Cold Spray Technology and Its Application in the Manufacturing of Metal Matrix Composite Materials with Carbon-Based Reinforcements

Abstract

Cold spray technology, as an emerging surface engineering technique, effectively prepares hard coatings by high-speed projection of powder materials onto substrates at relatively low temperatures. The principal advantage of this technology lies in its ability to rapidly deposit coatings without significantly altering the properties of the substrate or powder materials. Carbon-based materials, especially carbides and diamond, etc., are renowned for their exceptional hardness and thermal stability, which make them indispensable in industrial applications requiring materials with high wear resistance and durability at elevated temperatures. This review elucidates the fundamental principles of cold spray technology, the key components of the equipment, and the properties and applications of hard coatings. The equipment involved primarily includes spray guns, powder feeders, and gas heaters, while the properties of the coatings, such as mechanical strength, corrosion resistance, and tribological performance, are discussed in detail. Moreover, the application of this technology in preparing metal matrix composite (MMC) materials with carbon-based reinforcements, including tungsten carbide, boron carbide, titanium carbide, and diamond, are particularly emphasized, showcasing its potential to enhance the performance of tools and components. Finally, this article outlines the challenges and prospects faced by cold spray technology, highlighting the importance of material innovation and process optimization. This review provides researchers in the fields of materials science and engineering with a comprehensive perspective on the application of cold spray technology in MMC materials with carbon-based reinforcements to drive significant improvements in coating performance and broaden the scope of its industrial applications.

1. Introduction

The cold gas dynamic spray (CGDS) technology, also known as cold spray, is employed for depositing a variety of materials and has emerged as a promising technique in multiple material processing fields, including additive manufacturing, surface coating for corrosion and wear resistance, repair and restoration of damaged components, and the fabrication of functional and protective layers in the aerospace and automotive industries [1,2,3]. The cold spray process is fundamentally based on the energy exchange between the gas stream and the injected powder materials. The gas (N₂, He, or air) is accelerated to supersonic speeds through a converging/diverging nozzle, propelling the powder particles to velocities between 300 m/s and 1200 m/s [4,5]. Upon impact with the target surface (substrate), up to 90% of the kinetic energy of the particles is converted into heat, while the remaining energy facilitates viscoelastic and elastic deformation of the particles and substrate. The successful adhesion of the material depends on various phenomena triggered by high-strain-rate plastic deformation at the impact interface [6,7]. The strain and strain rates reported at the particle/substrate interface reach values of 10 and 10⁹/s [8], respectively, far exceeding those commonly observed in traditional powder manufacturing methods [9,10]. These extreme material deformation phenomena include fracturing of the surface oxide layer, localized melting, amorphization, diffusion, shear instability, and fluid dynamic plasticity, all of which are integral to the bonding process [11,12].

Due to its unique low-temperature process attributes, cold spray process technology prevents significant thermally induced phase transformations during the spraying process, effectively preserving the original microstructure and properties of the materials. This technology demonstrates considerable flexibility in designing coating compositions through raw material selection, allowing for the deposition of a wide range of coating types. Typical coating materials include ductile materials such as Cu [13,14], Al [15], Zn [16], Ni [17], Ti [18], Fe-based alloys [19,20], and Ni-based superalloys [21,22], metallic glasses [23], intermetallic materials [24], and ceramic particle-dispersed metal matrix composites (MMCs) such as WC-Co [25,26], SiC/Al [27], Cu–diamond [28], and cubic BN/NiCrAl [29]. Furthermore, ceramic coatings [30] can also be deposited through vacuum cold spraying or previously known aerosol deposition processes [31,32]. Due to its rapid deposition rate, high deposition efficiency, strong adhesion within the coating, and high bonding strength, cold spray technology offers the potential to produce coatings without thickness limitations, thus broadening its applications in the manufacture of standalone parts.

In recent years, the development of advanced coating techniques has garnered significant attention due to their applications in various industrial sectors. Among these techniques, cold spraying has emerged as a prominent method, offering several distinct advantages over traditional methods such as magnetron sputtering and electrodeposition. Cold spraying, a solid-state process, enables the deposition of coatings at relatively low temperatures, thereby avoiding issues related to high thermal stress and oxidation commonly associated with high-temperature processes. Magnetron sputtering, as demonstrated in the study by Sahu et al. [33], offers precise control over the composition and structure of thin films. However, it typically requires a vacuum environment and may involve higher operational costs and complexities. Similarly, electrodeposition [34] is a widely used technique due to its simplicity and cost-effectiveness, but it often faces limitations in coating uniformity and material versatility. In contrast, cold spraying provides the advantage of depositing a wide range of materials, including metals, alloys, and composites, without the need for a vacuum environment. This technique ensures high-density coatings with excellent mechanical properties, making it an attractive option for applications requiring robust and durable surfaces.

In various application domains, the fabrication of MMC materials with carbon-based reinforcements represents a significant branch of cold spray technology. These coatings, typically utilized to enhance material resistance to wear [35], corrosion [36], and high temperatures, are extensively applied in industries such as aerospace, automotive, mold manufacturing, and biomedicine. In the aerospace sector [37], the cold spray process is employed for the repair and enhancement of engine components and airframe structures. For instance, applying cold spray technology to aircraft engine blades significantly improves their corrosion and wear resistance, thereby extending their operational lifespan. One study [37] demonstrated that cold spraying WC onto Ti alloy blades substantially increased their hardness and wear resistance, thus enhancing overall flight efficiency and safety.

In the automotive industry [38], cold spray technology is utilized to manufacture or repair critical components such as pistons and bearings. Since cold spray treatment can increase surface hardness without altering the substrate material’s properties, it aids in enhancing the durability and performance of these components. Notably, when manufacturing pistons for high-performance racing vehicles, it can increase heat resistance and impact resistance while maintaining lightness.

The application in mold manufacturing, a core part of the precision manufacturing industry, involves crucial industrial sectors like automotive, aerospace, and consumer electronics [39]. Molds often endure high pressures and temperatures during use, requiring high wear resistance and corrosion resistance. The application of cold spray technology in this field is primarily reflected in extending mold life and performance. Cold spray treatment can apply one or more layers of hard materials on the mold surface such as WC or Cr-N, which possess high hardness and excellent wear resistance characteristics. By adding such hard coatings, significant reductions in mold wear during production can be achieved, extending maintenance intervals, thereby reducing downtime and production costs.

In the biomedical industry, cold spray technology is predominantly used for surface treatment of medical devices and implants to enhance their biocompatibility, corrosion resistance, and wear resistance [40]. Medical implants, such as artificial joints and bone scaffolds, require high corrosion resistance and biocompatibility. Cold spray technology can coat these implants with Ti or Ti alloys, materials known for their excellent biocompatibility and corrosion resistance. Ti coatings can also promote the integration of implants with human bone, enhancing the stability and lifespan of the implants.

In terms of enhancing wear resistance, cold spray technology can effectively form high-density, low-porosity coatings on target surfaces without compromising the structural integrity of the material itself [41]. The coatings typically employ materials with high hardness, such as WC, Al, or Ti alloys. Through cold spraying, these materials are propelled in powder form at high velocities to impact the substrate and mechanically bond to the surface, thereby significantly enhancing the coating’s wear resistance. For example, WC is a material with extremely high hardness, commonly used in applications requiring exceptional wear resistance, such as surface treatments for mining and machining equipment.

Regarding corrosion resistance, cold spray can be applied for the protection of metal substrates, particularly suitable for equipment protection in the marine and chemical industries [42]. By employing corrosion-resistant materials like pure aluminum, zinc, or copper, cold spray coatings effectively isolate environmental factors such as salt spray, moisture, and chemical corrosives, preventing them from chemically reacting with the substrate material. Moreover, cold spray technology can provide a robust protective layer without altering the properties of the substrate material, demonstrating greater adaptability, especially when dealing with complex or irregular surfaces.

For enhancing high-temperature resistance, cold spray technology also shows promising applications [43]. By using ceramic materials such as zirconia, a protective layer can be formed on the substrate material that remains stable in high-temperature environments, preventing damage due to oxidation and thermal expansion at high temperatures. This type of coating is particularly suitable for the aerospace and energy sectors, such as for the protection of turbine blades and combustion chambers, significantly improving the performance and reliability of these components under extreme temperature conditions.

This paper provides a comprehensive review of the mechanisms of microstructural evolution during the cold spray deposition process and its impact on the mechanical properties of the coatings. After a brief introduction to the fundamental working principles and components of the cold spray system, the review sequentially discusses the microstructural characteristics of the coatings, the bonding strength of the coatings, performance features of the coatings, and typical applications of hard coatings. A detailed examination follows, presenting the current state of applications for MMC materials with carbon-based reinforcements based on WC, B₄C, TiC, SiC, diamond, and other hard materials. Finally, the paper concludes with a summary of the content discussed and explores the latest research advancements and potential future directions for cold spray technology in materials science.

2. Technical Introduction

2.1. Cold Spray Principle

The principle of cold spray technology involves the acceleration of solid particles to supersonic speeds (300–1500 m/s) through a high-temperature, high-pressure gas stream facilitated by a de Laval nozzle. Solid particles, introduced into the accelerating gas stream via a powder delivery system, are accelerated and impact the substrate surface. The impact leads to severe plastic deformation, producing an adiabatic shear instability effect that results in the formation of a coating, as illustrated in Figure 1 [44]. Unlike traditional thermal spray techniques, the distinct characteristic of cold spray is its relatively low spraying temperature, which minimally affects the thermal properties of both the particles and the substrate. This allows for the fabrication of metastable and thermally sensitive materials. Over more than three decades, cold spray technology has evolved towards material diversification, technological complexity, and high-end application development.

2.2. Device Composition Cold Spraying

For the successful deposition of coatings, cold spray equipment primarily consists of several key components: the spray gun, powder feeder, gas heating device, high-pressure gas source and piping, control and operation system, robotic arm for holding the gun, and other auxiliary devices, as illustrated in Figure 1. The roles and principles of each component are as follows.

2.2.1. Spray Gun [45]

Within the cold spray system, the spray gun serves as the central component of the entire apparatus. Powder particles are mixed with the accelerating gas within the spray gun system and are accelerated to a specified velocity within the nozzle of the spray gun, impacting the substrate to form a coating. The spray gun system is designed to facilitate the high-speed and uniform ejection of powder particles, while also being convenient to manufacture and cost-effective. The system includes a converging–diverging-type nozzle, known as a Laval nozzle (or de Laval nozzle), which accelerates the gas from subsonic to supersonic speeds. Based on the downstream geometry, nozzles can be categorized into conical and bell-shaped types; additionally, the downstream cross-sectional area of the nozzle may be rectangular, circular, or elliptical.

2.2.2. Powder Feeder [45]

The powder feeder is an essential component of the powder delivery system, used to load the powder and transport it to the spray gun at a specified feed rate according to the requirements of the spraying process. The powder feeder must ensure stable delivery of the powder. Based on different feeding principles, powder feeders can be classified into various types: gravity feed, atomization feed, screw feed, rotary disk feed, scraper feed, capillary feed, and drum wheel feed. Currently, commercial powder feeders include Praxair’s 1264HP high-pressure feeder, Germany’s Impact Innovations’ interchangeable high-pressure feeder, and Japan’s Plasma Giken’s high-pressure feeder.

2.2.3. Gas Heater [45]

The gas temperature supplied from high-pressure sources generally does not exceed room temperature, thus the powder acceleration effect is not significant. Therefore, to ensure full expansion and enhance the acceleration effect of the gas, a safe heater is required to adequately preheat the working gas.

2.2.4. High-Pressure Air Source Device [45]

In selecting gases, their acceleration effects are primarily considered, followed by factors such as cost, safety, and reactivity. Hydrogen offers the best acceleration effects but is flammable and explosive and currently not usable; helium is second in terms of acceleration effect but is expensive; high-pressure compressed air is the most economical, though its acceleration effect is moderate, and its higher oxygen content can lead to oxidation of the heating tubes and deposited coatings; nitrogen is cheaper than helium and offers better acceleration effects, hence it is widely used as the working gas in most cold spray experiments. The gas supply can be from single or multiple high-pressure gas cylinders, high-pressure air compressors, or liquid nitrogen storage tanks (equipped with booster pumps and vaporizers). Helium is usually stored in fixed high-pressure cylinders, but due to its production costs being up to 50 times higher than nitrogen, using it as a routine gas in cold spraying is impractical. LERMPS laboratory in France has established a helium recycling system in their cold spray experiments for helium circulation, purification, and reuse.

2.2.5. Control and Operating System [45]

The cold spray control cabinet consists of a programmable controller, control circuits, and control pipelines for conveying the working gas. It primarily controls the working gas pressure and temperature, while also coordinating the operation of the touchscreen and controlling other peripheral devices, thus completing complex logic control functions.

Cold spray experiments can be set, recorded, displayed, and monitored through the control interface, which checks all functional units of the cold spray system and the operating conditions of the equipment, including working gas pressure and temperature, working gas flow, powder feed flow, powder feed rate, cooling water flow, and cooling water temperature.

2.2.6. Gun Manipulator [45]

The robotic arm typically requires 6-axis capability and must have strong load-bearing capacity and extensive coverage. Well-known commercial robotic arm manufacturers include KUKA, ABB, and FANUC, all of which offer models that can accommodate cold spray guns.

2.2.7. Other Auxiliary Devices [45]

Other auxiliary devices include water cooling for the spray gun, powder preheating devices, work turntables/clamping systems, ventilation and dust removal systems, and facilities such as sandblasting rooms, soundproof rooms, and tool rooms.

2.2.8. Key Control Parameters

The quality of cold spray coatings is influenced by many factors, broadly categorized into three aspects [46]:

(1)

The carrier gas parameters used for heating and accelerating the powder, such as chamber pressure, temperature, and type of gas.

(2)

Parameters of the cold spray nozzle, including the length of contraction and expansion, shape, material, throat area, and exit area.

(3)

Powder and substrate parameters, such as powder particle size, shape, density, thickness of surface oxides (or hydroxides), mechanical and thermal properties of the powder material, mechanical and thermal properties of the substrate material, substrate surface roughness, and substrate surface temperature [47].

2.3. Coating Structure Characteristics

The structural characteristics of coatings formed through cold spray deposition play a pivotal role in their overall performance and application. Cold spray deposition involves the impact and consolidation of solid particles, resulting in unique microstructures characterized by significant deformation and fragmentation. This section delves into the key microstructural features of cold-sprayed coatings, including grain refinement, recrystallization, phase transformations, and porosity, as well as the impact of deposition parameters on these characteristics.

It is well known that the microstructure of materials processed by cold spray deposition, either through upstream or downstream injection, originates from the deposition of solid particles. Therefore, their microstructures are characterized by particle fragmentation and significant deformation upon impact, with a fine-grained structure typically observed within the splats [48]. Figure 2a displays the splat-like microstructure obtained by Sirvent et al. [49] using a high-pressure cold spray (HPCS) system to deposit Al-Cu-Mg at 350 °C and 3.75 MPa. The pancake-like morphology is produced by sheets featuring equiaxed grains internally and elongated grains at the boundaries. Figure 2b depicts the presence of equiaxed grains within the interior of the plate. The appearance of elongated grains at the plate boundaries indicates that the powder underwent significant plastic deformation during the HPCS process. The microstructural configuration of the splat-like coatings reveals material homogeneity related to the stacking process, depending on the characteristics of the powder.

Cold spray deposition occurs in the solid state, and the phases present in the original raw material particles are generally retained without being oxidized or undergoing other deleterious reactions. Strain and heating rates can reach approximately 10⁹/s and 10⁹ K/s [50,51], respectively. These extreme conditions can promote grain refinement, strain hardening, and phase transformations, producing distinctive characteristic microstructures. Grain refinement occurs due to dynamic recrystallization. After the particles impact the substrate, intense shear stresses deform the impact zone. Consequently, recrystallization initially occurs at the particle–particle interfaces. When adiabatic shear instability (ASI) occurs, material flow and dislocation entanglement form walls, networks, and eventually subgrain boundaries [49,52]. The extent of recrystallization and the size of the formed subgrains depend on the material type and particle velocity. Thus, cold spray parameters, including temperature and pressure, influence recrystallization. Sirvent et al. [49] assessed the impact of temperature on cold spray deposition of Al-Cu-Mg-Mn using transmission electron microscopy (TEM) and observed that subgrain size increases with the spraying temperature, suggesting that materials deposited at lower temperatures form microstructures with less distortion (Figure 3).

Dynamic recrystallization can be regarded as a strain-mediated process. However, under the influence of high plastic deformation, additional mechanisms may be activated, leading to the formation of twinning phenomena in materials with relatively low stacking fault energy. Furthermore, strain can also be adjusted through solid-state phase transitions, facilitating the formation of amorphous and disordered structures [53]. These structures significantly influence the tribological properties of materials, especially when the formation of a mechanical mixing layer (MML) dominates the wear mechanisms.

During the impact process, interactions between particles or between particles and the substrate induce the formation of submicron structures at the platelet interface or substrate–platelet interface. These structures, formed under non-equilibrium conditions, are crucial for controlling the adhesion and cohesion of coatings. The mechanical performance and structural integrity of the coatings are significantly affected by these interactions, including those promoting localized melting and those cleaning the interfaces. Although obtaining direct evidence of melting presents challenges, the melting phenomena might be confined to minute interaction zones. The impact and deformation of particles might disrupt or remove contaminants or oxide layers on the powder surface [48]. Due to the high plastic deformation experienced by particles during the cold spray deposition process, the coating may exhibit high hardness and high dislocation density similar to work-hardened alloys. The enhanced hardness of cold-sprayed materials might influence their mechanical performance and tribological behavior, particularly in scenarios where wear mechanisms are predominantly controlled by plastic deformation [49,52].

Pursuing completely dense coatings is preferred. Consequently, porosity emerges as a frequently analyzed key microstructural feature in cold spray materials. The formation of porosity is attributed to insufficient kinetic energy of the particles or plastic deformation during the impact process. Porosity is typically observed at the interfaces between platelets and on the surface of the coating [54]. As particle velocity increases, the porosity decreases. Therefore, meticulous control of parameters such as pressure and temperature is essential to optimize the porosity of cold spray materials. Figure 4 presents optical images illustrating the variation in porosity within 316L stainless steel deposited using HPCS technology at temperatures of 900 °C and pressures of 5, 6, and 7 MPa [55]. It is evident that an increase in pressure accompanies a reduction in porosity.

The microstructural characteristics developed during the cold spray process significantly influence the mechanical behavior of the coatings. Key aspects of these characteristics include phase evolution, grain morphology, and their effects on mechanical performance. Phase evolution reveals the transformation behavior of materials during deposition, impacting phase composition and mechanical properties, such as enhanced hardness and wear resistance. Grain morphology, encompassing grain size, shape, and distribution, directly affects the coating’s mechanical properties. High-velocity particle impacts during cold spraying leads to severe plastic deformation, resulting in a fine, uniform grain structure that enhances strength, hardness, fracture toughness, and fatigue performance. Additionally, microstructural characteristics like residual stress state, porosity, and interfacial bonding strength significantly influence mechanical behavior. Understanding these aspects is crucial for optimizing the cold spray process and developing new materials. Detailed microstructural characterization, focusing on phase evolution and grain morphology, provides insights into improving the mechanical properties and performance of cold spray coatings.

The microstructural characteristics of cold spray coatings, influenced by particle deformation, recrystallization, and deposition parameters, are crucial for their performance. Proper control of temperature, pressure, and particle velocity is essential to optimize these structural features, reducing porosity and enhancing hardness. Understanding these microstructural dynamics aids in developing high-performance coatings for various industrial applications.

2.4. Coating Adhesive

Coating adhesive strength is a crucial determinant of the performance and longevity of coated materials. Various studies have focused on assessing and enhancing the adhesive properties of cold spray coatings, employing different methodologies and exploring the influence of numerous parameters. This review examines key research efforts that delve into innovative testing techniques and the critical factors affecting adhesive strength in cold spray applications.

White et al. [56] discussed the assessment of the adhesive strength of cold spray Al-Zn-Mg-Cu deposits using fracture mechanics methods, specifically comparing these to the conventional ASTM C633 adhesion strength test. The study highlights the limitations of the ASTM C633 test, primarily influenced by the cohesive strength of the epoxy resin used, which often restricts the measurable strength of the bond. By employing compact tension (CT) and mixed-mode four-point bending tests, the research revealed more comprehensive adhesion strength values, unhindered by the constraints of the ASTM C633 test. The interface CT test indicated that adhesive strength varied linearly with substrate roughness, suggesting a complex interaction between mechanical interlocking and metallurgical bonding influenced by surface roughness. Concurrently, the four-point bending tests demonstrated higher interface toughness values (Figure 5 illustrates the geometry of the notched-interface four-point bending test specimens), although they did not show a clear dependence on surface roughness. This discrepancy highlights the impact of mechanical stresses and residual stresses, which significantly alter the measured adhesion values.

The research by Huang et al. [57] focuses on developing coatings with ultra-high adhesive strengths using the cold spray process. The study found that effective bonding between copper coatings and various substrates such as aluminum and stainless steel can be achieved at specific particle velocities, which is crucial for the cold spray method. The adhesion strength largely depends on mechanical interlocking and metallurgical bonding at the interface, influenced by the velocity and temperature of the sprayed particles. Higher particle velocities lead to more intense plastic deformation and embed the particles into the substrate, thus enhancing the mechanical interlocking essential for strong adhesion. The study concludes that such ultra-strong adhesive strengths can be achieved by optimizing particle velocities, providing valuable insights for industrial applications requiring robust coating adhesion.

Boruah et al. [58] examine an improved adhesive-free testing method tailored to assess the interfacial adhesion strength of high-strength cold-sprayed Ti6Al4V coatings. This method aims to overcome the limitations of conventional adhesive tests, enabling precise measurement of adhesion strengths exceeding 70–90 MPa, which surpasses the typical limits of traditional tests. The study employs a parametric approach to evaluate the influence of various factors such as coating thickness, scanning speed, track spacing, tool path patterns, and substrate surface treatment on adhesion strength. The findings indicate that certain conditions, such as specific surface treatments and optimized spraying parameters, can significantly enhance the bonding strength between the coating and the substrate. These results underscore the critical role of process parameters in achieving robust interfacial bonding in cold-sprayed coatings, which is vital for the effective repair of high-value metal components.

In the research conducted by Bruera et al. [59], the adhesion performance of cold-sprayed copper coatings on stainless steel substrates with varying degrees of roughness and hardness was assessed using a comprehensive set of tests. The method combined surface treatments including polishing and sandblasting, followed by vacuum annealing to tailor the substrate’s properties. The coating’s adhesion was evaluated through single-particle deposition experiments and complete bond strength tests. The results demonstrated that substrate roughness and hardness significantly influenced the adhesion of copper particles, with rougher and softened surfaces exhibiting superior adhesion compared to hardened surfaces. This enhanced bonding was attributed to improved mechanical interlocking and potential metallurgical bonding, underscoring the critical role of substrate preparation in optimizing coating adhesion for industrial applications.

Goldbaum’s study [60] investigated the impact of deposition conditions on the adhesion strength of Ti and Ti6Al4V cold spray coatings. This study utilized micromechanical testing techniques to measure adhesion strength. Tested parameters included deposition velocity, powder size, particle position within the gas jet, gas temperature, and substrate temperature. The findings indicated that increased deposition velocity significantly improved bonding strength due to higher levels of plastic deformation and thermal effects at the interface, thus enhancing mechanical interlocking and metallurgical bonding. The results suggest that optimized spray conditions can facilitate continuous adhesion along the interface, with bond strengths approaching those of the bulk material’s shear strength, emphasizing the importance of precisely controlled spraying parameters to optimize the adhesion strength of cold spray coatings.

The reviewed research underscores the importance of advanced testing methods and optimized parameters in improving the adhesive strength of cold spray coatings. These findings highlight the critical role of particle velocity, surface roughness, and substrate preparation in achieving robust adhesion. The insights gained are vital for developing high-performance coatings for industrial use.

2.5. Coating Properties of Cold Spray Coatings

2.5.1. Mechanical Properties

The mechanical properties of cold-sprayed coatings are crucial for their application and performance. Various studies have explored the hardness, tensile strength, and adhesion of different coatings, revealing how deposition parameters and material compositions influence these properties.

Lioma et al. [61] employed experimental design methods to study the hardness of cold-sprayed WC-Ni and WC-12Co-Ni coatings on a low-carbon steel substrate. They observed that during the formation of the coatings, there was a low porosity and well-refined grain structure, with a minimal percentage of Ni, indicating that, compared to other coatings, WC-4wt%Ni and WC-12Co-4wt%Ni exhibited higher hardness. Kumar et al. [62] analyzed the results of applying Ni-20wt%Cr nanostructured powder particles of various sizes and compositions on SAE-213-T22 and SA 516-Grade 70 steel substrates in a boiler environment under cold spray conditions. Compared to traditional boiler components, the coatings on both SA 516-Grade 70 and SAE-213-T22 showed higher hardness and the presence of Cr₂O₃ phases, thus endowing the coated steel with excellent erosion and corrosion resistance. Ma et al. [63] studied the mechanical behavior of Inconel 718 coatings, which were deposited using high-velocity powder deposition technology on an Inconel 718 substrate under different pressures and temperatures, with helium and nitrogen as treatment gases. EBSD and SEM studies revealed the microstructure of the coatings. Mechanical properties, such as hardness, tensile strength, and adhesion were significantly improved under the helium-treated Inconel 718 alloy coating conditions at 435.4 MPa. Moreover, compared to the coatings treated with nitrogen, a thermal treatment at 990 °C enhanced the performance to 899.4 MPa. Figure 6 presents the stress–strain curves for cold-sprayed Inconel 718 coatings, applied according to ASTM E8 standards, under conditions of 3 MPa and 1000 °C with helium and 5 MPa and 1000 °C with nitrogen.

Chang et al. [64] examined the mechanical and electrical properties of cold-sprayed pure copper and Cu-Cr powder deposited on copper substrates, with varying percentages of Cr (13.06%–16.01%). The structural characteristics of the coatings were analyzed using scanning electron microscopy. They discovered that the copper coatings had good conductivity, 73.28% according to the International Annealed Copper Standard. Due to the superior interfacial adhesion between the Cu-Cr particles and the copper substrate, which reduced porosity and formed a metallurgical bond, the wear resistance and hardness of the Cu-Cr coatings were enhanced (145.7 HV). Garrido et al. [65] explored the application of cold-sprayed Ti6Al4V coatings on bulk Ti6Al4V substrates at two treatment temperatures, 800 °C and 1100 °C, followed by post-thermal treatment processes. The performance of the coatings was analyzed using SEM and EDX. The hardness of the coatings was measured using a Vickers microhardness tester. The mechanical properties of the Ti6Al4V-1100 °C coatings post-thermal treatment was enhanced, but under spray conditions, the Ti6Al4V-1100 °C coatings exhibited higher hardness (3.54 GPa) compared to both the post-treated and the sprayed Ti6Al4V-800 °C coatings (3.27 GPa and 3.22 GPa, respectively). Due to the impact conditions of temperature and pressure at 40 bar and 50 bar, as pressure and temperature increased during the coating process, the powder particles underwent perfect plastic deformation, resulting in excellent adhesive behavior between the particles and the substrate.

Cold-sprayed coatings exhibit enhanced mechanical properties such as hardness, tensile strength, and adhesion, influenced by factors like material composition and deposition conditions. Proper optimization of these parameters is essential for achieving coatings with superior mechanical performance.

2.5.2. Corrosion Behavior

The corrosion behavior of cold-sprayed coatings is a crucial factor determining their durability and performance in various environments. This section reviews studies on the corrosion resistance of different cold-sprayed materials, highlighting the impact of coating composition and deposition parameters.

Mohammad Dia [66] examined the effects of pure aluminum cold spray coatings on the corrosion and corrosion fatigue life of magnesium (3wt%Al–1%wtZn) extrusions in their study titled “The effect of pure aluminum cold spray coating on corrosion and corrosion fatigue of magnesium (3%wtAl–1%wtZn) extrusion.” The study noted that the coated samples exhibited improved corrosion resistance due to the low porosity within the coating and the formation of a protective oxide layer. However, the fatigue life of the coating in an NaCl solution was longer compared to in air. Kalsi et al. [67], in their research on “Performance of cold spray coatings on Fe-based superalloy in Na₂SO₄-NaCl environment at 900 °C”, investigated the corrosion behavior and hardness of cold-sprayed NiCrAlY and NiCoCrAlY on the high-temperature alloy Fe-based 800 H. Corrosion tests were conducted at 900 °C in a solution of Na₂SO₄ and 10% NaCl. The coatings provided increased hardness, approximately 576–636 HV for NiCrAlY and 582–640 HV for NiCoCrAlY. The corrosion performance of the coatings was assessed using energy-dispersive spectroscopy, X-ray diffraction, scanning electron microscopy, and X-ray imaging, with the NiCoCrAlY coating showing superior corrosion resistance over the NiCrAlY coating. Silva et al. [68] analyzed the mechanical, electrochemical, and wear characteristics of cold-sprayed Al and Al-Al₂O₃/Al coatings on low-carbon steel in their work “Corrosion characteristics of cold gas spray coatings of reinforced aluminum deposited onto carbon steel”. The coatings were characterized and their corrosion behavior was analyzed using SEM, Tafel plots, Bode plots, and Nyquist plots. They reported improved mechanical and wear performance in the Al-Al₂O₃/Al coatings due to excellent interfacial interlocking between the metal and ceramic particles on the coating surface. The Al-Al₂O₃/Al coatings showed enhanced salt spray corrosion resistance, protecting the substrate for up to 3000 h, as the electrolyte could not penetrate into the substrate due to the dense nature of the coating. Figure 7 depicts electrochemical impedance spectroscopy (EIS) plots, illustrating the impedance behavior of aluminum-based coatings immersed in chloride solutions over varying durations.

Singh et al. [69] studied the corrosion behavior of cold-sprayed Ni-20Cr, Ni-20Cr-TiC, and Ni-20Cr-TiC-Re coatings on an SAE 213-T22 steel substrate under high-temperature conditions in his research “Effect of additions of TiC and Re on high temperature corrosion performance of cold sprayed Ni-20Cr coatings”. The electrochemical performance of the coatings was evaluated in a 900 °C sodium sulfate solution containing 60 wt% V₂O₅. The corrosion surface of the coatings was analyzed using FE-SEM/EDS. The results indicated that the porosity ranges of the Ni-20Cr-TiC-Re coatings were 1.5–1.6%, while the Ni-20Cr-TiC-Re coatings ranged from 1.60 to 1.61%. Al-Mangour et al. [70] evaluated the corrosion behavior of cold-sprayed cobalt-chromium alloy L605 powders, depositing varying volumes of Co-Cr (25%, 33.3%, and 50%) on low-carbon steel substrates. The annealing of these coatings was carried out at 1100 °C to enhance their corrosion performance. Dynamic potential polarization experiments conducted in sodium chloride solution demonstrated that the 33.3% Co-Cr coating exhibited improved corrosion behavior in both sprayed and annealed samples compared to other coatings. Bara et al. [71] investigated the mechanical and electrochemical properties of various types of NiCr, NiCrTiC, and NiCrTiCRe powders coated on SA 516 steel using cold spray. The characteristics of these coatings were analyzed using X-ray diffraction (XRD) and field-emission scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS). To simulate boiler conditions, both coated and uncoated samples were maintained in a heated zone for 15 cycles. Their findings indicated that the NiCrTiCRe coating exhibited a higher hardness, of approximately 243–336 HV, and improved corrosion resistance due to the formation of a Cr₂O₃ layer, compared to other coatings. Chavan et al. [72] from New Mexico evaluated the corrosion performance of low-carbon steel coated with zinc powder through cold spray technology. According to their experimental results, the electrochemical behavior of the zinc coatings was superior due to excellent inter-particle bonding, and the presence of fewer pores and cracks within the microstructure of the coating, thereby providing an effective corrosion barrier to the low-carbon steel. Additionally, a heat treatment at 150 °C also significantly enhanced the coating’s corrosion resistance. Dzhurinskiy et al. [73] studied the corrosion behavior of various pure Al, 75Al-25Al₂O₃, 50Al-50Al₂O₃, 30 Al-50Al₂O₃-20Zn, and 35Al-40Al₂O₃-25Zn powders deposited on Al-Cu-Mg alloy materials using kinetic spraying. Electrochemical studies were conducted using open-circuit potential and dynamic potential polarization. The presence of Al₂O₃ increased the hardness of the coatings. The corrosion resistance of the Al-Al₂O₃-Zn coatings was found to be superior to that of Al-Al₂O₃ coatings due to the influence of the Al-Zn galvanic couple on the corrosion performance of the coatings. Koivuluoto et al. [74] examined the corrosion behavior of Ni, NiCu, and NiCu-Al₂O₃ coatings on carbon steel, applied using HPCS under various spraying parameters, in environments of sulfuric acid, sodium chloride, and hydrogen chloride. The electrochemical behavior was also analyzed using Tafel plots. Both the Ni and NiCu coatings showed improved corrosion resistance. Heat-treated coatings provided further enhanced corrosion resistance compared to coatings that were only sprayed. Zhou et al. [75] explored the electrochemical behavior of cold-sprayed hydroxyapatite/Ti (HAP/Ti) composite coatings, containing 20 wt% and 50 wt% HAP, on Ti substrates for medical applications. Post-coating, the samples underwent heat treatment and electrochemical testing. The study revealed that the 20 wt% HAP/Ti composite coating exhibited enhanced corrosion resistance compared to the Ti substrate under sprayed conditions. Additionally, subsequently heat treatment, which also increased the hardness of the coatings, significantly improved their corrosion resistance. Zhu et al. [76] assessed the mechanical, electrochemical, and conductivity properties of Al-Zn-Cu composite coatings on Al-Mg-Si alloy samples, with varying copper contents (25, 30, and 35 wt%). The microstructure of these cold-sprayed coatings was analyzed using scanning electron microscopy and energy-dispersive spectrometry. The results indicated that in coatings with 35 wt% copper in the Al-65wt%Zn matrix, the copper content significantly enhanced the deposition efficiency, hardness, electrochemical performance, and electrical conductivity of the coatings. Yang et al. [77] evaluated the electrochemical performance of cold-sprayed Al-Mg-Si/Al₂O₃ MMCs coatings with varying Al₂O₃ particle contents (20%, 40%, 60%) on ZM5 magnesium alloy. The corrosion behavior was assessed through cyclic polarization and electrochemical impedance spectroscopy tests. The findings indicated that minimal content of Al₂O₃ particles in the coating improved corrosion performance, whereas a higher Al₂O₃ content, that did not facilitate mechanical interlocking between the matrix and the reinforcement, led to deteriorated corrosion resistance. Figure 8 displays the linear polarization curves for Al-Mg-Si and Al-Mg-Si/Al₂O₃ composite coatings after immersion in a 3.5 wt% NaCl solution for durations of 1 h, 5 h, 12 h, and 24 h.

Zhang et al. [78] investigated the mechanical and corrosion properties of industrial pure Al-reinforced, Al-Cu-Mg-reinforced, Al-Mg-Si-reinforced, and Al₂O₃-reinforced powders deposited on Al-Cu-Mg substrates via a low-pressure cold spray system. Electrochemical corrosion experiments were conducted in a 3.5% NaCl solution. The corrosion behavior was analyzed using the scanning vibrating electrode technique along with Tafel plots, Bode plots, and Nyquist diagrams. The experimental analysis revealed that the Al-Mg-Si coating exhibited superior hardness, adhesion, and corrosion resistance. Wang et al. [79] studied the electrochemical properties of cold-sprayed SiC-particle-reinforced Al-Mg-Cr-Mn coatings with varying SiCp contents (15%–60%) on pure Al and Al-Mg-Cr-Mn substrates. Electrochemical testing was performed in an alkaline Na₂SO₄ solution. Corrosion studies of the coatings were analyzed using scanning electron microscopy and Tafel plots. The results showed that the SiCp/Al-Mg-Cr-Mn coatings significantly improved corrosion resistance due to the deposition of particles in a plastically deformed state, with a porosity of less than 1%.

Cold-sprayed coatings demonstrate enhanced corrosion resistance, significantly influenced by material composition and deposition conditions. Proper optimization of these factors is essential for achieving durable coatings suitable for various industrial applications.

2.5.3. Tribological Properties

The tribological properties of cold-sprayed coatings significantly influence their wear resistance and overall durability. This section reviews various studies that examine the wear behavior and mechanical properties of different cold-sprayed materials, focusing on the impact of material composition and processing conditions.

Spencer et al. [80] conducted a study on the wear and corrosion behavior of an Al-Si-Mg alloy with varying proportions (25%–75%) of Al₂O₃-particle-reinforced phases deposited on an Mg-Al-Zn alloy using the cold spray technique. The coated samples underwent post-spray heat treatment to enhance their corrosion resistance and hardness. The coatings were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was observed that the addition of Al particles improved both the electrochemical and tribological performance of the coatings. Zhang et al. [81] studied an Al-Mg-Si alloy’s surface coated with pure copper and copper–molybdenum disulfide (Cu-MoS₂) MMC coatings using cold spray. The coatings were subjected to wear tests over 1000 cycles. The results indicated that the Cu-MoS₂ coatings exhibited higher wear resistance compared to the pure copper coatings, attributed to the inclusion of ceramic particles which facilitated strong bonding between the coating and the substrate. Ling et al. [82] examined the tribological performance and hardness of graphite/copper–zinc composite coatings prepared using a low-pressure cold spray system. Graphite was incorporated at different weight percentages (10, 20, and 30) on brass plates. The wear tests were conducted using a pin-on-disc wear testing machine, and the microstructure and wear behavior were analyzed using SEM. The findings showed that coatings with 30 wt% graphite exhibited higher hardness and wear resistance due to the dense and durable bond formed in the coatings with over 20% graphite content.

Baiamont et al. [83] evaluated the wear and hardness behavior of cold-sprayed and plasma transferred arc (PTA)-welded Stellite 6 coatings on AISI 304 stainless steel substrates. The mechanical properties and wear performance were analyzed using a Vickers microhardness tester and a pin-ring wear tester. They reported that the cold-sprayed Stellite 6 coatings exhibited higher hardness and better wear resistance than the PTA samples, attributed to the coatings’ low porosity and higher density.

Zhang et al. [84] demonstrated that mixing Ni or Zn powders into Cu-Al₂O₃ coatings via cold spray promotes the formation of dense composite coatings with low porosity. The addition of Ni significantly enhanced the microhardness of the Cu-Al₂O₃ coatings but did not improve their friction-reducing properties. The inclusion of nickel increased the density of the coating and improved corrosion resistance due to nickel’s inherent corrosion-resistant properties.

Padmini et al. [85] studied the wear properties of Inconel 625 coatings on SAE 213 T11 and T12 substrates using kinetic spraying. Wear performance was analyzed under various loads and temperatures using the pin-disc method, and the microstructure of the coatings was examined using EDAX and XRD. Both T11 and T22 exhibited increased hardness. Compared to the SAE 213 T12 substrate, the tribological performance of the SAE 213 T11 substrate was significantly enhanced.

A study on the mechanical and corrosion properties of Ti6Al4V coatings by N.W. Khun et al. [86] examined the deposition of Ti6Al4V powder on a 9 mm thick Ti64 alloy plate using high-velocity powder deposition technology. The microstructure of the coating was analyzed using scanning electron microscopy (SEM). The results revealed that the Ti6Al4V coating exhibited higher hardness and wear resistance. However, the presence of porosity within the coating resulted in lower corrosion resistance compared to the Ti64 substrate.

Cong et al. [35] investigated the fabrication of Al/Al₂O₃ MMC coatings on steel substrates using low-pressure cold spray technology. The inclusion of ceramic particles significantly enhanced the tribological performance of the coatings. However, after 960 h of exposure to neutral salt spray and electrochemical tests, the corrosion resistance of the coatings decreased, which was attributed to the lower bonding strength of the coating.

Qiu et al. [87] analyzed the wear behavior of cold-sprayed MMC coatings made from A380 alloy powder with varying weight percentages of Al₂O₃ reinforcements on AZ31 magnesium alloy. The addition of Al₂O₃ significantly improved the tribological performance of the coatings, making it a favorable production coating for magnesium alloys. As the content of Al particles increased, the interfacial bonding strength between the matrix and the reinforcement also increased due to the dispersion effect of the Al particles, thereby increasing the wear rate of the coating.

Sun et al. [88] utilized HPCS technology to study the wear and mechanical properties of CoCrMo and Ti6Al4V coatings on Al-Mg-Si alloy samples. The coatings achieved higher adhesion strength with the substrate. Both types of coatings exhibited superior hardness, wear resistance, and corrosion resistance compared to the substrate. However, the CoCrMo coating demonstrated improved electrochemical, tribological, and mechanical properties compared to the Ti6Al4V coating.

Melendez et al. [89] explored the wear performance and hardness of cold-sprayed MMC coatings composed of 50WC + 50Ni, 75WC + 25Ni, 92WC + 8Ni, and 96WC + 4Ni (wt%) on a low-carbon steel substrate under low-speed cold spray conditions. Their findings indicated that with an increase in the WC content in the coatings, there was a notable enhancement in hardness and wear resistance compared to those produced by supersonic flame spraying and high-velocity cold spraying.

Seraja et al. [90] compared the outcomes of coatings applied on low-carbon steel using cold spray technology and high-velocity oxygen fuel (HVOF) techniques to evaluate oxidation and tribological mechanical behavior. The coatings were characterized using SEM, optical microscopy, and X-ray diffraction (XRD). The cold spray coatings did not undergo phase transformations, whereas more oxidation occurred in the HVOF coatings. As illustrated in Figure 9, compared to the samples coated using HVOF, the samples coated using cold spray exhibited superior wear and oxidation behavior.

Cold-sprayed coatings exhibit enhanced tribological properties, including increased hardness and wear resistance, influenced by material composition and processing techniques. For example, some research shows that both scratch width and depth values decrease with an increase in nanoindentation hardness, which indicates that the increase in the resistance to plastic deformation is associated with a decrease in the scratch width [33]. Optimizing these factors is crucial for developing durable coatings with superior performance in demanding applications.

2.5.4. Antibacterial and Antiviral Properties

The antibacterial and antiviral properties of cold-sprayed coatings have garnered significant interest for their potential in enhancing surface hygiene and reducing pathogen transmission. This section reviews various studies on the efficacy of cold-sprayed coatings, particularly focusing on copper and composite materials, in providing antimicrobial and antiviral protection.

Sundberg et al. [91] examined the microscale and nanoscale roughness of cold-sprayed copper surfaces using ultrafine-/fine-grain copper and nanostructured copper produced from spray-dried powders. Characterization was performed using atomic force microscopy (AFM) and three-dimensional confocal microscopy, aiming to provide a clearer, more coherent understanding of the enhanced antiviral activity of nanostructured copper cold-sprayed coatings compared to alternative copper-based antiviral surfaces and ultrafine-/fine-grain copper cold-sprayed surfaces.

Sanpo et al. [1,92] explored the antimicrobial properties of hydroxyapatite–silver doped with nanoscale silver particles embedded in PEEK, which were also applied via cold spray. The study indicated that as the concentration of hydroxyapatite–silver/PEEK nanocomposite increased, the antimicrobial effectiveness against Gram-positive bacteria significantly improved, with the copper addition to the chitosan showing a 70% reduction in E. coli, thereby demonstrating the feasibility of depositing biopolymers with copper on aluminum through cold spraying. In this work, the reduction in bacterial count achieved a seven-log decrease.

Hutasoit et al. [93] evaluated the antibacterial performance of cold-sprayed copper coatings against Staphylococcus aureus and Escherichia coli on stainless steel and aluminum substrates. The study found that the antibacterial efficacy varied with the characteristics of the copper coating, such as oxide content, thickness, and the electrochemical potential produced by different substrate materials. Specifically, coatings with a higher content of copper oxide exhibited lower antibacterial activity. Moreover, the electrocoupling effect between copper and the substrate significantly influenced the antibacterial performance, with copper/aluminum coatings showing immediate effects against Staphylococcus aureus, while copper/stainless steel coatings were more effective against Escherichia coli. The study highlighted that optimizing copper coating characteristics can enhance antibacterial surfaces.

Razavipour et al. [94] investigated the biofilm inhibition and antiviral response of copper surfaces treated via cold spray and shot peening techniques, focusing on the influence of surface morphology and microstructure. The study assessed the biofilm’s resistance to Pseudomonas aeruginosa and the antiviral efficacy against a slow virus model of SARS-CoV-2. It was found that the biofilm inhibition capabilities of copper surfaces were influenced by their microstructure and morphology, with treated surfaces initially exhibiting superior biofilm inhibition that gradually decreased over time, converging to the performance of untreated copper after 18 h. The shot-peened samples, due to their rough and ultrafine microstructures, showed enhanced biofilm control, especially after 18 h. The study also observed that surface morphology plays a crucial role in antiviral activity, with smoother surfaces exhibiting more effective immediate antiviral responses. The findings suggest that optimizing the characteristics of copper surfaces, including minimizing the content of copper oxide and considering electro-interactions, can enhance antimicrobial performance.

Sousa et al. [95] conducted a study focusing on the antimicrobial characteristics and pathogen contact killing/deactivation capabilities of cold-sprayed copper coatings, highlighting their potential to combat the spread of COVID-19 through deactivation of contaminants. The study discussed the application of copper cold-sprayed coatings on high-contact surfaces in biomedical and healthcare settings, aimed at preventing the spread of SARS-CoV-2 by rapidly deactivating viral particles upon surface contact. It emphasized the unique microstructural pathway mediated by copper ion diffusion within the copper cold-sprayed coatings, which effectively deactivates contaminants. The research highlighted copper’s microdynamic antimicrobial properties and detailed the development of copper-containing materials and coatings with enhanced antipathogenic efficacy. The conclusions suggest that optimizing copper cold-sprayed coatings could offer a strategic mitigation approach against COVID-19, enhancing public health safety by reducing the transmission of contaminants and potentially preventing future pandemics.

Saha et al. [96] developed a copper-coated rubber surface through cold spray technology to combat the spread of highly contagious viruses and bacteria, emphasizing its application on escalator handrails. The coating utilized irregularly shaped pure copper powders, designed to exhibit enhanced virucidal and antibacterial performance. Characterization involved assessing particle morphology, size distribution, and the bonding mechanisms between the copper powder and rubber, which was identified as purely mechanical interlocking. The antiviral efficacy was tested using a surrogate for SARS-CoV-2, showing significant deactivation effects on the dual-channel copper-coated surface. Similarly, antibacterial tests against Escherichia coli demonstrated a significant reduction in microbial presence, highlighting the coating’s potential in healthcare applications. The findings support the effectiveness of copper cold-sprayed coatings on rubber substrates in significantly reducing pathogen transmission on high-contact surfaces.

Cold-sprayed coatings, especially those incorporating copper, demonstrate enhanced antibacterial and antiviral properties. Optimizing the microstructure and composition of these coatings can significantly improve their effectiveness, offering a promising solution for reducing pathogen transmission on high-contact surfaces.

2.6. Typical Applications of Cold Spray Coatings

2.6.1. Protective Coating

Cold spray technology is widely recognized for its ability to produce dense, uniform, and low-porosity coatings, offering excellent protective properties. This section explores the applications of cold spray in creating anti-corrosion and wear-resistant coatings, highlighting their effectiveness in enhancing material durability and performance.

(1) Anti-corrosion coating: Cold spray technology is recognized for its capacity to produce coatings that are dense, uniform, and have low porosity with controllable thickness, thereby offering excellent corrosion protection for the underlying substrates. This process involves the acceleration of powder particles at temperatures below their melting point using a high-velocity gas jet, allowing the particles to deform plastically upon impact and form a coating [79,97].

In a notable application, Li et al. [98] successfully utilized cold spray technology to deposit a high-bond-strength, dense Ti coating on an SS304 stainless steel substrate. The coated samples were subsequently subjected to heat treatment in the range of 950 to 1050 °C. Following the heat treatment, the electrochemical performance of the Ti coating in a 3.5% NaCl solution was evaluated and found to be equivalent to that of bulk Ti and superior to that of the base SS304 material. This highlights the potential of cold spray technology in enhancing the lifecycle and durability of critical components through improved corrosion resistance.

Further extending the utility of cold spray technology, Silva et al. [68] developed an Al-Al₂O₃ composite coating on carbon steel surfaces. The resultant coating demonstrated exceptional performance in a rigorous 3000 h salt spray test, showcasing its superior corrosion resistance. This example illustrates the effectiveness of composite coatings produced via cold spray technology in providing long-lasting protection and performance enhancement, particularly under harsh environmental conditions.

(2) Wear-resistant coating: Cold spray technology, distinguished by its low operational temperatures, offers significant benefits in the deposition of protective coatings. This process, which operates below the melting points of the constituent materials, is particularly effective in minimizing residual stresses, which are predominantly compressive. Such characteristics enable the uniform distribution of phases within the coating. Cold spray technology can fabricate monophasic coatings as well as composite coatings reinforced with ceramic phases, thereby providing substrates with excellent resistance to frictional wear and erosion [99,100,101].

In a practical application, Peat et al. [102] successfully utilized cold spray technology to deposit a WC–cobalt chrome (WC-CoCr) composite coating on Al-Mg-Si substrates. This method significantly enhanced the substrate’s resistance to wear and erosion, demonstrating the practical benefits of cold spray in improving material durability under aggressive usage conditions.

Traditionally, Ni-WC composite coatings have been fabricated using laser cladding technology. This method involves melting the materials at high temperatures, which can lead to several issues such as uneven distribution of WC particles, decarburization of WC, and the introduction of significant thermal residual stresses. These factors often culminate in premature coating failure. In contrast, cold spray technology avoids these pitfalls by applying materials without reaching their melting points, resulting in a more uniform particle distribution, and reduced residual stresses. Consequently, coatings produced via cold spray exhibit superior wear resistance compared to those prepared using traditional high-temperature methods [103].

Cold spray technology excels in producing protective coatings with superior anti-corrosion and wear-resistant properties. By operating below the melting points of materials, it ensures uniform particle distribution and minimizes residual stresses, significantly extending the lifespan and durability of coated substrates.

2.6.2. Functional Coating

Cold spray technology, operating at relatively low temperatures, is highly suitable for creating functional coatings in various industries. This section explores its applications in biomedical coatings, electronics, and catalysis, highlighting the unique benefits and enhanced properties it offers.

(1) Biomedical: Cold spray technology, due to its operation at relatively low temperatures, is particularly suitable for the deposition of biologically sensitive coatings. This technology has garnered extensive attention from biomaterials researchers for its efficiency and cost-effectiveness in producing biocompatible coatings. It holds significant promise for future applications in the fabrication of functionalized drug-delivery coatings and personalized medical devices without compromising the thermal sensitivity of the materials involved.

Cinca et al. [104], Hasniyati et al. [105], and Kergourlay et al. [106] have leveraged cold spray technology to apply hydroxyapatite coatings onto Ti alloy surfaces. This application aims to reduce immune rejection and enhance material compatibility with biological tissues. Hydroxyapatite, a mineral that is integral to human bone, when coated on Ti implants using cold spray, facilitates better integration with bone tissue while minimizing adverse immune responses.

Kim et al. [107] utilized cold spray technology to produce thin films of ZIF-8, a type of zeolitic imidazolate framework (ZIF). This process preserved the intrinsic properties of the ZIF-8 material while endowing it with excellent impact resistance. This is particularly advantageous for applications requiring the unique molecular sieving and gas adsorption capabilities of ZIFs, as the cold spray process does not alter the chemical structure or porosity of the material.

Furthermore, El-eskandrany et al. [108] applied cold spray technology to deposit copper-based composite coatings. These coatings are noted for their strong bio-inhibitory properties, making them suitable for anti-fouling and antimicrobial applications. The ability of copper to inhibit microbial growth is effectively utilized in these coatings, which can be applied to surfaces in healthcare settings to reduce the risk of bacterial and fungal contaminations.

(2) Electronics industry: Cold spray technology is distinguished by its ability to produce coatings with exceptionally low oxygen content and porosity. This characteristic results in coatings with excellent mechanical properties and superior thermal and electrical conductivity. The chemical composition and microstructure of the coatings produced can be kept consistent with the original materials, making this technology highly advantageous for applications in the electronics industry [109].

Dardona et al. [110] have innovatively combined micro cold spray with direct write (DW) technology to deposit copper–graphene composite traces. Although the conductivity of these traces is lower than that of bulk copper, this novel material holds significant potential for use in low-power electronics industries. Furthermore, the incorporation of graphene enhances copper’s resistance to corrosion, broadening the functional scope of the composite material.

Cold spray technology [111] has also been utilized to prepare high-heat-capacity coatings on superconducting materials. This application aims to enhance the low-temperature superconducting stability of these materials, which is crucial for maintaining their performance under operational conditions. The ability to apply coatings at low temperatures without altering the underlying material properties is particularly critical in superconducting applications, where even minor structural or compositional changes can significantly impact superconducting characteristics.

(3) Catalysis, electrode coating: Cold spray technology facilitates the deposition of coatings that exhibit significant plastic deformation due to high-density dislocations and peening effects. These structural characteristics of cold spray coatings confer enhanced catalytic activity due to their unique morphology [112,113].

Wang et al. [114] utilized cold spray technology to apply a CuO/ZnO/Al₂O₃ catalytic coating on an aluminum substrate. Their experimental results indicated that the cold spray catalytic coatings displayed higher activity compared to coatings produced by other techniques. The high impact velocity in cold spraying ensures a dense coating with substantial interfacial bonding and reduced porosity, which likely contributes to the increased catalytic efficiency observed.

Lee et al. [115] adopted cold spray technology to fabricate nanocopper oxide films in an outdoor setting for use as photoelectrochemical cathodes. The photocurrent density produced by these films reached the highest levels recorded in the literature, demonstrating the potential of cold spray technology in enhancing the functional properties of photocatalytic materials. Furthermore, the process’s cost-effectiveness significantly reduces production costs, offering a sustainable method for large-scale application in renewable energy technologies.

Li et al. [116,117] employed vacuum cold spray technology to fabricate nanoporous TiO₂ coatings and their scattering layers for use in dye-sensitized solar cells (DSCs). Their findings indicated that increasing the airflow from 0.003 m³/s to 0.0075 m³/s enhanced the short-circuit photocurrent density of the N719 dye-sensitized cells from 83 A/cm² to 98 A/cm². Under the conditions of 0.0075 m³/s airflow, the dye-sensitized solar cells with TiO₂ coatings deposited exhibited a maximum total energy conversion efficiency of 4.2%. This improvement highlights the advantages of cold spray technology in creating high-efficiency energy-harvesting devices by optimizing the structural and optical properties of photocatalytic coatings.

Cold spray technology provides significant advantages in producing functional coatings with enhanced properties for biomedical, electronic, and catalytic applications. Its ability to maintain material integrity while improving performance makes it a valuable tool across diverse fields.

2.6.3. Repair Coating

In aerospace component repair and remanufacturing, traditional methods like argon arc welding and thermal spraying often result in defects due to high thermal input. Cold spray technology, operating at lower temperatures, offers a promising alternative by avoiding these heat-induced issues, making it particularly suitable for repairing complex and thin-walled components.

In the field of aerospace component repair and remanufacturing, traditional methods such as argon arc welding, thermal spraying, and laser deposition have been predominant. However, these methods suffer from significant drawbacks due to high thermal input, including oxidation inclusions, thermal distortion, and cracking, which severely limit the effectiveness and applicability of repairs. Particularly, complex-shaped thin-walled components often cannot be repaired using these techniques and are consequently scrapped, resulting in substantial resource and cost wastage.

Cold spray technology, utilizing solid-state deposition, offers a solution to the limitations imposed by excessive heat input associated with traditional repair techniques. This technology operates at relatively low temperatures, thereby avoiding the thermally induced defects typical of conventional methods. Its application in aerospace component repair holds significant potential, as demonstrated in various practical scenarios.

One illustrative application is the repair of micro-fretting damage on the surface of an A357 cast aluminum part from an F/A-18E/F Super Hornet fighter jet [63], where cold spray technology was employed to fully restore the dimensions and functionality of the damaged area. Additionally, the use of cold spray for repairing components of Seahawk helicopters has been shown to reduce costs by 35% to 50% compared to manufacturing new parts [63].

The U.S. Department of Defense has shown considerable interest in cold spray technology, investing significant research funding to advance this technique for maintaining military equipment such as fighter jets, naval ships, and tanks. Furthermore, the establishment of the MIL-STD-3021 [63] standard demonstrates the strategic integration of cold spray technology within defense maintenance protocols.

Beyond military applications, major aerospace manufacturers have also invested heavily in research on both theoretical and applied aspects of cold spray technology. Prominent companies such as Boeing, General Electric (GE), Pratt & Whitney, and Honeywell in the United States, and international firms like Airbus and Safran in France, as well as Rolls-Royce and TWI in the United Kingdom, are exploring cold spray techniques to repair high-value aerospace components.

Cold spray technology provides an effective solution for aerospace repairs, minimizing thermal defects and reducing costs. Its adoption by major aerospace and defense organizations highlights its potential to revolutionize maintenance protocols for high-value components.

2.6.4. Additive Manufacturing Applications

Cold spray additive manufacturing (CSAM) offers significant advantages in the field of additive manufacturing by enabling the production of complex standalone components and the restoration of worn parts. This section explores the potential of CSAM in various applications, highlighting its ability to produce intricate structures and repair defective components efficiently.

In the realm of additive manufacturing (AM), CSAM presents a significant technological advantage compared to other AM methods. CSAM, therefore, holds considerable potential within the solid-state additive manufacturing sector for producing standalone components or for the restoration of worn parts [118]. Figure 10 shows damaged transmission gearboxes and housings that were successfully restored using CSAM. The repaired parts exhibited high adhesion strength, wear resistance, and corrosion resistance, which extends their service life. Figure 11 depicts a modification technique where a new part is affixed to a bearing cover using CSAM. Post-CSAM, a seamlessly integrated new section is obtained, with indistinct boundaries between the new element and the original part [60]. The capability to manufacture such intricate structures is pivotal to CSAM technology, as it substantially broadens its application spectrum.

Mechanical components degrade over time due to electrochemical and tribological effects, or other factors. In the absence of suitable repair technologies, these defective parts often remain irreparable and must be replaced [119]. CSAM technology emerges as an economical method with substantial capability to restore such defective parts. This capacity is attributable to its ability to disregard the thermal defects of the underlying substrate and to maintain the inherent characteristics of the coating materials. CSAM has been effectively employed in restoring various damaged and defective components across multiple disciplines [5].

In the CSAM process, the raw material does not deposit directly onto the defective area due to its complex surface geometry, and the capacity of the degraded surface in the defective area influences the adhesive behavior. Consequently, pre-processing is required above the damaged area to reconstruct the degraded region. Surface restoration necessitates various machining operations on the repair area to achieve a better surface finish, acceptable for CSAM application [120]. Even with CSAM, the deposited products must undergo machining processes to achieve standard geometric shapes. Figure 12 illustrates a typical restoration process of a component using CSAM.

Using CSAM technology, an effective restoration was achieved on the corroded inner bore surfaces of aluminum alloy valve actuators. In contrast to traditional repair techniques depicted in Figure 13a, CSAM does not inflict thermal damage on the underlying substrate and enhances the capability of the restoration. The CSAM-repaired actuators were assembled into engine parts and employed in actual applications after passing all performance tests [121]. Furthermore, in the automotive industry, a mechanized center utilized CSAM to repair the corroded aluminum alloy oil pump casings of Caterpillar 3116 and 3126 engines, as illustrated in Figure 13b. This application showcases CSAM’s efficacy in restoring critical components to operational standards without compromising the material integrity of the underlying substrates.

CSAM technology provides an effective solution for both manufacturing new parts and restoring damaged components, minimizing thermal damage and maintaining material integrity. Its ability to create and modify complex structures broadens its application across multiple industries, showcasing its versatility and efficiency.

3. Cold Spraying Technology for the Preparation of Hard Composite Materials

3.1. Introduction to MMC Materials with Carbon-Based Reinforcements

MMC materials with carbon-based reinforcements are high-performance engineering materials composed of various matrices and reinforcing materials, extensively utilized across critical industries such as aerospace, automotive, electronics, and healthcare. These materials integrate metal, polymer, or ceramic matrices with hard substances such as silicon carbide, tungsten carbide, and alumina, offering exceptional mechanical strength, wear resistance, and chemical stability. They maintain superior performance in high-temperature, high-pressure, and highly corrosive environments, making them suitable for extreme operating conditions. Faced with challenges of high production costs and substantial energy consumption, future research will focus on developing more economical and efficient manufacturing methods and new composite material systems to further enhance material performance, reduce production costs, and explore new application domains. Additionally, environmental sustainability has become a crucial consideration for future developments; efforts to improve material recycling and reuse techniques aim to minimize environmental impact and enhance the overall lifecycle efficiency of materials. These initiatives will enable broader and more sustainable applications of MMC materials with carbon-based reinforcements in modern industries.

3.2. WC-Reinforced MMC Coatings

Tungsten carbide (WC) is a compound characterized by its exceptional hardness and high melting point, making it one of the most durable materials available for industrial applications. It exhibits a remarkable combination of mechanical properties, including high compressive strength, significant resistance to wear and abrasion, and excellent thermal conductivity. WC also maintains its hardness at elevated temperatures, which is critical for applications involving high thermal loads. Furthermore, tungsten carbide is chemically inert, providing robust resistance to corrosion and oxidation in harsh environments. These properties make WC an ideal choice for cutting tools, wear-resistant components, and applications requiring long-lasting performance under extreme conditions. Consequently, WC-based coatings produced through cold spray technology are increasingly employed to enhance the durability and functionality of various industrial components, providing an effective solution for extending the service life of equipment subjected to severe operational demands.

Alidokht et al. [122] employed the cold spray technique to fabricate Ni-WC composite coatings. Their research investigated the impact of WC particle content on the sliding wear performance of the coatings. Reciprocating sliding wear tests indicated that the incorporation of WC particles into the Ni matrix enhanced the sliding wear performance of the composite coatings. The improvement in sliding wear resistance within the composite coatings was attributed to the initial cycles providing Ni resistance to plastic deformation and adhesive wear, and in longer cycles, it was related to the formation of a stable and cohesive mechanically mixed layer (MML) on top of the wear tracks.

Chen et al. [123] utilized an HPCS system to prepare WC-reinforced martensitic age-hardened steel 300 composites. Based on X-ray CT reconstruction, the distribution of WC particles within the CSWC/MS 300 composite samples was directly observed and analyzed. The volumetric fraction and equivalent diameter distribution of WC particles indicated a high retention rate (85.4%) under higher propellant gas pressure (N₂, 5 MPa, 900 °C). As the propellant gas pressure increased, both the plastic deformation of the sample and the retention rate of WC increased, thereby enhancing the sample’s tensile strength and wear resistance.

Zhang et al. [124] fabricated Cu-MoS₂-WC composite coatings using the cold spray method. Sliding wear tests conducted in dry nitrogen demonstrated that Cu-MoS₂-WC exhibited lower friction against Al, a more uniform wear track throughout, and a lower and more consistent wear rate. The presence of WC particles contributed to material transfer within the contact and microstructure of the third body. The wear track morphology showed extensive delamination in Cu-MoS₂ tracks, whereas Cu-MoS₂-WC tracks appeared smoother with minimal delamination. WC particles positioned directly under the wear tracks provided a scenario of a hard material with a soft ‘skin’, achieving lower friction. The average coefficients of friction for Cu-MoS₂, Cu-MoS₂-WC, and pure Cu over 1000 cycles are depicted in Figure 14. Analysis of the data presented in Figure 14 reveals a significant reduction in the average coefficient of friction (CoF) for the samples treated with cold-sprayed WC. This observation underscores the efficacy of WC as a surface treatment in enhancing the frictional properties of copper-based composites. MoS₂ demonstrated similar redistribution behaviors on the wear tracks of both coatings. However, after 1000 cycles, a greater quantity of MoS₂-containing fragments adhered to the mating surfaces of Cu-MoS₂-WC, likely forming a transfer film that initially separates the sliding contacts, thereby reducing friction.

Silva et al. [125] studied the morphology, mechanical properties, and corrosion resistance of WC-Co coatings deposited by cold gas spray onto an Al-Cu-Mg alloy. The results underscored the substantial potential of the CGS technique in fabricating carbide coatings. The sprayed coatings exhibited a uniform distribution of ceramic phases without cracks, low porosity, and no evident interconnected pores. The WC-12Co coating maintained good corrosion resistance after soaking in 3.5% NaCl solution for at least 400 h. Prolonged immersion in 3.5% NaCl led to cobalt-phase dissolution and WC particle loss, forming interconnected pores that allowed electrolyte ingress and substrate corrosion. Lesser thickness, higher porosity, and potentially greater defect presence accounted for the weaker performance of this coating. Conversely, the density, low porosity, and thickness of the WC-25Co coating protected Al-Cu-Mg from corrosion in 3.5% NaCl solution for at least 10,700 h, as evidenced by the near-constant resistance and capacitance during testing. The WC-25 Co coating demonstrated very high corrosion resistance during 3000 h of exposure in a neutral 5 wt% NaCl salt spray test at 35 °C.

Gao et al. [126] employed nanostructured WC-12 Co powder to investigate the deposition behavior of nanostructured WC-12Co coatings by studying the surface morphology and cross-sectional structure of single WC-12Co particle impacts on substrates of varying hardness, including stainless steel and different hardness nickel-based self-fluxing alloy coatings. Observations from the top and cross-section of individual WC-12Co particles indicated that particle deposition penetration depended on substrate hardness. When the substrate surface was covered by WC-12Co particles, the newly formed substrate, i.e., the coating’s hardness, was significantly enhanced. The substantial increase in surface hardness led to the rebound of impacting particles and erosion of deposited particles, which hindered effective coating formation.

Andrew et al. [127] studied the effects of WC particle size on the deposition of WC-Co metal–ceramic coatings using cold spraying. Powders of both micron- and nanostructures had similar cobalt contents. Altering the WC particle size significantly affected the deposition efficiency of the coating process. Micron-structured WC-Co raw materials did not permit coating accumulation under the same or higher thermal spraying parameters than those used for the nanostructured WC-Co raw materials. Furthermore, micron-structured WC-Co coatings exhibited joint erosion and deposition effects on the surface. Smaller WC particles (<1 μm) were observed near the substrate interface, and larger WC particles (1–2 μm) near the coating surface. These observations suggest the presence of a critical WC particle size for deposition through cold spraying and the emergence of size standards due to the formation and cohesion mechanisms within the coating. The nanostructured test samples displayed:

(1)

A dense microstructure with virtually no porosity.

(2)

A crack-free interface between the coating and substrate, indicating good adhesion.

(3)

No observable phase changes. XRD patterns of each powder and its corresponding coating showed no observable peak differences, but broadening of the diffraction peaks in the coatings indicated grain refinement during the coating process. Additionally, all nanostructured sprayed WC-Co coatings exhibited Vickers hardness values above HV 1000.

The advancements in WC-based cold spray coatings have demonstrated significant improvements in wear resistance, mechanical strength, and corrosion protection, making them highly suitable for industrial applications requiring durability and reliability. The use of nanostructured powders and optimized spray parameters has yielded dense, crack-free coatings with excellent adhesion and hardness. However, challenges such as optimizing particle retention, reducing porosity, and improving long-term stability remain. Future research focusing on these areas, along with material innovations, will further enhance the performance and applicability of WC-based cold spray coatings.

3.3. B₄C-Reinforced MMC Coatings

Boron carbide (B₄C) is renowned for its exceptional hardness, ranking just below diamond and cubic boron nitride, and its low density, which makes it one of the hardest and lightest materials available for industrial applications. It possesses outstanding mechanical properties, including high wear resistance and excellent chemical stability, which enable it to withstand harsh environments and aggressive chemical agents. Additionally, B₄C has a high neutron absorption cross-section, making it valuable in nuclear applications for radiation shielding and control. These unique properties make B₄C an ideal candidate for enhancing the durability and functionality of various components. As a result, B₄C-based cold spray coatings have emerged as a promising technology, leveraging the material’s hardness and stability to produce robust, wear-resistant surfaces. This application not only extends the service life of industrial components but also offers significant performance improvements in demanding operational settings.

Tariq et al. [128] applied CSAM technology to deposit a 6 mm thick B₄C/Al composite neutron shielding coating on an Al-Cu-Si cylindrical substrate. The microstructure, mechanical behavior, and neutron shielding performance of the self-supporting coating were investigated under different heat treatment conditions at 200, 300, 400, and 500 °C. The results indicated that with increasing heat treatment temperatures, the coating’s ductility and strength progressively improved, owing to the gradual bonding/healing of interlayer boundaries through recovery and recrystallization mechanisms. The coating heat-treated at 500 °C exhibited the highest ductility (1.4%) and strength (60 MPa), with a minimal porosity level of 1.9%. Neutron shielding results demonstrated thermal neutron attenuation with increasing sample thickness. Furthermore, neutron shielding performance slightly improved in samples heat-treated at 500 °C. Figure 15 presents the electron backscatter diffraction (EBSD) Euler angle maps for the samples in their as-sprayed condition and after undergoing heat treatment, observed in the transverse (XZ)-plane relative to the direction of spraying.

Thunaipragasam et al. [129] utilized a metallized cold spray process to prepare B₄C/Al-Zn-Mg-Cu and Al + plasma electrolytic oxidation (PEO) composite coatings on Mg-Al-Zn alloy surfaces. The phase structure, mechanical properties, wear resistance, and corrosion resistance of the ceramic coatings were also examined. The results showed that the PEO ceramic coating consisted of α-Al and γ-AlAl, retaining a small amount of residual B₄C. The PEO-B₄C coating achieved a hardness of 13.8 GPa and an elastic modulus of 185.5 GPa, which were 21.0% and 23.5% higher, respectively, than those of comparable cold spray coatings. Due to the lower wear rate (4.84 × 10⁻⁵ mm³/Nm) and CoF (0.64) of the PEO-B₄C coating, its wear resistance was, respectively, 58% and 15.7% better compared to equivalent cold spray coatings. The corrosion current density of the PEO-treated B₄C-Al-Zn-Mg-Cu coating was 3.735 × 10⁻⁶ A/cm², similar to that of the untreated cold spray coating. Finally, compared to untreated cold spray B₄C-Al-Zn-Mg-Cu, the mechanical properties and wear resistance of the coating were significantly enhanced by PEO treatment.

Zhao et al. [130] utilized cold spray and annealing processes to fabricate B₄C/Al-316L neutron-absorbing composite plates. The microstructure and mechanical properties of B₄C/Al composites and B₄C/Al-316L composite plates were examined. Key findings are summarized as follows:

(1)

Monte Carlo N-Particle Transport Code (MCNP) results indicate that B₄C/Al composites exhibit excellent neutron-shielding properties, with neutron mitigation rates decreasing exponentially as the thickness of the B₄C/Al composite material increases.

(2)

Post-annealing, cold-sprayed B₄C/Al composites exhibited grain growth in Al, reduction in internal stresses, and a decrease in microhardness.

(3)

Annealing at 600 °C resulted in metallurgical bonding in B₄C/Al composites, enhancing the tensile strength of cold-sprayed B₄C/Al composites from approximately 97 MPa to about 143 MPa.

(4)

Annealing improved the fracture strength of B₄C/Al-316L composite plates. At an annealing temperature of 500 °C, the fracture strength and shear strength were approximately 254 MPa and 78 MPa, respectively.

(5)

B₄C/Al-316L composite plates (A-400) demonstrated a yield strength of approximately 163 MPa at high temperatures (400 °C), which is considered high for neutron absorbers. As the annealing temperature increased, the fracture strength of B₄C/Al-316L composite plates slightly decreased.

Filippov et al. [131] focused on cold spray and laser cladding composite technologies, primarily examining the microstructure, elemental content, and morphology of laser tracks. During this phase, the authors concentrated on the interaction between the laser device and the material without affecting the layer growth product. The results indicated that after cold spraying nickel particles in the coating were deformed and the content of ceramic particles B₄C significantly reduced. After laser cladding, no boundaries were present between nickel, and significant changes occurred in the ceramic particles. A combination of cold spraying and laser cladding techniques yielded a metal–ceramic microstructure of Ni and boron (B) carbide with up to 90% concentration of ceramic components. Compared to the initial mixture, the B carbide content in the coating was significantly reduced. The study results showed a dependence of the concentration and focus depth on the track structure of the laser tracks.

Vladislav et al. [132] investigated the impact of heat treatment on the microstructure and properties of aluminum-based coatings with varying B₄C contents, deposited on stainless steel substrates using cold spray. Mixtures containing 30, 50, and 70 wt% B carbide were used as the initial powder for coating deposition. The coatings were heat-treated in an Ar atmosphere at 400 °C for 2 h. The heat treatment did not significantly alter the coating structure. Compared to the as-sprayed coatings, heat treatment allowed for a reduction in microhardness, which could be explained by the grain growth in aluminum and the reduction in dislocations. The bond strength of the coatings obtained from the mixture with 30 wt% B₄C remained almost unchanged after heat treatment and was comparable to the bond strength of the as-deposited coatings. However, annealed coatings prepared from mixtures with 50 and 70 wt% B₄C were characterized by a reduced bond strength compared to the as-sprayed coatings.

Filippov et al. [133] examined the microstructure of metal–ceramic coatings prepared using successive applications of cold spray and laser cladding technologies. The results showed that under certain cold spray parameters, a Ti coating with uniformly distributed B carbide particles could be achieved. The laser treatment mode employed led to the decomposition of B carbide, mixing with the Ti substrate, and formation of borides and TiC. The initial high content of B carbide (35 wt%) in the ceramic led to a lack of free Ti on the surface as well as between the molten layer and the cold-sprayed coating.

Shikalov et al. [134] prepared aluminum-based composite coatings with different B₄C contents and sizes using the cold spray method. The results indicated that as the ceramic component content in the raw mechanical mixtures increased, its content in the coating also increased. With the same content in the raw materials, larger carbide powders corresponded to a higher content in the coating. The maximum content of B carbide in the coatings was 27 vol.% when the mixture was sprayed with a coarse powder content of 72 vol.%. An increase in the B₄C content in the aluminum matrix of the coating led to a significant increase in the microhardness of the coating. With the same content of B carbide in the coating, higher microhardness values corresponded to coatings obtained from mixtures with fine carbide powders. The presence of fine ceramic powders in the coating had almost no effect on the CoF, with results very close to those of pure aluminum coatings. The presence of coarse ceramic powders in the coating, regardless of their content, significantly reduced the CoF.

Fomin et al. [135] studied the effects of laser cladding and cold spray process parameters on the attainment of metal–ceramic coatings. Under the selected parameters of cold spraying and laser beam modes, metal–ceramic seams with different weight ratios and particle sizes of the initial Ni-B carbide mixtures were obtained. Furthermore, the interaction between B carbide and molten metal produced new ceramic particles. The study revealed how the structure of laser tracks depends on the size of the ceramic particles within the range of 3–75 µm. The form of the resonator cavity was primarily dependent on the laser mode and ceramic content. The dagger mode prevented the formation of deeper seam cavities but the high temperature gradient at the seam edges led to cracks and porosity. In the thermal conduction mode, laser treatment had low porosity and cracks. An important factor affecting cavity shape was particle size. Samples with large B₄C particles displayed smooth surfaces, their form resembling a biconvex lens with uniformly dispersed ceramic particles. The structure of this seam was most suitable for forming multilayer metal–ceramic coatings and simple-shaped three-dimensional products.

The advancements in B₄C-based cold spray coatings have demonstrated significant improvements in wear resistance, mechanical strength, and neutron shielding, making them highly suitable for demanding industrial applications. The integration of heat treatments and hybrid techniques, such as laser cladding, has further enhanced the properties of these coatings, leading to better ductility, reduced porosity, and improved microstructure. However, challenges such as optimizing particle retention, reducing microhardness variability, and improving long-term stability remain. Future research should focus on refining spray parameters, exploring new material combinations, and conducting long-term performance studies to fully realize the potential of B₄C-based cold spray coatings.

3.4. TiC-Reinforced MMC Coatings

Titanium carbide (TiC) is a highly durable ceramic material characterized by its exceptional hardness, high melting point, and superior thermal conductivity. It exhibits excellent wear resistance and mechanical strength, making it suitable for applications requiring extreme durability. TiC also offers good electrical conductivity and chemical stability, which allows it to perform well in corrosive environments and under high thermal loads. These properties make TiC an attractive candidate for various industrial applications, including cutting tools, abrasives, and wear-resistant components. Consequently, TiC-based cold spray coatings have gained attention as a promising technology for enhancing the surface properties of industrial components. By leveraging the hardness and stability of TiC, these coatings provide robust, wear-resistant surfaces that significantly improve the longevity and performance of tools and equipment exposed to harsh operational conditions.

Koricherla et al. [136] conducted a study on the wear behavior of cold-sprayed Ti and Ti-TiC composite coatings at room and elevated temperatures, with a particular focus on the impact of TiC on friction, wear rate, and the frictional chemical phases. By comparing coatings with varying TiC contents (volumetric ratios of 14%, 24%, and 35%) to a pure Ti baseline, it was found that Ti-TiC composites significantly enhanced wear resistance due to their higher hardness and the formation of protective oxide layers at elevated temperatures. Notably, the Ti-35%TiC coating demonstrated the best performance, with a reduced friction coefficient and wear rate at higher temperatures, exhibiting material gains achieved through oxidation and material transfer at the interface. This research underscores the potential of this coating to enhance the wear resistance of aerospace and automotive components.

Venkata Naga Vamsi Munagala [137] detailed the effects of metal powder characteristics (spherical and irregular Ti6Al4V powders) on the deposition characteristics of cold-sprayed Ti6Al4V-TiC coatings through experimental and numerical studies. Coatings deposited with irregular powders exhibited higher ceramic retention rates and lower porosity. Finite element simulations indicated that surface porosity and roughness significantly influence ceramic retention by promoting substrate void closure and compaction, leading to increased substrate pitting depth and energy dissipation. The study concluded that metal powder characteristics critically impact the final performance of cold-sprayed MMC coatings, with porous powders offering distinct advantages over spherical powders.

Munagala’s [138] research focused on enhancing the wear resistance of Ti alloys through the cold spray process to create MMC coatings reinforced with a hard second phase. Specifically, Ti6Al4V and Ti6Al4V-TiC coatings were deposited on low-carbon steel substrates and their dry sliding wear behavior was tested across a temperature range of 25–575 °C. The study found that both coatings generally showed a reduction in wear rate and CoF with increasing temperature, with the composite coating exhibiting superior wear resistance at all temperatures. At elevated temperatures, the formation of a protective oxide glaze composed of WO₃, TiO₂, and CoWO₄ significantly enhanced wear resistance. The study detailed the influence of temperature and TiC reinforcement on altering friction and wear mechanisms, confirmed through electron channeling contrast imaging (ECCI) and nanoindentation to characterize the wear tracks and subsurface microstructures. Figure 16 shows the SEM images of the counterfaces used for sliding tests on composite coatings at different temperatures.

Chen et al. [139] investigated successfully fabricated in situ-synthesized TiC/Ti-Al composite coatings using a method that combined cold spraying with heat treatment, aimed at enhancing the mechanical properties and wear resistance of TiAl intermetallic compounds. The coatings were deposited at various gas temperatures (250 °C, 450 °C, and 550 °C) and subsequently annealed at 650 °C for different durations, followed by a final hold at 1100 °C. The results demonstrated that higher deposition temperatures facilitated lower porosity and improved microhardness and wear resistance. Moreover, the in situ-synthesized TiC significantly enhanced the microhardness, fracture toughness, and wear resistance of the coatings, especially after the final high-temperature annealing at 1100 °C. This research illustrates that cold spraying combined with heat treatment is an effective approach for developing Ti-Al composite coatings with superior wear resistance and mechanical strength.

Vidyuk et al. [140] explored the efficacy of spark plasma sintering (SPS) as a post-spray treatment for enhancing the structure and properties of cold-sprayed TiC-Cu composites. By applying SPS at 900 °C with a pressure of 40 MPa, the study successfully synthesized TiC within the copper matrix and significantly improved the cohesion and microhardness of the composite material. This treatment not only induced the in situ synthesis of TiC but also effectively densified the material and eliminated interface microcracks, demonstrating SPS’s potential to enhance both the mechanical and structural properties of cold-sprayed coatings, making it a promising technology for developing wear-resistant composite materials.

Kusiński et al. [141] studied the development, characterization, and application of nanoscale TiC/Ti coatings using supersonic cold gas spraying (SCGS). The aim was to optimize the SCGS process to deposit dense nano-phase TiC-Ti coatings on Ti alloy surfaces to enhance biocompatibility and wear resistance, crucial for biomedical applications such as knee joint implants. High-energy ball milling was used to produce nanostructured powders, which were then deposited to maintain the nanostructure and achieve coatings with low porosity and high hardness. The study highlighted the potential of these coatings in medical applications, as they possess higher wear resistance, hardness, and biocompatibility, in compliance with ISO standards.

Recent research on TiC-based cold spray coatings has demonstrated substantial progress in enhancing the wear resistance, mechanical strength, and thermal stability of industrial components. Notable advancements include the improved wear performance at elevated temperatures, the beneficial effects of in situ synthesis and heat treatments on coating properties, and the successful integration of TiC into various metal matrices, resulting in robust and durable composite coatings. However, challenges such as optimizing particle retention, reducing porosity, and achieving consistent coating quality across different substrates remain. Future work should focus on refining spray parameters, exploring advanced post-spray treatments like spark plasma sintering, and developing new composite formulations to further enhance the performance and reliability of TiC-based cold spray coatings in demanding applications.

3.5. SiC-Reinforced MMC Coatings

Silicon carbide (SiC) is renowned for its exceptional hardness, high thermal conductivity, and remarkable chemical stability, making it one of the most robust and versatile ceramic materials available. It exhibits excellent wear resistance and maintains its mechanical properties at elevated temperatures, which is crucial for high-stress applications such as abrasives, cutting tools, and heat exchangers. Additionally, SiC is highly resistant to oxidation and corrosion, allowing it to perform effectively in aggressive chemical environments. These superior properties make SiC an ideal candidate for various industrial applications that demand long-lasting performance and durability. As a result, SiC-based cold spray coatings have gained significant attention, utilizing the material’s hardness and stability to create robust, wear-resistant surfaces. This technology extends the service life of industrial components and enhances their performance in challenging operational settings, offering a practical solution for industries seeking to improve equipment longevity and efficiency.

Wang et al. [142] focused on the application of SiCp/Al-Mg-Cr-Mn composite coatings on magnesium substrates using the cold spraying method for corrosion protection. The key research method involved preparing SiCp/Al-Mg-Cr-Mn composite coatings on magnesium substrates through cold spraying and analyzing their microstructure, corrosion properties, and thermal properties through various tests including galvanic corrosion immersion, thermal cycling, and electrochemical measurements. The study concluded that the cold-sprayed SiCp/Al-Mg-Cr-Mn composite coatings provided superior corrosion resistance compared to Al-Mg-Cr-Mn coatings alone, attributing the enhanced performance to the densification of the coating under the peening effect of hard particles and the positive effects of SiC particle size and distribution. The coatings also exhibited good adhesion and thermal properties, indicating potential for protective applications on magnesium substrates in corrosive environments. Sansoucy et al. [27] focused on developing and evaluating SiC-reinforced Al-12Si alloy coatings via cold spray technology. They explored how varying volumes of SiC in the feedstock powder affected the mechanical properties and microstructure of the coatings. The study found that up to 45% of the SiC mixed with the aluminum matrix was retained, demonstrating a uniform distribution within the matrix. Although the addition of SiC increased the coatings’ porosity, it did not significantly affect the thickness or adhesion strength of the coatings. This suggests that cold spray technology can effectively produce composite coatings with desirable properties, such as uniform particulate distribution and consistent coating thickness, despite increased porosity with higher SiC content. Lee et al. [143] examined the effect of SiC particle size on the properties of Al-SiC composite coatings. It investigated the incorporation of SiC particles of different sizes into aluminum substrate coatings using cold spray technology, focusing on the interface between the coatings and the substrate. The research concluded that SiC particle size significantly influences the formation of craters on the substrate surface, with larger particles causing more pronounced cratering. This implies that particle size can affect the mechanical interlocking, and hence, the performance, of composite coatings. Sansoucy et al. [27] presented an investigation into the effects of SiC particle reinforcement on Al-12Si alloy coatings using the cold spray technology. The study explored how the inclusion of 20% and 30% volume of SiC particles in the feedstock powder affectsedthe composite coatings’ bond strengths, microstructures, porosity, and SiC content. It was observed that around 45% of the SiC particulate blended with the aluminum alloy was retained in the coatings, demonstrating a uniform distribution within the Al-12Si matrix. Despite the addition of SiC particles, which slightly increased the coatings’ porosity, the overall coating thickness, adhesion strength, and distribution of SiC particles within the matrix were not adversely affected. This research underscores the potential of cold spray technology in producing SiC-reinforced aluminum alloy coatings with enhanced properties for various applications. The paper investigated the feasibility of using low-pressure cold spray technology to deposit SiC-based cermet coatings on Ti6Al4V substrates. Adebiyi et al. [144] explored how varying the gas temperature (450 °C, 500 °C, and 550 °C) affected the microstructure, porosity, and hardness of the coatings. They found that the coatings maintained the initial phases of the feedstock powder without experiencing detrimental phase transformations or decompositions. The process resulted in coatings with partially homogeneous SiC distribution, minimal porosity, and enhanced microhardness (up to 652–712.7 HV_0.3), indicating improved surface properties, without significantly altering the bulk material. Kumar et al. [145] investigated Al-SiC MMC coatings, emphasizing the retention of SiC particulates and their impact on the coatings’ properties. Utilizing different SiC volume percentages in the feedstock, the study examined how SiC particulate content influenced the microstructure, mechanical properties, and wear resistance of the coatings. The research concluded that increasing the SiC content enhanced the coatings’ hardness, mechanical strength, and wear performance. Heat treatment further improved these properties, with the most significant wear resistance observed in coatings with 52% SiC particulate volume, especially after vacuum heat treatment. This comprehensive analysis showcased the potential of Al-SiC MMC coatings for applications requiring enhanced wear resistance. Figure 17 illustrates the wear rates under varying conditions.

Gyansah et al. [146] explored the fabrication of 5 mm thick SiC/Al MMCs using cold spray, focusing on how varying SiC contents and subsequent heat treatments affected their microstructure, thermophysical properties, and flexural strength. Through detailed experimental analysis, it was shown that increasing the SiC content significantly improved the composites’ flexural strength due to mechanisms such as zigzag crack propagation and high SiC particulate retention. Heat treatment further enhanced these properties, with improvements attributable to the coarsening of Al splat and mechanisms like crack branching and interface delamination. The results underscore the effectiveness of cold spray in producing high-performance SiC/Al composites, suggesting their potential for advanced engineering applications. Chen et al. [147] focused on the development of 316L stainless steel and 316L-SiC composite coatings applied on the AZ80 magnesium alloy using cold spray technology. They aimed to enhance the alloy’s wear and corrosion resistance. The findings revealed that both coatings significantly improved wear resistance, with the composite showing superior performance due to the added SiC. Although SiC addition slightly reduced corrosion resistance compared to pure 316L, both coatings effectively decreased the magnesium alloy’s corrosion current density, demonstrating their potential in protective applications.

Khomutov et al. [148] investigated the creation and attributes of Al-Zn-Mg-Cu-SiC composite deposits via cold spray additive manufacturing, focusing on the effects of SiC content and post-treatment processes. Utilizing nitrogen gas at specific pressures and temperatures, the researchers achieved a maximum SiC volume concentration of 20%–25%. The SiC particles, uniformly distributed at the Al-Zn-Mg-Cu splat boundaries, influenced the redistribution of hardening phases during heat treatment and hot isostatic pressing (HIP), enhancing the composite’s properties. The findings indicate that while T6 heat treatment and HIP do not significantly enhance ductility, they do improve the thermal expansion coefficient, making it lower than that of bulk Al-Zn-Mg-Cu. This research highlights the potential for optimizing Al-Zn-Mg-Cu-SiC composite properties through careful control of SiC content and post-treatment processes. Yang et al. [149] investigated the effects of SiC particle size on the microstructure and mechanical properties of cold-sprayed Al-Mg-Cr-Mn/SiCp composite coatings. By varying the SiC particle size from 2 to 67 μm, they analyzed how these differences affected particle velocity and kinetic energy, ultimately impacting the coatings’ characteristics. The results indicated that smaller SiC particles resulted in lower content within the coatings due to reduced kinetic energy, affecting the coatings’ microhardness and cohesion strength. Larger particles, however, contributed to improved mechanical properties, suggesting that a balance between SiC content and particle size is critical for optimizing coating performance.

Recent advancements in SiC-based cold spray coatings have demonstrated significant improvements in corrosion resistance, wear resistance, and mechanical strength of industrial components. Research highlights include enhanced coating performance through optimized SiC content and particle size, improved adhesion and reduced porosity, and the effective use of heat treatments to further enhance material properties. These coatings offer robust protection and longevity in challenging environments, making them suitable for various applications. However, challenges such as achieving consistent SiC distribution, managing increased porosity with higher SiC content, and optimizing the balance between particle size and coating performance remain. Future research should focus on refining spray parameters, exploring advanced post-spray treatments, and developing new composite formulations to maximize the benefits of SiC-based cold spray coatings.

3.6. Diamond-Reinforced MMC Coatings

Diamond is distinguished by its unparalleled hardness, which is the highest of any known material, alongside exceptional thermal conductivity and outstanding optical properties. This crystalline form of carbon also exhibits significant chemical inertness, making it resistant to corrosion and capable of maintaining its properties under extreme conditions. Its excellent wear resistance and low friction coefficient make diamond ideal for applications that require cutting, grinding, and polishing of hard materials. These unique characteristics render diamond an invaluable material for various high-performance industrial applications. Consequently, diamond-based cold spray coatings have emerged as a groundbreaking technology, leveraging the extraordinary hardness and durability of diamond to create superior wear-resistant surfaces. This application not only enhances the longevity and performance of industrial tools and components but also provides significant improvements in efficiency and operational effectiveness in demanding environments.

Barry Aldwell et al. [150] introduced a novel technique for diamond coating deposition using cold spray (CS). Diamond powders precoated with copper and nickel were used to facilitate the CS process at Trinity College Dublin. The method enabled the deposition of high-diamond-content coatings without phase changes or graphitization. Experiments with varying inlet pressures (1 MPa and 2 MPa) demonstrated that the higher pressure resulted in better consolidation and higher diamond content, with peak thicknesses of 0.05 mm and 0.9 mm, respectively. SEM and EDX analyses confirmed that the coatings’ chemical composition matched the original feedstock, with the 2 MPa coating containing 45.74% carbon by weight. The study highlighted the efficiency of the process, achieving a coating build rate of 1.8 kg/h. This innovative approach offers a scalable, efficient alternative to traditional methods, preserving the properties of diamond and enabling applications in various industrial fields, including additive manufacturing. Shuo Yin et al. [151] investigated the fabrication of diamond-reinforced metal matrix composites (DMMCs) using the cold spray (CS) technique. This method involves accelerating micron-sized copper-clad diamond powders to supersonic speeds and depositing them onto aluminum alloy substrates. The study demonstrated that CS could produce thick DMMC coatings with high diamond content while avoiding graphitization. The coatings retained the original diamond phase, with the highest diamond mass fraction reaching 43 wt%. This process showed significant improvements over traditional methods by preventing diamond fracture and maintaining high deposition efficiency. The wear tests indicated excellent wear-resistance properties, emphasizing the potential of CS for industrial applications. T Zhe Wang et al. [152] explored the use of cold spraying to fabricate diamond–copper composite coatings for marine applications. The method involved coating diamond particles with copper using an electroless plating technique, and then, cold spraying the composite powder onto stainless steel substrates. The resulting coatings, containing 31.79% diamond by weight, exhibited significantly enhanced wear and corrosion resistance in artificial seawater compared to pure copper coatings. Specifically, the diamond–Cu coatings reduced the friction coefficient from 0.32 (Cu coating) to 0.10 and demonstrated improved electrochemical stability. These results suggest that diamond–Cu composite coatings offer promising applications as durable anti-corrosion layers for marine structures.

Yin et al. [151] examined the production of diamond-reinforced metal matrix composite (DMMC) coatings using cold spray technology. The research focused on fabricating thick DMMC coatings on aluminum alloy substrates with copper-clad diamond, pure copper, and their mixtures as feedstock powders. The cold spray method proved effective in preventing diamond graphitization, retaining the original diamond phase within the coatings. The research found that copper-clad diamond powders, due to their structure, allowed for a higher diamond content within the coatings compared to traditional pre-mixed powders. Additionally, wear tests on these coatings showcased their excellent wear-resistance properties, attributed to the diamond reinforcement. This study highlights the potential of cold spray technology in developing DMMC coatings with enhanced wear resistance for industrial applications. Aldwell et al. [153] introduced a novel method for depositing metal–diamond composite coatings using cold spray technology, focusing on the use of diamond powders precoated with Cu and Ni without additional binders. They demonstrated the feasibility of producing thick coatings on Al alloy substrates with a high diamond fraction and without phase change. The method presents a more efficient deposition process compared to traditional pre-mixing, allowing for the retention of diamond concentration from feedstock to coating, thus showcasing a promising approach for engineering applications requiring high-performance coatings. Chen et al. [154] explored the fabrication of Al/diamond MMCs using cold spray additive manufacturing with advanced core–shell-structured diamond powders. They highlighted the composite’s enhanced tribological properties, emphasizing that the core–shell-structured diamond was easier to deposit than pure Al, especially under low particle impact velocity. The research found that Al/diamond composites exhibit superior wear resistance, comparable to high-performance alloys. The uniform distribution of diamond reinforcements and high diamond retainability in the composites are key to their improved wear resistance, demonstrating the potential of cold spray manufacturing for advanced composite materials. Figure 18 displays slices from X-ray computed tomography (XCT) reconstructions in the vertical, horizontal, and transverse planes, each illustrating the distribution of diamond particles within the slices.

Li et al. [155] delved into the fabrication and tribological assessment of Ti–diamond composite coatings using cold spray technology. They emphasized achieving high deposition efficiency and uniform distribution of diamond particles within the coatings on a Ti6Al4V substrate. Various characterizations confirmed that the coatings preserved the original phases without phase transformations. The inclusion of diamond particles enhanced the coating’s hardness and wear resistance, showing optimal performance at a 10 wt% diamond content. This research underscores the potential of integrating diamond into MMCs for improved wear properties through cold spray techniques. Sesana et al. [156] examined the effectiveness of cold spray techniques in depositing nickel and Ti-coated diamond powders on various substrates for applications in the ceramic ball production industry. The research focused on analyzing the deposition mechanisms, adhesion response, and abrasion power of the coated samples against Si₃N₄ ceramic balls. Pin-on-disk tests were used to evaluate the wear resistance under different operational variables. The study concluded that diamond-coated powders, particularly with nickel and titanium coatings, show promising abrasion performance, potentially offering an innovative approach to ceramic ball manufacturing processes. Kovarik et al. [157] investigated the mechanical and fatigue properties of diamond-reinforced Cu and Al MMCs (DMMCs) prepared by cold spray, comparing two different diamond mass concentrations embedded in two metal matrices. The study found that the inclusion of diamond particles significantly improved properties like strain at fracture, ultimate strength, fatigue crack growth resistance, and fracture toughness compared to pure metals. This enhancement was attributed to a combination of changes in the properties of the metallic matrix due to the peening effect of reinforcement particles and stress redistribution due to the particles’ presence.

Recent advancements in diamond-based cold spray coatings have demonstrated substantial improvements in wear resistance, mechanical strength, and corrosion resistance across various industrial applications. Innovations such as using Cu- and nickel-clad diamond powders have enabled the production of high-diamond-content coatings without phase changes, preserving the exceptional properties of diamond. These coatings have shown significant potential in enhancing the performance of tools and components, particularly in demanding environments. However, challenges remain in achieving consistent coating thickness, optimizing particle retention, and managing porosity. Future research should focus on refining spray parameters, exploring advanced post-spray treatments, and developing new composite formulations to maximize the benefits of diamond-based cold spray coatings for industrial applications.

3.7. Other Hard Materials (FeMnCrSi, etc.)

3.7.1. BN Based

Boron nitride (BN) is a unique ceramic material known for its exceptional thermal conductivity, electrical insulation, and high chemical stability. It exists in several crystalline forms, with hexagonal boron nitride (h-BN) being the most common, exhibiting a structure similar to graphite. This configuration imparts h-BN with excellent lubricating properties and a low friction coefficient, making it ideal for high-temperature lubricants and release agents. BN is also highly resistant to oxidation and corrosion, allowing it to maintain its properties in harsh chemical environments. These attributes make BN a valuable material for various industrial applications, including electrical insulators, heat sinks, and protective coatings. Consequently, BN-based cold spray coatings have garnered significant attention for their potential to create wear-resistant, thermally conductive, and electrically insulating surfaces. This technology not only enhances the durability and performance of industrial components but also provides an effective solution for improving thermal management and electrical insulation in challenging operational settings.

Luo et al. [158] developed large-sized cubic BN (cBN)-reinforced nanocomposites through cold spray deposition, utilizing a blend of 40 vol.% cBN-NiCrAl nanocomposite particles with large-sized cBN particles. The deposition behavior, microstructure, and mechanical properties, including hardness, fracture toughness, and two-body dry abrasive wear behavior, were examined. The results indicated that incorporating large cBN particles into the nanocomposite significantly improved its abrasive wear resistance without complex processes, offering a promising method for enhancing the wear resistance of composites.

Smid et al. [159] explored different methods for preparing Ni-hBN (nickel-hexagonal boron nitride) powders for cold-sprayed self-lubricating coatings. They examined admixed, milled, and precoated (encapsulated) powder formulations, focusing on their deposition behavior, microstructural homogeneity, and mechanical properties, including bond strength, microhardness, and wear behaviors. The findings suggest that while admixed powders are economical and simple to prepare, milled and precoated formulations offer better deposition and properties by facilitating prior contact between Ni and lubricant powders. The optimal hBN content for effective deposition in Ni coatings was identified as approximately 6 wt%, with observed reductions in friction and some trade-offs in bond strength and lubricant uniformity at higher hBN contents. For more detailed insights, please refer to the article in Tribology Transactions. Luo et al. [160] presented a novel approach for enhancing the interface of cBN/NiCrAl nanocomposites through the formation of a nanoscale active layer via cold spray technology. They revealed that this method significantly improved thermal conductivity due to the creation of strong interfacial bonding at a relatively low annealing temperature of 825 °C, which is considerably lower than traditional sintering temperatures. This process limits excessive interfacial reactions, avoiding deterioration of mechanical properties. The findings open new avenues for tailoring the interface structures of composites, potentially broadening their application in industries requiring materials with superior thermal and mechanical performance. Zhao et al. [161] investigated the incorporation of hexagonal boron nitride (h-BN) into yttria-stabilized zirconia (YSZ) coatings using suspension plasma spraying (SPS). The research aimed to enhance the tribological properties of ceramic coatings by embedding h-BN as a lubricating additive. Through experimental analysis, it was found that adding h-BN to YSZ coatings effectively reduced their friction coefficient and wear rate. Moreover, the particle size and content of h-BN played crucial roles in the final coating’s performance, with finer h-BN particles contributing more significantly to tribological improvement. The optimum h-BN content was identified based on particle sizes, demonstrating the potential of SPS in creating coatings with improved wear resistance and lubrication for industrial applications.

Recent advancements in BN-based cold spray coatings have demonstrated significant progress in enhancing wear resistance, thermal conductivity, and lubrication properties. Studies have shown that incorporating large cBN particles into nanocomposites and optimizing Ni-hBN powder formulations improve abrasive wear resistance and reduce friction, making these coatings ideal for high-performance industrial applications. Additionally, novel approaches to enhancing interfacial bonding in cBN/NiCrAl composites and embedding h-BN in YSZ coatings have further improved thermal and tribological properties. However, challenges remain in achieving consistent deposition, maintaining bond strength, and optimizing lubricant uniformity at higher BN contents. Future research should focus on refining coating formulations, improving deposition techniques, and exploring advanced post-spray treatments to maximize the potential of BN-based cold spray coatings for industrial applications.

3.7.2. Cobalt/Nickel-Based Cemented Carbide

Cobalt/nickel-based cemented carbides are composite materials renowned for their exceptional hardness, high fracture toughness, and excellent wear resistance. These materials are composed of hard carbide particles, such as tungsten carbide (WC), bonded together by a metallic cobalt or nickel matrix. The resulting composite exhibits a unique combination of hardness and toughness, which makes it ideal for demanding applications such as cutting tools, mining equipment, and wear-resistant components. The cobalt or nickel binder phase also imparts significant corrosion resistance and enhances the material’s overall thermal stability, allowing it to perform reliably under extreme conditions. Given these superior properties, cobalt/nickel-based cemented carbide cold spray coatings have emerged as a promising technology for extending the service life of industrial components. By leveraging the hardness and durability of cemented carbides, these coatings provide robust, wear-resistant surfaces that improve the performance and longevity of tools and equipment in harsh operational environments.

Yoshiaki et al. [162] investigated the enhancement of a thermally sprayed cemented carbide layer (WC-CrC-Ni) on a steel substrate using friction stir processing (FSP). Initially, the WC-CrC-Ni layer was deposited via thermal spraying. Subsequently, FSP, employing a sintered WC-Co tool, was utilized to modify this layer. The study’s outcomes revealed that FSP effectively eliminated defects within the layer, leading to a denser arrangement of WC particles and a significant improvement in microstructural properties. This process resulted in a notable increase in the layer’s hardness, achieving approximately 2000 HV, which surpassed the hardness of the unmodified sprayed layer by about 1.5 times. Moreover, the bond strength between the cemented carbide layer and the steel substrate was enhanced due to the diffusion of metallic elements and mechanical interlocking at the interface, without compromising the fracture toughness. This research demonstrates that FSP is a potent method for improving the structural and mechanical properties of thermally sprayed cemented carbide layers, offering promising implications for the development of advanced coated materials with superior performance characteristics. Lupu et al. [163] focused on improving the wear and corrosion resistance of 52,100 steel by applying a Ni/CrC coating using the cold spray technique. The study revealed that the applied coating was dense, adhered strongly to the steel substrate without any cracks, and significantly enhanced the steel’s mechanical properties. Through microscratch resistance and electro-corrosion tests, the coating demonstrated excellent wear resistance and a significantly reduced oxidation rate compared to uncoated steel, indicating a marked improvement in corrosion resistance. The microstructural analysis confirmed the effective mechanical interlocking between the coating and substrate, attributed to particle deformation upon impact. Microindentation tests yielded high hardness values and a Young’s modulus indicating the coating’s potential to protect and extend the service life of steel components in demanding industrial environments. Overall, the research showcased the effectiveness of the Ni/CrC cold spray coating in bolstering the durability and performance of 52,100 steel. Jasthi et al. [164] investigated the microstructure and mechanical properties of Cu-Cr-Nb (GRCop-42) and Fe-Ni-Cr (HR-1) alloy deposits created using an HPCS process, focusing on the impact of varying the process gas between helium (He) and nitrogen (N₂) and the effect of post-deposition heat treatments. The research utilized a blend of GRCop-42 powder with incremental additions of HR-1 powder (ranging from 0 to 100 wt%) to examine the alterations in mechanical behavior and structural integrity of the deposits. The findings revealed that using He as a process gas yielded deposits with significantly lower porosity and improved mechanical properties compared to those produced with N₂. Initially, all deposits exhibited brittle characteristics. However, subsequent heat treatments considerably enhanced their ductility and tensile strength. The study observed the coarsening of Cr₂Nb precipitates in GRCop-42 deposits and the formation of the η phase in HR-1 deposits after heat treatment, alongside σ phase formation at the interfaces in composite deposits. The incorporation of up to 50 wt% HR-1 into GRCop-42 deposits effectively reduced porosity and boosted mechanical properties, attributed to the hard HR-1 particles inducing in situ peening effects. This research underscores the potential of cold spray additive manufacturing for producing high-performance composite materials, though it also highlights the necessity for optimized heat treatment protocols to mitigate undesirable phase transformations. Figure 19 shows the SEM micrographs of GRCop-42 with varying contents of HR-1 produced using N₂ process gas after the aging heat treatment.

Anderson et al. [165] evaluated the tribological performance of FeMnCrSi alloy coatings applied to carbon steel via cold gas spray (CGS), emphasizing the impact of deposition parameters. By varying the working gas to nitrogen (N₂) at temperatures of 900 °C, 1000 °C, and 1100 °C, it was discovered that coatings deposited at 1000 °C showcased enhanced hardness, elastic modulus, cavitation resistance, and sliding wear resistance. Similarly, coatings deposited with helium (He) at 600 °C exhibited high compressive residual stress and robust mechanical properties, highlighting the significance of precise deposition parameters. The coatings demonstrated high density and low porosity, with post-deposition heat treatments further improving ductility and tensile strength. These findings illuminate the potential of CGS FeMnCrSi coatings to significantly improve the wear and cavitation resistance of steel substrates, providing insights into optimizing CGS parameters for superior protective coatings.

Dong et al. [166] investigated the application of FeCrAl and Mo coatings on Zr substrates through cold spraying, aimed at enhancing high-temperature performance by preventing interdiffusion. Utilizing transmission electron microscopy (TEM) and scanning electron microscopy (SEM), the study uncovered that cold spraying leads to significant plastic deformation, creating microbands and interlocked nanoscale structures for improved mechanical interlocking at the Mo/Zr interface. Energy-dispersive X-ray spectroscopy (EDS) analyses confirmed the fine-scale mixing of Zr and Mo, indicative of a strong bond. Mechanical testing revealed that coated Zr samples exhibited reduced strength degradation at elevated temperatures compared to uncoated samples, attributed to the work-hardening effect from Mo particle impacts. The research demonstrated that cold-spraying effectively deposits Mo as a diffusion barrier, significantly enhancing the microstructural and mechanical integrity of Zr substrates for potential nuclear applications.

Significant progress has been made in the development of cobalt/nickel-based cemented carbide cold spray coatings, showcasing their enhanced wear resistance, hardness, and corrosion protection. Studies have demonstrated that techniques like friction stir processing (FSP) and optimized cold spray parameters can effectively improve the microstructure and mechanical properties of these coatings, resulting in denser, defect-free layers with superior performance. Additionally, the application of these coatings on various substrates has shown marked improvements in durability and longevity under extreme conditions. Despite these advancements, challenges such as achieving consistent coating quality, managing porosity, and optimizing post-deposition treatments remain. Future research should focus on refining spray techniques, exploring advanced post-processing methods, and developing new composite formulations to maximize the benefits of cobalt/nickel-based cemented carbide cold spray coatings for industrial applications.

4. Summary and Prospects

Cold spray technology, as an emerging surface engineering technique, has garnered significant attention from both the scientific community and industry due to its unique process characteristics and potential applications in material preparation. A comprehensive analysis of relevant literature and experimental studies summarizes the major advancements and challenges in the field of MMC coating preparation using cold spray technology.

4.1. Summary

(1)

Cold spray technology deposits coatings by accelerating particles to subsonic or supersonic velocities onto a substrate, causing plastic deformation. Unlike traditional thermal spray techniques, cold spraying avoids material oxidation or thermal stress issues associated with high temperatures, enabling the deposition of pure metals, alloys, and even composite materials without adversely affecting the substrate properties. Recent research highlights the application of cold spray technology in areas such as the repair of aerospace components, wear-resistant coatings for automotive parts, surface modification of biomedical implants, and anti-corrosion coatings for various mechanical equipment. These applications not only demonstrate the versatility of cold spray technology in the domain of MMC coatings but also its potential to enhance the performance and extend the lifespan of industrial products.

(2)

However, despite significant progress in preparing MMC coatings, challenges remain concerning the uniformity of the coatings, adhesion strength, and control over microstructure. Current research focuses predominantly on optimizing coating properties by manipulating process parameters such as particle temperature and velocity, spray distance, and the size and shape of particles. These studies contribute to a better understanding of particle behavior during the cold spray process and their interaction with the substrate, providing a scientific basis for fabricating coatings with the desired performance. The main advantages of this technology include low thermal input, high deposition rates, and broad material compatibility. Nonetheless, cold spray faces technical challenges such as insufficient bond strength and porosity control. Moreover, ensuring coating uniformity on substrates with complex shapes remains a technical bottleneck.

4.2. Prospects

(1)

Microstructure optimization and material property control: To enhance the performance of cold spray technology in the field of MMC coatings, research is concentrated on optimizing the microstructure and controlling the material properties of the coatings. By selecting different powder materials such as metals, alloys, or composites, researchers aim to improve the hardness, wear resistance, and adhesion strength of the coatings. The use of a mixture of micron-sized and nano-sized powders has been proven to increase the density and bond strength of the coatings.

(2)

Advancements in equipment technology: Modern cold spray systems are evolving towards more efficient and intelligent solutions. Innovations include modular designs that offer easier customization and upgradability, integration of advanced process control technologies to adjust and optimize spraying parameters in real time, and the development of more sophisticated powder delivery systems to ensure stable and uniform spraying. Additionally, by altering spraying parameters such as the distance between the nozzle and the substrate, the velocity of powder ejection, and the pressure of the carrier gas, researchers can finely tune the microstructure of the coating, thereby controlling its performance. Post-treatment techniques such as heat treatment, laser treatment, or electrochemical treatment are also utilized to enhance coating properties.

(3)

Expansion into new applications: Beyond traditional industrial applications, the use of cold spray technology is gradually expanding into the biomedical, electronics, and energy sectors. In the biomedical field, researchers are exploring the use of cold spray technology to prepare biocompatible coatings to improve the interfacial characteristics of implant materials. In the electronics industry, the preparation of conductive coatings via cold spraying can provide more efficient heat dissipation solutions for electronic devices. In the energy sector, corrosion-resistant coatings prepared by cold spraying can enhance the stability and lifespan of energy equipment in harsh environments.

(4)

Innovations with machine learning and AI: Further innovation includes the use of machine learning and artificial intelligence algorithms to predict and optimize the spraying process. Through training with extensive experimental data, these algorithms can predict the impact of different spraying parameters on coating performance, thereby optimizing the process.

In summary, while significant progress has been made in the development of hard coatings using cold spray technology, numerous challenges remain to be overcome. With ongoing advancements in materials science, process technology, and equipment manufacturing, cold spray technology is poised to find broader applications across various fields, driving the development of new materials and processes.