		a = 4.2507
				(A2) WC		a = 2.9011 c = 2.8328
				W2C		a = 2.9554 c = 4.6641
				CoO		a = 4.2451
				(A3) WC		a = 2.9027 c = 2.8345
				W2C		a = 2.9624 c = 4.6924
				CoO		a = 4.2506
9	Kurbanbekov S. et al.	HVOF	Cr3C2-NiCr	Cr23C6	Cr23C6 (cubic, Fm-3m)	a = 10.6600	[101]
				Cr3C2	Cr3C2 (orthorhombic, Pnma)	a = 5.5400 b = 2.8330 c = 11.4940
				CrNi3	CrNi3 (cubic, Fm-3m)	a = 3.5400
				NiCrO4	NiCrO4 Tetragonal lattice (space group I41/amd)	a = 5.5380 b = 5.5380 c = 8.4350
10	Zhang C. et al.	SAPS	Cr3C2-NiCr	Cr3C2, Cr23C6, NiCr, (Ni,Cr)7C3 and Ni	-	-	[102]

presents XRD analysis results, comparing different spraying methods in terms of phase composition and microstructure. This comparison provides insight into the relationship between deposition parameters and coating characteristics, assisting in selecting the most suitable technique for specific performance requirements.

Xie M. et al. [93] analyzed the XRD spectra of Cr₃C₂-NiCr coatings obtained by the HVOF method, comparing them with the original powder. The results indicate that the crystalline phases in the sputtered coating largely match those of the original powder, with the primary phases being Cr₃C₂ and the NiCr binder. However, a small Cr₇C₃ diffraction peak was detected in the coating, likely due to the partial decarburization of Cr₃C₂ under high-temperature flame spraying conditions [103]. Both Cr₃C₂ and Cr₇C₃ contribute to the coating’s high hardness and wear resistance at elevated temperatures [97].

Zhou Z. et al. [98] investigated the phase composition of Cr₃C₂-NiCr coatings before corrosion. Their XRD results showed the presence of Cr₃C₂, NiCr, Cr₇C₃, Cr₂₃C₆, Ni, and Cr. At high temperatures and in oxidizing environments, phase transformations occur, leading to the decomposition of initial phases and the formation of new compounds during the spraying process [104,105,106].

Similarly, Rakhadilov B. et al. [100] examined WC-Co coatings and found that regardless of the oxygen feed rate, the coatings contained WC, W₂C, and CoO phases. The presence of W₂C and CoO was attributed to the thermal decomposition of WC and its reaction with oxygen, leading to partial carbon loss. Increasing the oxygen flow rate altered the intensity of diffraction peaks, with W₂C peaks at 38.4° and 40° becoming more pronounced in coating A3, while coatings A1 and A2 showed no significant phase intensity changes when the oxygen rate was increased from 150 to 170 L/min.

Kurbanbekov S. et al. [101] analyzed HVOF-sprayed Cr₃C₂-NiCr coatings and identified Cr₂₃C₆ (cubic, Fm-3m) and Cr₃C₂ (orthorhombic, Pnma) as the main phases, both contributing to high mechanical strength. Additionally, CrNi₃ (cubic, Fm-3m) was detected, enhancing corrosion resistance. The NiCrO₄ phase was absent in the initial powders. Local SEM analysis confirmed the coating’s microstructure, highlighting its suitability for high-temperature, wear, and corrosion applications on the E110 alloy.

The observed differences between Cr₃C₂-NiCr coatings obtained by HVOF and HVAF methods stem from variations in the temperature and cooling rate, which significantly impact phase composition and microstructure. High spraying temperatures promote the partial decarburization of Cr₃C₂, leading to the formation of Cr₇C₃ and Cr₂₃C₆, altering mechanical properties. The rapid cooling in HVOF promotes the formation of amorphous phases, increasing hardness but reducing ductility, whereas HVAF preserves the original crystalline powder structure, improving wear resistance. Prolonged high-temperature exposure leads to carbide coarsening, reducing binder plasticity and negatively affecting erosion resistance. Optimizing spraying parameters, such as the powder feed rate and oxygen flow rate, can help balance hardness, fracture toughness, and corrosion resistance.

4. Microstructure and Morphology of Coatings

The conducted studies have shown that the used spraying modes provide a dense and uniform coating with a thickness of 75–110 μm (Figure 6). The coatings are uniformly distributed, without a columnar structure and delamination, confirming their strong adhesion properties. SEM and EDX analyses were performed to characterize the microstructure and elemental composition of the coatings. The results revealed the presence of Cr, Ni, C, and O, with Cr content ranging from 17.03 to 83.84 at.%, Ni content from 44.83 to 78.50 at.%, C content from 0.58 to 7.04 at.%, and O content from 3.45 to 9.13 at.%. The presence of oxygen is attributed to oxidation during the spraying process. In region 1, Cr predominates (60 at.%), whereas in region 2, Ni (25 at.%) and C (5 at.%) are more abundant. The energy dispersive analysis further confirmed the sequential layer distribution in the metal coating [101].

The HVOF process parameters have been optimized to achieve an optimal coating performance. This included controlling the pressure, powder feed rate, and distance between the atomizer and the substrate (

Table 3. HVOF modes for coating application [101].

No.	Distance, mm	Fuel, bar	Air, bar	Oxygen, bar
Sample (a)	350	1.7	2.2	2.8
Sample (b)	350	1.7	3.3	2.8
Sample (c)	350	1.7	3.7	2.8
Sample (d)	350	2.4	3.3	2.8

) [101].

Elemental analysis of the coatings revealed the presence of Cr, Ni, C, and O, consistent with the Cr₃C₂-NiCr powder composition. Chromium dominated in region 1 (up to 60 at.%), while region 2 showed higher concentrations of nickel (25 at.%) and carbon (5%). Oxygen content (up to 9.1 at.%) likely resulted from exposure to air during spraying. These findings confirm the expected layer structure of the coating [101].

Wear resistance studies have demonstrated the performance of Cr₃C₂-NiCr coatings under different stress conditions. Kashparova et al. investigated abrasion resistance using aluminum oxide as an abrasive under a 22 N load. Under high stress, the wear rate of Cr₃C₂-NiCr was 5.54⸱10⁻² mm³/m, which was higher than that of WC-Co (2.23⸱10⁻² mm³/m). Under low stress, the difference was even greater: 8.72⸱10⁻³ mm³/m versus 1.80⸱10⁻³ mm³/m. SEM micrographs showed that under low stress, the surface was smooth with uniform phase wear, while under high stress, plastic deformation, deep scratches, and the mechanical mixing of abrasive particles were observed [107].

A further microstructural analysis of the coatings under (Figure 7) low-stress conditions revealed abrasive wear characterized by fine grooves formed due to loading and particle contact. The wear mechanism involved uniform phase interaction between Cr₃C₂ carbides and the NiCr matrix, without significant matrix displacement or adhesion. However, localized carbide pullout and cracks perpendicular to the wear direction were observed [107].

The surface is uniformly grooved along the sliding direction (A), with even abrasion of both Cr₃C₂ carbides and the NiCr matrix. No signs of matrix displacement or smearing are observed. However, some carbides are locally pulled out (B), and occasional cracking occurs perpendicular to the abrasion direction (C) [107].

The SEM cross-sectional micrographs of the three coatings are presented in Figure 8. These images illustrate their porous microstructure, where chromium carbide particles are embedded in the matrix phase. The coating thickness was approximately 150 μm, and no cracks were observed. Two types of pores were reported in HVOF-sprayed coatings: isolated and uniformly distributed pores. Among the examined coatings, coating B demonstrated the highest density and uniformity with the lowest porosity, suggesting superior mechanical performance [108].

Shi C. et al. analyzed nanomodified multimodal and conventional Cr₃C₂-NiCr coatings applied via HVOF spraying on a copper crystallizer, focusing on microstructure and mechanical properties. Nanomodified particles, consisting of nano-, submicro-, and micrograins, formed a three-layer structure with a metal binder shell. Multimodal coatings exhibited high density (porosity 0.3 ± 0.12%), homogeneity, increased microhardness (985.85 HV_0.3 vs. 837.19 HV_0.3 for conventional coatings), and strong adhesion (75.32 MPa vs. 45.59 MPa). Compared to traditional hard chromium galvanic coatings, multimodal Cr₃C₂-NiCr coatings showed superior durability, wear resistance, and environmental advantages [109].

The SEM micrographs of Cr₃C₂-NiCr powders (Figure 9) illustrate the differences between conventional powder (CP) and nanomodified multimodal powder (MP). CP exhibits spherical morphology (Figure 9a), ensuring good flowability in HVOF, while MP consists of disk agglomerates (Figure 9d) with a fine-grained structure (Figure 9e). Cross-sectional analysis indicates that CP contains voids (Figure 9c), whereas MP features a denser microstructure with nanoscale carbide grains embedded in a metal binder (Figure 9f). A NiCr shell, 0.5–1 μm thick, can be identified on the outer layer of MP (Figure 9f, region ②) [109].

Figure 10a,e depict rough surfaces with oxide particles, pores, and partially melted or unmelted particles. Figure 10a also shows stress-induced cracks formed during cooling. The increase in Cr content in the 80 wt% Cr₃C₂-NiCr + 20 wt% NiCr coatings (Figure 10f) was confirmed through EDS analysis, whereas a decrease in Cr content was observed in coatings formed from powder sifted through a 400-mesh sieve (Figure 10j) [110].

Irfan Hu et al. examined the morphology of conventional and nanomodified multimodal Cr₃C₂-NiCr coatings after laser cladding (Figure 11). The conventional coating exhibited an uneven surface due to low NiCr content (Figure 11a,b), resulting in non-uniform shrinkage during solidification. Cr₃C₂ particles were partially melted and evenly distributed within the binder phase, with carbide particle agglomeration. In contrast, the nanomodified coating (Figure 11c,d) formed core–shell structures and a composite binder phase. The addition of rare-earth metal oxides, such as Ce (Figure 11b), refined the grain structure and enhanced wear resistance. The presence of micron, submicron, and nanoparticles of Cr₃C₂ bound by the NiCr phase contributed to higher coating density [111].

Thus, differences in Cr₃C₂-NiCr coating performance stem from variations in spraying techniques (HVOF, laser cladding), initial powder morphology (spherical, agglomerated, nanomodified), and processing conditions. HVOF coatings exhibit high density, uniform phase distribution, and low porosity, contributing to superior wear resistance and adhesion strength. Laser cladding, on the other hand, enhances coating structure by forming core–shell architectures and composite binder phases, leading to increased hardness and wear resistance. The incorporation of rare-earth metals (Ce) promotes grain refinement and further improves mechanical properties. These variations arise from differences in energy interaction, melt dynamics, and cooling conditions, ultimately influencing the final coating performance.

5. Tribological Performance

Particular attention is paid to coating performance, including wear mechanisms, adhesion strength, and resistance to fatigue failure. Wear behavior is analyzed based on friction conditions, material composition, and structural features. Adhesion strength is examined in relation to substrate–coating interactions and residual stress distribution. One study also explores the mechanisms of fatigue failure, emphasizing crack formation and propagation under cyclic loading.

The experimental results demonstrated that the application of Cr₃C₂-NiCr HVOF coating on gray cast iron significantly enhanced wear resistance. Specifically, the coating with 80% Cr₃C₂ and 20% NiCr exhibited a hardness of 1410 HV, which is 3.5 times greater than that of the uncoated sample (410 HV). Similarly, the 75% Cr₃C₂ and 25% NiCr coating also achieved a high hardness of 1350 HV. Pin-on-disk wear tests indicated that both compositions effectively reduced the friction coefficient and wear rate compared to gray cast iron. Notably, the 80% Cr₃C₂ and 20% NiCr composition demonstrated the best performance, attributed to its higher carbide content, which enhances wear resistance.

Thus, the Cr₃C₂-NiCr HVOF coating with an 80% Cr₃C₂ and 20% NiCr composition is highly effective in minimizing wear and friction, making it a suitable protective solution for moving mechanical components [112].

Table 4. Comparative analysis of the friction coefficient of coatings depending on temperature.

No.	Authors	Method	Coating	Load (N)	Coefficient of Friction (f)/T, ℃	Ref.
1	Shunmuga Priyan et al.	HVOF	80%Cr3C2 + 20%NiCr	10	0.2018	[112]
				20	0.2155
				30	0.2346
			75%Cr3C2 + 25%NiCr	10	0.2161
				20	0.2341
				30	0.2516
2	Pankaj Chhabra et al.	APS	Cr3C2-NiCr	25	0.63 (RT)	[113]
					0.37 (400 °C)
					0.43 (800 °C)
				50	0.51 (RT)
					0.36 (400 °C)
					0.41 (800 °C)
3	Li W. et al.	HVOF	CrN	316	0.6 (25 °C)	[114]
					0.6–0.2 (150 °C)
					0.4 (350 °C)
					0.3 (550 °C)
			CrN/Cr3C2-NiCr	316	0.6 (25 °C)
					0.3 (150 °C)
					0.4 (350 °C)
					0.3 (550 °C)
4	Rakhadilov B et al.	HVOF	86WC-10Co-4Cr	-	0.488–0.463	[115]
5	Huang C. et al.	APS	NiCr/(Cr3C2-BaF2CaF2)	-	0.8–0.25 (0–800 °C)	[116]
6	Chhabra P. et al.	APS	Cr3C2-NiCr	25	0.59 (RT)	[117]
					0.39 (400 °C)
					0.42 (400 °C)
				50	0.58 (RT)
					0.34 (400 °C)
					0.38 (400 °C)

presents a comparative analysis of the friction coefficient of coatings depending on temperature and various load values obtained by different application methods.

The analysis of Cr₃C₂-NiCr coatings’ performance focuses on wear behavior, adhesion to the substrate, and resistance to fatigue failure, ensuring a structured and coherent discussion.

The dependence of the friction coefficient (CoF) and wear volume on spraying distance has been investigated in the works of Rakhadilov B. It was found that increasing the spraying distance from 100 mm to 300 mm leads to a decrease in the friction coefficient from 0.488 to 0.463 and a reduction in wear volume from 0.079 mm³ to 0.036 mm³. The coating exhibited maximum wear resistance at 300 mm and a minimum at 100 mm, which was attributed to a decrease in the WC phase content [116].

At room temperature, the average CoF values for uncoated H11 steel were recorded as approximately 0.48 and 0.40 at 25 N and 50 N loads, respectively. In contrast, coated H11 steel exhibited higher CoF values of approximately 0.63 and 0.51 under the same conditions. Furthermore, CoF values for coated specimens fluctuated over time, whereas uncoated specimens displayed instability throughout the test. The specific wear rate for uncoated H11 was 609.91 ± 13 mm³/Nm × 10⁻⁶ at 25 N and 487.18 ± 10 mm³/Nm × 10⁻⁶ at 50 N. For coated specimens, these values were significantly reduced to 5.52 ± 0.11 mm³/Nm × 10⁻⁶ and 9.82 ± 0.19 mm³/Nm × 10⁻⁶, respectively, demonstrating improvements of 110 and 48 times, attributed to the superior hardness and adhesion strength of the coating [117].

The manufacturing process of Cr₃C₂-NiCr powders also plays a significant role in coating performance. A study comparing two powders, Woka 7302 and Praxair 1375, revealed substantial differences in morphology and phase composition. Coatings produced from Woka 7302 powder, which has a denser microstructure, showed reduced decarburization, lower oxide formation, and higher strength than those produced from Praxair 1375. Additionally, variations in the O₂/H₂ ratio in the HVOF torch influenced the Young’s modulus, increasing it from 0.40 to 0.50, and enhanced coating strength while having minimal impact on Vickers hardness [118].

S. Mahade et al. investigated Cr₃C₂-NiCr as a potential alternative to WC-Co coatings for automotive brake disks, focusing on its environmental benefits. Coatings deposited using HVAF and HVOF methods were analyzed for microstructure, phase composition, mechanical properties, and porosity. Wear tests with alumina balls under different loads (5, 10, and 15 N) revealed that HVAF coatings performed better under severe conditions, while HVOF coatings were more effective under milder conditions (Figure 12). The study provided detailed insights into wear degradation mechanisms, contributing valuable data for the development of wear-resistant microstructures [119].

Furthermore, surface roughness measurements using an optical profiler showed that HVAF-deposited coatings exhibited slightly higher roughness than HVOF-deposited coatings [119].

The cyclic impact testing of Cr₃C₂-NiCr coatings produced by HVOF, as studied by Bobzin, K. et al., revealed variations in failure mechanisms under different load conditions (300–1000 N). At 300–800 N, cohesive failure within the coating was observed, while at 1000 N, delamination occurred at the coating–substrate interface. Minor changes in crater volume were attributed to local plastic deformation and microabrasion, whereas significant material losses and segment delamination occurred at 800 N and above. These findings highlight the effectiveness of impact testing in evaluating the wear resistance of thermally sprayed coatings [120].

The effects of heat treatment on the performance of HVOF-sprayed Cr₃C₂-NiCr coatings were also explored. Coatings deposited on 304 stainless steel were heat-treated at 700–800 °C for 1 h to 16 days. Initially, the as-sprayed coatings contained Cr₃C₂, an amorphous phase, and a Ni-rich metallic phase. Upon heat treatment, Cr₇C₃ and Cr₂₃C₆ carbides formed at the coating–substrate interface, eliminating the amorphous phase. These carbides created a network, enhancing coating hardness after initial softening. Oxidation followed a parabolic rate law up to 800 °C, with an activation energy of 164 kJ/mol. Interdiffusion led to the formation of Cr₂₃C₆ carbides at the interface, and changes in microhardness with increasing distance confirmed carbon diffusion, aligning with theoretical models [121].

In summary, variations in Cr₃C₂-NiCr coating performance are influenced by the powder composition, deposition method, heat treatment, and testing conditions. HVOF coatings containing 80% Cr₃C₂ and 20% NiCr exhibited the highest hardness (1410 HV) and the lowest friction coefficient due to their high carbide content. Powder production significantly impacts morphology and phase composition, with Woka 7302 powders forming denser microstructures with reduced decarburization compared to Praxair 1375. HVAF coatings provide superior wear resistance under high loads, whereas HVOF coatings perform better under mild conditions. Additionally, modifications in O₂/H₂ ratios influence mechanical properties, while spraying distance affects CoF and wear resistance. Optimal coating selection must consider application conditions, deposition methods, and powder characteristics.

6. Analysis of Results of Experiments at High Temperatures

Jin D. et al. (2016) [122] investigated a 250 μm thick Cr₃C₂-NiCr coating deposited on a Zr-2.5Nb alloy using the HVOF method. Autoclave tests in air and water vapor at 700–1000 °C demonstrated the good adhesion of the coating, although the presence of micropores was noted. Compared to the uncoated alloy, the coating exhibited significantly higher oxidation resistance. However, cracks were observed at the interface with the substrate.

At 700 °C, the coating maintained its structural integrity, while at temperatures above 800 °C, chromium oxide nanoparticle formation was detected. Heat treatment at 1000 °C resulted in increased porosity, though the coating remained well adhered to the substrate even after exposure to 900 °C for 1 h. Importantly, no diffusion of elements between the coating and the alloy was observed.

In another study, AISI 1045 steel (American Iron and Steel Institute, Washington, DC, USA), was selected as the substrate, with dimensions of 60 mm × 30 mm × 10 mm. A Cr₃C₂-NiCr composite powder was used for coating, where Cr₃C₂ was encapsulated with NiCr to minimize carbon loss during the spraying process [123]. The NiCr-to-Cr₃C₂ ratio was 3:7, with a powder composition of 19.38 at% C, 75.87 at% Cr, and 4.74 at% Ni. The powder particles ranged in size from 25 to 30 μm and exhibited a spherical and porous morphology, which facilitated uniform heating and improved fluidity during deposition [124].

Unlike NiCr solid-solution phases, Ni and Al exist as separate phases and undergo reactions at high temperatures. To analyze the phase transformation of the NiAl coating under thermal exposure, XRD patterns of the NiAl bonding layer were examined after annealing at different temperatures, as shown in Figure 13. Initially, the main phases of the sprayed coating were Ni and Al. After annealing at 500 °C, new phases—AlNi₃, Al₃Ni₂, and AlNi—emerged, while the Al phase disappeared. As the temperature increased, the intensity of the NiAl phase grew, whereas the Al₃Ni₂ phase gradually diminished.

According to the Ni-Al binary phase diagram, nickel and aluminum begin reacting above 660 °C, generating significant heat. As the temperature rises, the atomic percentage of aluminum decreases, leading to the formation of nickel–aluminum compounds. Additionally, copper–aluminum compounds can be identified at diffraction angles of 20° and 83°, as shown in Figure 13 [125].

The cross-sectional morphology of Cr₃C₂-NiCr coatings is presented in Figure 14. The image reveals that the Cr₃C₂ phase is embedded within the NiCr binder phase, with some defects observed at the phase boundaries. After annealing at 500 °C, a noticeable reduction in internal defects was observed, leading to increased coating density. However, as the annealing temperature increased, secondary carbide phases began to form within the coating. At 700 °C, the secondary carbides grew significantly in size, indicating temperature-dependent microstructural changes.

Additionally, as shown in Figure 15, the hardness of the sprayed coating is approximately 924 HV. After annealing, the hardness increases due to the enhanced coating density. A further increase in hardness is observed with the precipitation of secondary carbides in the NiCr phase. However, as the size of the secondary carbide particles grows, the hardness value slightly decreases. The maximum hardness, around 1176 HV, is achieved at an annealing temperature of 600 °C.

Furthermore, the hardness variation curve indicates that at the NiCr-Cr₃C₂/bond layer interface, the hardness of the NiCr/NiCr-Cr₃C₂ composite coating decreases more than that of the NiAl/NiCr-Cr₃C₂ composite coating. However, at the bond layer/CuCrZr substrate interface, the hardness reduction in the NiAl adhesion layer is more pronounced compared to the NiCr adhesion layer. These findings suggest that the NiCr/NiCr-Cr₃C₂ composite coating exhibits element migration and changes at the NiCr-Cr₃C₂/bond layer interface, whereas in the NiAl/NiCr-Cr₃C₂ composite coating, significant element migration and transformation occur at the bond layer/CuCrZr substrate interface [125].

Figure 16 presents the cross-sectional morphologies and element distributions at the interface between the NiCr bonding layer and the CuCrZr substrate after annealing treatments. At 500 °C, no significant element diffusion is observed. However, at 600 °C, while a distinct diffusion zone is still absent, the interface appears increasingly blurred. When the annealing temperature reaches 700 °C, copper diffusion into the coating becomes evident, indicating the formation of a metallurgical bond at the NiCr bonding layer/CuCrZr substrate interface [125].

As shown in Figure 16, the corrosion and erosion of the sprayed coating lead to the formation of irregular black corrosion product areas (~20 μm) with microcracks. EDS analysis indicates that these corrosion products primarily consist of Ni/Cr oxides, with Cl concentrated in these areas, which accelerates coating degradation. After annealing at 500 °C, the corrosion morphology remains unchanged. However, at 600 °C, the affected areas decrease significantly (~5 μm), the surface becomes smoother with no visible microcracks, and Cl content is reduced, indicating an improvement in the coating’s protective properties. Beyond 600 °C, the corrosion-affected areas expand again (~12 μm), and small microcracks reappear. Thus, the optimal corrosion resistance of the coating is achieved at 600 °C [125].

In conclusion, annealing has different effects on the adhesion strength of gradient coatings. For NiCr/Cr₃C₂-NiCr, adhesion strength increases, whereas for NiAl/Cr₃C₂-NiCr, it decreases due to the absence of metallurgical bonding at the Cr₃C₂-NiCr/NiAl interface. In the Cr₃C₂-NiCr coating, the precipitation of secondary carbides enhances hardness and wear resistance, reaching a peak at 600 °C before declining. The tribological performance of the coating is largely independent of the bonding layer. The corrosion resistance of the coatings generally improves after annealing, particularly at 600 °C. However, for the NiAl coating, interfacial weakening and reactions between Ni and Al contribute to an increased corrosion rate [125] (see Figure 17).

A typical cross-section of the developed microwave-clad coating is shown in Figure 18. The microwave cladding process allows precise control over heating parameters, such as the power level and heating time, enabling the optimization of the cladding process. This control facilitates the formation of a refined microstructure while minimizing porosity, resulting in a dense, defect-free coating with improved bond strength. Compared to other cladding techniques like laser or thermal spraying, microwave cladding offers superior metallurgical bonding. Its unique volumetric heating, high heating and cooling rates, and precise temperature control contribute to enhanced coating performance, making it more efficient than alternative cladding methods [125].

The developed microwave coating demonstrates high adhesion to the substrate due to the partial mutual diffusion of elements, ensuring a gap-free interface. The wavy boundary (Figure 18a) results from vortex melt flows during microwave heating. The melting rate of both the coating and the substrate is influenced by melt dynamics. The coating is free from pores and interphase cracks, maintaining a defect-free structure. An enlarged view (Figure 18b) reveals a uniform distribution of hard chromium carbides within a soft nickel matrix [126]. Under microwave influence, nickel particles melt first, promoting structural homogeneity. Slow solidification prevents defects, leading to the uniform dispersion of strengthening phases such as primary chromium carbides and complex metallic carbides, which enhance the coating’s mechanical properties [127].

The volumetric heating effect is directly linked to hybrid microwave heating, which minimizes temperature gradients across the irradiated surface. The uniform carbide distribution in the coating may result from the slow solidification of the molten material. Unlike traditional methods, no cellular dendrites are observed in the developed microwave deposit, likely due to a uniform thermal gradient that prevents cellular structures from transitioning into dendritic forms. Similar uniform carbide distributions have been reported in other studies [128,129].

The differences in test results for Cr₃C₂-NiCr coatings are attributed to various factors, including the deposition method, heat treatment conditions, bonding layer composition, and failure mechanisms. HVOF-sprayed coatings exhibit high hardness and wear resistance; however, under increased load (1000 N), cohesive failure and delamination from the substrate occur. Heat treatment enhances coating density and hardness by precipitating secondary carbides, peaking at 600 °C. Beyond this temperature, interface weakening and increased porosity are observed. Corrosion resistance improves significantly after annealing, particularly at 600 °C, but higher temperatures induce reactions between coating components, such as Ni and Al, which degrade protective properties. As temperature increases, elemental interdiffusion intensifies, affecting mechanical and tribological characteristics. In contrast, microwave cladding provides a superior metallurgical bond, reduces porosity, and enhances coating adhesion to the substrate.

7. Conclusions

This study comprehensively analyzed the mechanical, tribological, and corrosion properties of Cr₃C₂-NiCr coatings, highlighting the influence of deposition methods, heat treatment, and bonding layer composition. The results demonstrated that optimizing spraying parameters and post-treatment conditions significantly enhances coating performance. Below are the key conclusions drawn from this review.

Comparison of Spraying Techniques and Their Impact on Coating Properties

• Differences in Cr₃C₂-NiCr coating performance stem from variations in spraying techniques (HVOF, laser cladding), initial powder morphology (spherical, agglomerated, nanomodified), and processing conditions;
• HVOF coatings exhibit high density, uniform phase distribution, and low porosity, contributing to superior wear resistance and adhesion strength;
• Laser cladding enhances coating structure by forming core–shell architectures and composite binder phases, leading to increased hardness and wear resistance;
• The incorporation of rare-earth metals (e.g., Ce) promotes grain refinement and further improves mechanical properties.

Effect of Deposition Method on Coating Performance

• The differences in test results for Cr₃C₂-NiCr coatings are attributed to various factors, including the deposition method, heat treatment conditions, bonding layer composition, and failure mechanisms;
• HVOF-sprayed coatings exhibit high hardness and wear resistance; however, under increased load (1000 N), cohesive failure and delamination from the substrate occur;
• In contrast, microwave cladding provides a superior metallurgical bond, reduces porosity, and enhances coating adhesion to the substrate.
• Influence of Temperature and Cooling Rate on Phase Composition
• The observed differences between Cr₃C₂-NiCr coatings obtained by HVOF and HVAF methods stem from variations in the temperature and cooling rate, which significantly impact phase composition and microstructure;
• High spraying temperatures promote the partial decarburization of Cr₃C₂, leading to the formation of Cr₇C₃ and Cr₂₃C₆, altering mechanical properties;
• The rapid cooling in HVOF promotes the formation of amorphous phases, increasing hardness but reducing ductility, whereas HVAF preserves the original crystalline powder structure, improving wear resistance;
• Prolonged high-temperature exposure leads to carbide coarsening, reducing binder plasticity and negatively affecting erosion resistance.

Effect of Powder Composition and Spraying Parameters

• HVOF coatings containing 80% Cr₃C₂ and 20% NiCr exhibited the highest hardness (1410 HV) and the lowest friction coefficient due to their high carbide content;
• Powder production significantly impacts morphology and phase composition, with Woka 7302 powders forming denser microstructures with reduced decarburization compared to Praxair 1375;
• HVAF coatings provide superior wear resistance under high loads, whereas HVOF coatings perform better under mild conditions;
• Modifications in O₂/H₂ ratios influence mechanical properties, while spraying distance affects the coefficient of friction and wear resistance.

Influence of Heat Treatment on Mechanical Properties

• Heat treatment enhances coating density and hardness by precipitating secondary carbides, with hardness peaking at 600 °C;
• Beyond this temperature, interface weakening and increased porosity are observed;
• Corrosion resistance improves significantly after annealing, particularly at 600 °C, but higher temperatures induce reactions between coating components, such as Ni and Al, which degrade protective properties;
• As temperature increases, elemental interdiffusion intensifies, affecting mechanical and tribological characteristics.

Optimization of Spraying Parameters for Enhanced Performance

• Optimizing spraying parameters, such as the powder feed rate and oxygen flow rate, can help balance hardness, fracture toughness, and corrosion resistance;
• HVOF is superior to other spraying methods due to its high particle velocity, providing a dense, wear-resistant coating with low porosity and minimal oxidation;
• Unlike plasma and arc spraying, HVOF reduces the risk of thermal damage to the substrate and provides better adhesion;
• Compared to gas thermal spraying, it forms denser coatings with high hardness;
• These characteristics make HVOF the optimal choice for wear and corrosion protection in aviation, power generation, and mechanical engineering.

Future Research Directions

• Further research in this direction can focus on developing new composite coating compositions and optimizing their spraying parameters to improve performance characteristics;
• Investigating the role of additional alloying elements and nanostructured powders may lead to coatings with enhanced durability and thermal stability for extreme environments.