Advanced HVOF-Sprayed Carbide Cermet Coatings as Environmentally Friendly Solutions for Tribological Applications: Research Progress and Current Limitations

Abstract

Thermally sprayed carbide cermet coatings, particularly those based on tungsten carbide (WC) and chromium carbide (Cr₃C₂) and produced with the high velocity oxygen fuel (HVOF) process, are used in tribological applications as environmentally friendly alternatives to electroplated hard chrome coatings. These functional coatings are especially prevalent in the automotive industry, offering excellent wear resistance. However, their mechanical and tribological performances are highly dependent on factors such as feedstock powders, spray parameters, and service conditions. This review aims to gain deeper insights into the above elements. It also outlines emerging advancements in HVOF technology—including in situ powder mixing, laser treatment, artificial intelligence integration, and the use of novel materials such as rare earth elements or transition metals—which can further enhance coating performance and broaden their applications to sectors such as the aerospace and hydro-machinery industries. Finally, this literature review focuses on process optimization and sustainability, including environmental and health impacts, critical material use, and operational limitations. It uses a life cycle assessment (LCA) as a tool for evaluating ecological performance and addresses current challenges such as exposure risks, process control constraints, and the push toward safer, more sustainable alternatives to traditional WC and Cr₃C₂ cermet coatings.

1. Introduction

The surface characteristics of construction block elements greatly affect the field of applications since they are exposed to several functional factors, including mechanical, frictional, thermal, and/or corrosive conditions [1]. The application of cyclic motions, high stress, and elevated temperature conditions can lead to degradation and even the damage of the surface properties of materials [2,3]. Under these conditions, the use of coatings, principally of composite coatings, is a powerful practice for surface protection [4,5,6,7,8]. The thermal spraying of different types of coatings is classified as a cutting-edge technology and environmentally friendly solution for the design of composite coatings with high levels of protection from wear and corrosion phenomena. Thermal spraying is widely used in the industrial sector for a wide range of applications due to the flexibility of the process and the range of useable coating materials [9,10,11,12,13]. The field of applications covers energy, transport, electric and electronic devices, and biomedical and materials extraction and processing [14].

The global market revenue generated from materials, equipment, and coatings development was estimated at USD 10.46 billion in 2023 and is expected to grow at a compound annual growth rate of 4.8% from 2024 to 2030 [15]. Coating manufacturers are influenced by the strict regulations of REACH, U.S. EPA, and OSHA, which are concerned with the removal of hard chrome plating due to the production of carcinogen byproducts. They are finding and adopting sustainable solutions that can further promote the growth of the industrial market [15]. Other aspects, such as energy efficiency, weight saving, and the economy of resources, are also being considered. Depending on these factors, the optimization and advancement of the thermal spray facilities [16] and the design of new materials occur [17]. The general characteristics provided by thermal spray coatings include wear resistance against adhesion, abrasion, erosive, and cavitation forces; electrical insulation; chemical resistance; and improved adhesion between the top layers of the different components and the substrate.

The thermal spray process principally involves heating a spraying feedstock from the inside or the outside of a spray gun. The velocity and the difference in temperature between gas and particles greatly affect the heat transfer and the momentum flow to the powder particles [18]. Normally, the sprayed coatings have a thickness range of between 50 µm and 1 mm [19]. Several parameters influence the interactions between particles and the environment, even if the time of the interaction does not exceed a few microseconds. The spraying feedstocks are accelerated towards the surface of the substrate with elevated thermal and kinetic energies. As presented in Figure 1, different spraying materials can be used in the form of flex wire or wire, rods, and powders [20]. Metal alloys, steels, ceramics, and even plastics can be employed as spraying materials.

The heterogeneous and lamellar structures of the coatings are obtained, with some differences due to phase composition and the original chemicals of the initial feedstock. Thermal spraying has evolved over time, and it can be classified into three main processes according to source of energy, as shown in Figure 2. A comparison between the most common techniques is illustrated in

Table 1. Comparison between various thermal spray processes [21,22,23,24,25,26].

Process	Type of Feedstock	Heating Source	Temperature (°C) of the Gun	Particle Velocity (m/s)	Coating Materials	Porosity (vol.%)	Hardness Rc	Bond Strength
HVOF	Powder	Oxypropylene/Hydrogen/Propane/LPG	3100	1350	Metallic and ceramic	0.1–2	50	Excellent
Plasma spraying (APS)	Powder	Plasma arc	16,000	120–600	Metallic/Ceramic/com-pound/plastic	1–7	50	Very good to excellent
Detonation gun	Powder	Gas detonation Oxygen/Acetylene/Nitrogen	4500	800	Metallic/Ceramic/com-pound/plastic	0.1–1	**	Excellent
Electric arc	Wire	Arc between electrodes	6000	240	Ductile Materials	10–20	35	Good
Flame spraying	Powder Wire	Oxyhydrogen/Oxyacethylene	2800	180	Metallic and ceramic	10–20	20	Fair
Cold spray	Powder	Compressed gas/Helium/Nitrogen/Air	500–600	500–1200	Metallic and ceramic	<2	**	Fair

** Undetermined.

. This paper focuses on the high-velocity oxy-fuel (HVOF) spraying technique, which enables the deposition of a diverse range of materials onto substrate surfaces, including carbides, nitrides, metal alloys, and ceramics. The selection of an appropriate coating for wear and corrosion protection, as well as for the repair of worn or surface-damaged components and the replacement of hard chrome, is critical. The strategic choice of these coatings leads to commercially competitive solutions in surface engineering applications.

Thermally sprayed cermet coatings are used in a wide range of tribological applications, where enhanced performances, cost savings, and extended service life are offered [27,28]. Significant environmental and economic benefits can be achieved through a strong focus on tribology, leading to approximately a 15–20% reduction in friction and wear [29]. Industrialized countries are being called upon to advance the development of mechanical components in vehicles, tools, machinery, and equipment to support greater energy efficiency and more sustainable solutions. Tribological research plays a crucial role in promoting sustainability by minimizing friction and wear, thereby improving energy efficiency of mechanical systems. Reducing surface degradation directly contributes to extended component lifespans and improved operational durability. As a result, optimizing tribological performance supports the shift toward environmentally sustainable technologies by decreasing material waste, reducing maintenance requirements, and mitigating system failures or operational delays. The development of cermet coatings on sliding surfaces presents a promising approach for achieving low wear losses, which is essential for enhancing durability and energy efficiency. By significantly reducing material degradation during operation, these coatings contribute to the advancement of a sustainable circular economy within the transport, energy, and manufacturing industries, where prolonged component life and reduced energy consumption are critical performance objectives. In fact, environmental and health problems associated with residues and wastes from lubricants can be generated and should be technically resolved [30]. Advanced tribology research is focused on the development of new materials to reduce the use of lubricants and aims to decrease the generation of abrasive particles to avoid pollution and health problems. Efforts should be made to support the transition to renewable and more sustainable fuels that promote cleaner combustion, minimized environmental emissions, and extended replacement intervals of mechanical parts in the transport and energy sectors [31].

Advanced thermal spray processes are extensively used in the automotive industry in engine parts and transmission [32,33]. Some applications have acquired maturity while other applications are still under development and represent a considerable market for the future. For instance, HVOF spray is an intriguing process for coating metal matrix/carbides materials used in piston rings and cylinder bores, especially in high-performance diesel engines. Carbide-alloy cermet coatings made of tungsten carbide or chromium carbides are promising substitutes for hard chromium due to their capacity to combat harsh environmental conditions, intensive wear at high loads and temperatures, and cyclic motions. In addition to the intrinsic properties of the feedstock materials, the coating characteristics are significantly influenced by the spraying parameters, the morphological attributes of the powders (such as particle size and shape), and the operational conditions during deposition.

After providing a brief historical context of HVOF-sprayed carbide cermet coatings and their performance requirements, this review article initially presents the application of HVOF thermal-sprayed cermet coatings—particularly those based on WC and Cr₃C₂ carbides—in the automotive sector, focusing on their tribological properties and environmental benefits. Adopting a holistic yet critical perspective, it examines how the composition of the hard phase and matrix powders, along with spraying parameters and operational service conditions, influence the tribological performance of these carbide-based cermet coatings. The review also highlights major developments in this field while addressing current challenges and opportunities associated with HVOF carbide cermet coatings in broader applications, including aerospace systems, nuclear reactors, hydro machinery, and other areas where properties such as erosion resistance, corrosion resistance, and toughness are critically needed. Finally, this paper provides a critical discussion of the limitations of the HVOF process and its future scope from an ecological point of view. It is intended to inform both new and experienced researchers about key aspects of the discipline and their relevance to thermally sprayed cermet coatings.

2. Historical Overview and Performance Standards for HVOF Cermet Coatings

Cermets are composite materials consisting of hard ceramic particles embedded within a metallic binder, designed to synergistically combine the high hardness and thermal resistance of ceramics with the toughness and ductility of metals [34]. The first generation of cermets emerged shortly after Schröter’s patent on WC-Co hard metals [35]. Subsequently, Schwarzcopf and Hirshl pioneered the industrial-scale of TiC-Mo₂C systems, incorporating Ni, Cr, and other metallic constituents into the binder phase [36]. Continuous research and technological progress have focussed on enhancing the mechanical, thermal, and functional properties of these coatings through the development of novel material systems, improved deposition processes, and advanced application techniques [37,38,39,40,41,42]. Among the thermal spray methods developed for cermet deposition, the high-velocity oxy-fuel (HVOF) process and its variants have demonstrated outstanding performance. HVOF-deposited coatings are characterized by high density, low porosity, and excellent hardness, which provide superior resistance to abrasion and erosion wear [43,44].

Significant historical applications include the use of HVOF coatings during the 1990s on tailhook components of A-4 aircraft, which were subject to severe wear during carrier landings. These components originally exhibited a service life of no more than 10 landings. The introduction of HVOF-deposited WC-8Co and CrC-25NiCr coatings, alongside a Plasma Transferred Arc (PTA)-deposited Satellite 6 coating, demonstrated considerable improvements in impact resistance, wear durability, and resistance to thermal cracking [45].

Since the initial adoption of HVOF technology, there has been a continuous evolution in the formulation of cermet feedstock powders. The most commonly used carbide phases remain to be tungsten carbide (WC) and chromium carbide (Cr₃C₂), typically combined with metallic binders such as nickel (Ni), cobalt (Co), or chromium (Cr), in proportions ranging from 4% to 30% by volume [46,47]. Advanced thermal spray deposition technologies have also emerged. The Activated Combustion High-Velocity Air-Fuel (AC-HVAF) process, for example, offers improved fuel efficiency and reduced oxidation of carbide particles. Verstak et al. [48] demonstrated that AC-HVAF-deposited WC-Co and WC-Co-Cr coatings showed wear resistance improvements of between 10 and 20 times over electroplated chrome and spray-fused overlays, as illustrated in Figure 3.

Lakhdari et al. [49] conducted a comparative study of two thermal spray techniques: high-velocity oxygen fuel (HVOF), used to deposit WC-12Co coatings, and the electric arc spray (EAS), used to apply Fe-based (W,Ti)C coatings for protecting industrial parts against wear and corrosion. The WC-12Co coating deposited by the HVOF process exhibited superior mechanical properties and abrasive wear resistance, exhibiting a high hardness of 1375 HV and a significantly lower wear rate of approximately 3.6 × 10⁻⁴ mm³·N⁻¹·m⁻¹. These enhanced properties were attributed to the coating’s high density, superior hardness, and uniform dispersion of WC particles within the metallic matrix. In contrast, the Fe-(W,Ti)C coating deposited by wire arc spraying showed lower overall performance but emerged as a cost-effective alternative for applications where moderate performance is acceptable. Berghaus et al. [50] developed nanostructured WC-12%Co coatings using the Suspension Plasma Spray (SPS) process. However, the ultrafine powders exposed to temperatures exceeding 2200 °C led to the degradation of WC into brittle phases (W₂C/W₃C) and the formation of an amorphous cobalt binder. The resulting coatings exhibited a hardness of 700 HV_0.3—markedly lower than those produced via HVOF, which typically range from 1000 to 1550 HV_0.3.

3. Advanced HVOF-Sprayed Coatings for the Automotive Sector

3.1. Internal Combustion Engine

To meet the growing demands for higher wear resistance and extended service life in the automotive industry, the piston ring–cylinder liner pair must perform reliably under harsh conditions, including high temperatures, loads, and sliding speeds [51]. This interface contributes up to 26% of total engine friction losses [52], making its tribological performance a key factor in engine efficiency [53,54,55]. Wear of the piston ring can lead to engine failure, emphasizing the critical role of this contact in ensuring overall reliability [56]. As schematized in Figure 4, the piston ring pack is composed of compression and oil control rings, which must facilitate effective combustion sealing and heat transfer [57,58]. Repeated thermal cycling from start–stop operations can cause fatigue and material degradation [59,60]. Figure 5 presents the essential characteristics required for the efficient operation of a piston ring [61]. Piston ring materials must combine high wear and corrosion resistance, thermal conductivity, and elasticity to prevent premature failure and ensure long-term engine performance [62].

Cast iron and steel were the first materials used for piston rings [63]. Nowadays, surface modification technologies are attracting more attention, especially with the increase in peak pressure and the use of harsher thermal and mechanical loads, for instance, in heavy-duty diesel engines [64]. Advanced surface modification methods, including physical vapor deposition [65,66], chrome plating [67], electrochemical machining technology [64], atmospheric plasma spray coating [68], and high-velocity oxy-fuel coating [69], have been proposed to enhance the surface properties of piston rings and cylinder liners.

Carbide cermet coatings applied with HVOF spraying present a viable alternative to hard chromium coatings in piston ring applications, owing to their superior wear resistance at elevated temperatures, as well as their durability under harsh environmental conditions and cyclic loading [70,71,72]. Extensive research has refined the HVOF process for the deposition of ultra-hard WC-based cermet coatings, offering enhanced resistance to wear, corrosion, and high temperatures [69,71,73,74]. Piston rods and plungers have been coated with WC-10Co4Cr powders using the HVOF process as an alternative to hard chrome plating [73]. These coatings demonstrated higher microhardness, superior corrosion resistance, and higher adhesion to the substrate compared to conventional hard chrome. Additionally, Hazra et al. (2012) reported that WC-12Co coatings deposited via HVOF exhibited significantly better wear resistance than both plasma-sprayed counterparts and electroplated hard chrome [74]. The results were attributed to the reduced porosity and enhanced hardness of the HVOF-deposited coatings. Failure in this coating occurs due to localized microfracturing. HVOF-sprayed coatings with a composition of 75% Cr₃C₂and 25% NiCr exhibited excellent mechanical and tribological performance, making them strong candidates to replace electroplated chromium coatings [69]. The results align with those reported in previous studies, further supporting the reliability of the findings [75,76]. WC-CoCr coatings deposited by the HVOF process demonstrated high wear resistance and excellent mechanical properties [75]. These improvements were attributed to optimal carbide retention and effective particle heating during deposition. Moreover, since exhaust gas emissions are closely linked to incomplete combustion in internal combustion engines, enhancing in-cylinder combustion efficiency is essential for reducing pollutant formation [77]. To this end, combustion chamber components such as intake valves, exhaust valves, and pistons have been coated with WC-based materials using the HVOF process to improve combustion efficiency in diesel engines [78]. Scanning Electron Microscopy (SEM), presented in Figure 6, revealed a continuous interface between the coating layer and the substrate. It has been demonstrated that CO emissions of the coated engine decreased by 16% compared to the standard engine. Additionally, smoke emissions were reduced by 6% in the coated configuration.

3.2. Brake Assemblies

HVOF-applied cermet coatings show significant potential in brake systems, which consist of friction material pads rotating against automotive discs [79,80]. It is hypothesized that improving the wear resistance of brake discs could reduce wear particle emissions, thereby mitigating their negative environmental impact [81]. The tribological contact in brake systems is unique, characterized by high speeds, temperatures, and contact forces. The disc–lining pair operates under dry sliding conditions for approximately 50% of the braking duration [82]. As a result, ideal brake systems must provide stable friction behavior across all operating conditions. Brake discs and linings must meet various requirements, including a stable friction coefficient [83], strong anti-wear properties [84], and minimized noise and particulate emissions [85].

It has been reported that Cr₃C₂-NiCr and WC-CoCr coated discs, when rotating against a commercial friction material, exhibited extended running-in stages [80]. This stage is necessary to form an effective friction layer, produced from oxidized and compacted fragments of the friction material at ambient temperature. High wear resistance in brake disc coatings is achieved through a combination of low porosity and high microhardness [82]. Uncoated brake discs have higher surface roughness due to increased abrasion and lower hardness, whereas HVOF-coated discs (20NiCrBSi-WC12Co) demonstrate superior braking performance with lower roughness and higher hardness. The composition of the friction layer influences the wear mechanisms [83]. SEM revealed a small amount of wear particles and smooth secondary plateaus on the surface of the coated disc, as shown in Figure 7.

The conditions affecting thermal conductivity and the formation of the friction layer are critical for enhancing the tribological performance of brake discs [84,85,86,87]. Additionally, the polishing process significantly impacts both surface roughness and the tribological behavior of WC-10CoCr₄ coatings applied by the HVOF process on brake discs [88]. The industrial feasibility of the polishing operation has been demonstrated, achieving an average surface roughness of approximately 1 µm. The joint development of grinding processes and coating materials is crucial for reducing environmental impact and production costs [89]. WC-FeCrAl and WC-FeCr HVOF cermet coatings are proposed as eco-friendly alternatives to cobalt- or nickel-based matrices for application on cast iron brake discs [89].

3.3. Rolling Stock Parts

Thermal spray processes are also employed to enhance the service life and wear resistance of rail vehicle components that require high corrosion resistance and durability [90,91]. Materials such as nickel, iron, zinc, molybdenum-based alloys, and WC have been applied using various thermal spray techniques, including HVOF, Flame Spray (FS), and Twin Wire Arc Spray (TWAS). Figure 8 shows the “pivot roller,” which demands high corrosion resistance, high hardness, and excellent wear resistance, achieved through three selected coatings. The results indicate that WC-CoCr coatings sprayed with HVOF exhibited superior properties compared to other variants, including flame-sprayed NiCrBSi and HVOF-sprayed CrC-NiCr [90,91].

HVOF-sprayed WC-Co, WC-Co/NiSiCrFeB, and NiSiCrFeB coatings have been applied to AISI 1095 steel for use in conveyor rolls within the automotive sector, where repeated contact with softer sliding steel components leads to cumulative wear [92]. WC-Co/NiSiCrFeB and WC-Co coatings demonstrate superior suitability for moderate to hard counterface materials, such as medium-and high-carbon steels, compared to NiSiCrFeB.

4. Tribological Performances of Carbide-Alloy Cermet Coatings

Spray parameters, including spray distance, particle velocity, gas pressure, and powder feed rate, significantly influence the microstructure, porosity, and adhesion strength of HVOF hard coatings. The composition, distribution, size, and uniformity of the hard phase, along with the chemical composition of the feedstock powders, also impact the coating’s wear resistance, hardness, and overall tribological performance. Additionally, test conditions such as temperature, applied load, and atmospheric environment play a key role in determining the coating’s behavior during friction and wear testing. The interaction of these factors ultimately governs the durability and effectiveness of HVOF coatings in demanding operational environments. Figure 9 illustrates the key factors influencing the functional performance and service life of cermet coatings. This section explores several parameters that significantly affect the tribological behavior of carbide cermet coatings.

4.1. Service Conditions

In their studies, Wesmann et al. [93] investigated the tribological performance of self-mated 86WC-10Co₄Cr coatings under various test conditions. The friction and wear behaviors of the coatings were significantly influenced by sliding temperature and atmospheric conditions, as experiments were conducted in both air and nitrogen environments. These parameters played a critical role in determining the coatings’ tribological performance. The lowest friction was observed in a nitrogen atmosphere. The friction response increased by 0.4 when the temperature rose from room temperature to 200 °C, which had the largest impact on friction. Prasad et al. [94] studied the dry sliding behavior of 30% by weight WC-10Co reinforced with 70% FeNiCrMo (Metco 41 C). The two powders were mechanically blended using a milling process and then subjected to thermal spraying via HVOF to form coatings. The effects of temperature variations (room temperature, 200 °C, and 300 °C) and applied normal loads (10 N and 20 N) were evaluated through pin-on-disc friction testing. The presence of hard carbide phases, including Ni₃C, Cr₃C₂, and SiC, together with intermetallic compounds like Co₃W₃C and Mo₂C, significantly improved coating hardness and minimized material degradation. At elevated temperatures, oxide films formed on the coating surfaces, decreasing the friction coefficient at both loads compared to the substrate. Additionally, HVOF-sprayed WC-(W,Cr)₂C-Ni cermet coatings, using WC-Cr₃C₂-Ni feedstock powders, were applied to stainless steel substrates [95]. The tribological properties of these coatings were subsequently compared to those of WC-10Co-4Cr, which served as a reference material. The dry sliding tests were performed at four different temperatures (25 °C, 400 °C, 600 °C, and 750 °C). It has been reported that the overall damage across the wear traces of both coatings was quite limited at room temperature. At temperatures up to 600 °C, material detachment was observed in the WC-(W,Cr)₂C-Ni coating, primarily due to abrasive interactions between the transfer layer asperities and the thermally softened hard metal surface. In contrast, at 750 °C, the tribological behavior was dominated by the formation of a uniform and thin oxide scale composed of CrWO₄ and NiWO₄, which acted as a lubricating film. This minimized direct contact between the coating and the counter body, reducing the coefficient of friction compared to that at 600 °C. Severe oxidative deterioration led to a significant decline in the functional performance of WC–CoCr coatings, consistent with findings reported in previous studies [96].

4.2. Feedstock Powders

The feedstock material is a critical parameter in defining the HVOF application. For example, CoCr alloys, often used as the base material for WC cermet coatings, are widely applied in pipeline valves as a superior alternative to hard chrome plating. They provide significantly improved wear resistance, thereby extending the service life of gate valves under harsh operational conditions [97,98]. WC-Co and WC-CoCr cermet coatings developed using HVOF were applied to steel alloy substrates [99]. WC-CoCr coatings demonstrated better wear resistance due to the decomposition of WC into new phases during coating, which contributed to higher hardness.

The influence of powder size on the tribological characteristics of 86WC–10Co–4Cr coatings was studied by Rakhadilov et al. [100]. Friction and wear tests were conducted using a ball-on-disc tribometer with a 100Cr6 ball (6 mm diameter), as shown in Figure 10. The applied load was 10 N. Coatings developed from smaller powder particles (20–30 µm) exhibited the best friction and wear resistance, while larger particles (30–40 µm or 40–45 µm) did not show improved performance. Coatings made from 20–30 µm powder particles demonstrated the highest microhardness (780 HV_0.1) and wear resistance due to an increased WC phase content [101,102]. It was also shown that the extent of WC transformation during the spraying process was primarily influenced by powder characteristics (particle size, morphology, and carbide size), oxygen content in the environment, and spray parameters [102]. Jonda et al. investigated the effects of spray distance and feedstock material on the erosion resistance of WC-based cermet coatings applied to AZ31 magnesium alloy using the HVOF technique [103]. Results showed that spray distance had a significant effect, with coatings sprayed from 320mm displaying lower porosity, higher hardness (up to 1328 HV_0.2), smoother surfaces, and superior erosion resistance compared to those deposited from 400mm. In contrast, type of feedstock material, including WC-20Cr₃C₂-7Ni, WC-10Co₄Cr, or WC-12Co, had minimal influence. Moreover, the study also noted the presence of imperfections such as cracks, pores, and voids, attributed to thermal stresses during thermal projection and the short residence time of particles in the relatively low-temperature flame (~2930 °C).

Cr₃C₂-based coatings also belong to hard cermets, which are well known for their excellent resistance to oxidation with lower wear resistance comparative to WC-based coatings [104,105]. In the development of wear-resistant coatings, the selection of appropriate binder metals is critical to achieving the desired combination of mechanical performance, thermal stability, and environmental safety. The tribological study of different Cr₃C₂ coatings deposited by the HVOF process—made from Cr₃C₂-25NiCr, Cr₃C₂-50NiCrMoNb, and Cr₃C₂-37WC-18NiCoCr—demonstrated excellent performance under dry sliding wear conditions at both room temperature and 700 °C [106]. The Cr₃C₂-25NiCr coatings exhibited wear rates comparable to those at room temperature due to their excellent crack resistance behaviour. The Cr₃C₂-37WC-18NiCoCr coatings experienced rapid oxidation of WC particles exacerbated by the formation of WO₃ protrusions over the WC phase. The higher matrix content of the Cr₃C₂-50NiCrMoNb coatings decreased the wear resistance of the coatings. Wear mechanisms also evidenced material removal by brittle cracking, leading to delamination of the top layer, particularly in the Cr₃C₂-25NiCr coatings. This delamination increased wear rates and contributed to the instability of the steady-state friction coefficient.

Traditionally, Ni- and Co-based metal alloys have been widely used due to their excellent toughness and compatibility with hard ceramic phases such as Cr₃C₂. However, increasing regulatory and health concerns have prompted the exploration of alternative materials. Fe-based coatings have emerged as a viable and sustainable substitute, offering comparable performance while reducing the health and environmental risks associated with Ni and Co. Nickel is recognized as a skin sensitizer under Regulation (EC) No 1907/2006 (REACH) [107], and cobalt has been classified as a category 1B carcinogen by inhalation by the Cobalt REACH Consortium [108]. These classifications necessitate stricter handling protocols and restrict their use in environmentally sensitive or occupationally exposed settings.

Cobalt is also considered a critical raw material [109] and its use is limited by supply insecurity and low availability. From an economic perspective, both nickel and cobalt are regarded as expensive materials with significant price fluctuations [110]. The adoption of Fe-based binders not only aligns with evolving safety regulations but also offers advantages in terms of cost-effectiveness and raw material availability. Ongoing research has demonstrated that, when properly developed, Fe-based coatings can achieve tribological and mechanical properties comparable to those of their Ni- and Co-based counterparts. Chandramouli et al. [111] recently developed Fe-based coatings (stainless steel 316L SS316L) containing 30 wt.% of Cr₃C₂ or 30 wt.% of WC using the HVOF process to enhance their wear resistance at a high temperature. Both coatings exhibited reduced friction coefficients and wear rates at 600 °C. That was attributed to the presence of protective oxide phases formed after wear tests, such as FeO, Fe₂O₃, Cr₂O₃, NiMoO₄, and NiWO₄. These phases exhibited strong adhesion to the underlying surfaces, thereby changing the dominant wear mechanisms toward oxidation-modified adhesive wear, as shown in Figure 11. Terajima et al. [112] demonstrated enhanced wear resistance in amorphous Fe-based (FeCrMoCB) coatings by incorporating 2–8 wt.% WC-12Co particles. Compared to the unreinforced coating, the additions reduced porosity by filling splat interface cavities. As the WC-Co content increased, microhardness improved from ~700 HV to ~900 HV. Tribological tests using an alumina ball showed a reduction in the friction coefficient from 0.8 to ~0.6 and in specific wear rate (Ka) from 8 × 10⁻¹⁴ to 1.5 × 10⁻¹⁴ m²/N.

Other factors related to the feedstock material, such as the characteristics of the carbide phases, can significantly influence the selection of appropriate process parameters. HVOF-sprayed Cr₃C₂-25wt.%NiCr coatings were produced by using five size ranges of carbide powders from 0.5 µm to 10 µm and five size ranges of matrix powder from 5 µm to 45 µm [113]. The experimental investigation outlined the necessity of adjusting the spray parameters to achieve dense and cohesive coatings that are crack-free. The primary sliding abrasive wear results were greatly affected by the size of the abrasive grit SiC and the coating type, while the application of different normal loads had limited impact on the wear behavior of the coatings. Overall, the fine-structured coatings provided higher resistance to sliding abrasive wear. Mattheus et al. [114] employed the HVOF process to apply various carbide-based coatings, including Cr₃C₂-NiCr and WC-Co, with particle sizes ranging from −30 to +10 µm and down to sub-5 µm. Their findings demonstrated that these coatings outperformed conventional hard chrome plating in terms of wear resistance and provided enhanced corrosion resistance in saline environments. The use of ultrafine powders (<10 µm) further improved coating properties by increasing density, homogeneity, and dimensional accuracy. These coatings found applications in piston rings and hydraulic components, where they significantly reduced wear and enhanced marine durability.

4.3. Thickness Characteristics of Protective Coatings

The mechanical and tribological performance of HVOF-sprayed carbide cermet coatings is strongly influenced by both coating thickness and compositional parameters. Changes in coating thickness significantly influence stress distribution, load-bearing ability, and wear behavior, while composition governs microstructural integrity, hardness, and thermal resistance. The variation in coating thickness has significant effects on the microstructural, mechanical, and tribological characteristics of WC-CoCr coatings deposited onto A6082-T6 aluminum alloy [115]. The coating thickness was varied from 50 to 150 µm by adjusting the number of torch scans on the substrate. Thicker coatings were denser and exhibited fewer defects. In fact, it was shown that newly deposited particles were more efficiently flattened when a higher number of layers was applied, resulting in harder surfaces with reduced porosity. The WC-CoCr coatings also demonstrated improved resistance to cyclic impact and wear, as shown in Figure 12. After cyclic impact testing, the thicker coating produced with five torch scans exhibited signs of transverse cracking and near-surface delamination, with a rough interface between the coating and substrate. In contrast, extensive delamination was observed on the surface of the coating produced with only two scans, the lowest number of torch passes.

On the other hand, functionally graded coatings (FGCs) prepared using the HVOF process represent an innovative concept designed to develop damage-tolerant systems for applications involving severe wear and high temperatures without the need for a bonding agent [76]. A functionally graded coating (FGC) deposited by HVOF spraying, with a gradual transition from AISI316 stainless steel to WC-12Co across six layers, has been reported to enhance damage tolerance through improved crack resistance and reduced thermal stresses [76]. The smooth gradation in composition and hardness also enables the deposition of thick coatings with high wear resistance and reduced cracking under static and dynamic loading, outperforming monolithic coatings of similar composition. According to Matejicek et al., FGCs are suitable materials for improving the tribological properties of cylinder liners, offering an alternative to single homogeneous cermet coatings [116]. The development of an FGC consisting of two WC-Co/NiAl composite layers has been reported in previous work [117], where pure WC-Co was used as the topmost layer. The incorporation of NiAl layers reduced the risk of delamination compared to pure WC-Co coatings. This improvement was attributed to reduced stress buildup during spraying, which enhanced the load-bearing capacity of the FGC. The WC–Co/NiAl FGC also demonstrated good wear resistance under cyclic loading conditions, with reduced cracking phenomena. Furthermore, the introduction ofnickel-25% chromium (Ni-25Cr) and WC-12Co intermediate layer in an FGC—featuring WC-10Co-4Cr as the topmost layer—improved the wear resistance of cast steel cylinder liners without compromising heat dissipation [118].

5. Challenges and Future Scope of HVOF Carbide Cermet Coatings

The increasing use of components in harsh environments, such as nuclear reactors, marine vessels, and aerospace propulsion systems, as well as the required efficiency in hydro-machinery, has prompted the evolution of carbide cermet coatings. This section synthesizes recent advances in this field, with a particular focus on the intrinsic properties that may limit their outstanding performance in demanding service environments. HVOF coatings exhibit high hardness (often >1200 HV) for outstanding abrasive and erosive wear resistance [119,120], along with notable fracture toughness that helps prevent crack initiation and propagation under dynamic loads [121]. Their strong resistance to thermal cycling allows them to endure rapid temperature changes without degradation, making them ideal for high-temperature applications such as gas turbines and aerospace components [122,123]. These properties stem from an optimized microstructure with low porosity, dense layering, and strong adhesion, achieved through HVOF processing [124,125].

5.1. Fine-Grained Microstructures

Guo et al. [126] produced fine-grained tungsten coatings (~312nm grain size, 5–10 μm thick) via chemical vapor deposition (CVD) for W-based cermet nuclear fuel systems, achieving reduced hydrogen permeability and maintaining mechanical integrity. The study offers relevant insights for optimizing HVOF-applied coatings. This highlights the pivotal role of refined grain boundaries and dense microstructures in limiting hydrogen diffusion and enhancing structural durability under extreme conditions [127,128].

Similar principles apply to carbide cermet coatings deposited by HVOF, where microstructural control is achieved through tailored powder characteristics (e.g., particle size distribution, morphology, and agglomeration) and finely tuned spray parameters, including flame temperature, particle velocity, and standoff distance [129,130]. These factors govern splat formation, coating density, and porosity, which are fundamental elements affecting diffusion resistance and mechanical strength. Studies on WC-Co and Cr₃C₂-NiCr systems, for instance, have demonstrated that coatings with submicron or nanostructured powders exhibit superior resistance to high-temperature oxidation, wear, and permeation by aggressive media [93,131]. Wang et al. investigated the influence of carbide grain size in three types of WC-CoCr powders on the microstructure and mechanical properties of HVOF-sprayed coatings. Results showed that ultrafine powders led to greater WC decomposition into W₂C due to higher dislocation densities. Notably, the presence of apparent twin structures within WC grains played a key role in enhancing microhardness and fracture toughness. Among the tested powders, the nanostructured variant produced coatings with the most favorable properties, offering minimal decarburization along with superior hardness and toughness [132].

Moreover, research in hybrid or modified HVOF processes, including suspension HVOF and nanostructured feedstock powders, further underscores the trend toward microstructural refinement to enhance functional performance [133,134,135]. These findings reinforce the broader principle that fine-grained, dense coating architectures, whether produced by CVD or HVOF, are essential for attaining low diffusivity, high toughness, and long-term reliability in chemically and thermally aggressive service environments.

5.2. Advances in Multi-Element Alloying

5.2.1. Incorporation of Earth Elements

The mechanical properties of cermet composite coatings are strongly influenced by the alloy matrix composition. Modifying the metal elements in the matrix (e.g., Ni, Co, Cr, and Mo) can enhance the tribological performance of cemented carbide coatings [136]. Recently, various commonly used rare earth elements, such as cerium oxide (CeO₃), lanthanum oxide (La₂O₃), and erbium oxide (ErO₃), have been incorporated into surface protective coatings using a range of surface processing techniques, including laser cladding, electrodeposition, and thermal spraying [137,138]. Rare earth elements can provide desirable refining and purifying effects, which modify the solidification process and microstructure of various coatings, lower the melting point of alloys, and promote the formation of compound-rich eutectic structures, thus enhancing the erosion resistance of cermet coatings [138,139,140,141,142]. The addition of rare earth elements has proven effective in reducing thermal residual stresses by refining microstructures and stabilizing phases [137,139]. Their inclusion also helps minimize the mismatch in the coefficient of thermal expansion (CTE) between the coating and substrate, thereby improving interfacial stress distribution and enhancing overall system reliability. In this context, Zhang et al. demonstrated that crack formation in WC/60Ni cermet coatings with CeO₂ addition of 1 wt.%, 2 wt.%, and 3 wt.% was strongly influenced by thermal residual stresses [139]. A CeO₂ content exceeding 2 wt.% led to the formation of refractory compounds, which reduced molten pool fluidity, increased thermal gradients, and elevated residual stresses that promoted cracking. During solidification, rare earth enrichment enhanced energy absorption, resulting in the release of more WC particles. While higher WC content increased hardness, it also reduced ductility and promoted carbide precipitation, which intensified tensile stresses and crack sensitivity. Using the high-pressure high-velocity oxy-fuel (HP-HVOF) spraying technique, WC-10Co-4Cr coatings were deposited onto SS410 substrates in both their undoped form and with the addition of rare earth oxides—specifically 0.9 wt.% each of CeO₃, La₂O₃, and ErO₃ [143]. The incorporation of rare earth elements led to notable enhancements in the mechanical properties of the coatings. Specifically, microhardness values increased significantly, ranging from 996 to 1382 HV, while bond strength improved to between 71 and 78 MPa. A reduction in porosity was also observed, with the addition of 0.9 wt.% ErO₃ exhibiting the lowest porosity at 0.89%. Slurry erosion testing showed that the experienced ErO₃-doped sample exhibited the lowest mass loss under all tested conditions, indicating superior erosion resistance. Building on the standard Cr₃C₂-NiCr coating, novel Cr₃C₂-NiCrCoMo/nano-CeO₂ (NCE) coatings are being developed to improve oxidation resistance through the formation of a protective spinel-containing oxide layer [144]. The study of Du et al. investigated the effect of nano-CeO₂ content on the tribological performance and microstructure evolution of NCE coatings at temperatures of 400 °C, 600 °C, and 800 °C [144]. The NCE coating exhibited enhanced high-temperature wear resistance, with reduced friction coefficient and wear depth attributed to oxide lubrication and improved adhesion of the oxide film. Nano-CeO₂ also reduced the Ostwald ripening threshold, promoting the growth and aggregation of nano-Cr₃C₂, which further enhanced wear resistance. The addition of nano-sized rare earth particles in chromium carbide composite coatings may produce a strengthening effect due to reinforcement by dispersed phases, which impedes dislocation motion [145]. In their study, Vishnoi et al. investigated the influence of rare earth oxide additives—erbium, lanthanum, and cerium oxides (each at 0.2 wt.%)—on the performance of WC-10Co-4Cr composite coatings deposited on martensitic stainless steel (SS410) using the HVOF process [146]. The introduction of these rare earth elements significantly enhanced key surface properties; the coating exhibited a fourfold increase in hardness compared to the substrate, maintained low porosity (1–2%), and demonstrated improved hydrophobicity (contact angle ~134°). These improvements suggested that rare earth enhances the coating’s erosion resistance by strengthening the surface and reducing water interaction.

5.2.2. Incorporation of Transition Metals

Tribo-oxidation processes play a significant role in the wear mechanism, particularly at high temperatures, and contribute to the overall performance of the coating. In WC- and C₃C₂-based coatings, single-phase oxides such as Cr₂O₃, WO₃, and CoO, as well as binary-phase oxides, including CoWO₄, Cr₂WO₆, CrWO₄, and NiWO₄, have been identified. Certain oxides can function as solid lubricants by forming a thin oxide layer on the surface during sliding wear, thereby enhancing wear resistance [147,148,149]. Wear-resistant coatings primarily rely on high hardness and a low coefficient of friction to reduce mass loss and material deformation. Lower friction decreases the mechanical force acting on the surface and reduces heat generation, thereby limiting the rise in temperature. Excessive frictional heating can increase surface temperatures to critical levels, potentially leading to softening and deformation of the material.

Molybdenum, a soft metal with a high melting point, contributes to wear resistance through the formation of a MoO₂ oxide. During dry friction, molybdenum readily oxidizes to form a MoO₂ layer, which exhibits excellent lubricating properties and suppresses the adhesion of contacting surfaces, significantly reducing friction [150,151]. Additionally, molybdenum has been incorporated into carbide-based alloys to improve performance and enhance the wettability between the binder and matrix phases [152]. A key contribution of Behera et al.’s work was the identification of beneficial transition metal oxides that form dynamically during wear [153,154]. Their study explored the influence of Mo on the high-temperature wear and friction behavior of HVOF-sprayed 70%WC-Co/25%Mo/5%C and 70%WC-CrC-Ni/30%Mo coatings applied to Superni76. Ball-on-disc wear tests were performed at 300 °C and 600 °C under loads of 10N and 30N, using an Al₂O₃ ball as the counter body. Oxide films were formed and acted as solid lubricants under low loads and provided thermal stabilization at elevated temperatures. Similar observations were also made when studying the tribological behavior of HVOF-deposited WC-CrCo coatings with 10 wt.% Mo on Ti-31 alloy [154].

5.3. New Techniques for HVOF Spraying

5.3.1. Mixing Methods for Feedstock Powders

Another significant challenge stems from the dissimilar physical properties of the carbide cermet coatings’ constituent phases, such as their melting points and thermal conductivities, which require different spray parameters during deposition, leading to non-uniform melting and suboptimal coating performance [155]. Despite its widespread use, ball milling presents several technical and practical limitations; it is time-consuming and costly. Moreover, achieving a homogeneous mixture is not always guaranteed, particularly when dealing with powders of different sizes, shapes, and densities. Exploring new compositions of powders is also labor-intensive. A significant advancement in the HVOF technique involves using spray drying to agglomerate fine powders [156]. Novel WC-Co powder featuring a fully densified, ultrafine-grained microstructure was developed specifically for HVOF spraying applications [157]. A proposed alumina-assisted treatment, under spray drying conditions, has proven to be an effective method for producing fully densified, spherical WC-Co particles without inducing phase transitions or inter-particle bonding [157].

Other methodologies involved the insitu mixing of powders, eliminating the need for pre-mixed feedstock [158]. In a hybrid HVOF thermal spray process, graphene nanoplatelets (GPN) were mixed insitu with WC-Co powders [158]. This technique demonstrated the potential to enhance the wear resistance of conventional WC-Co by incorporating GNP powders. Solution precursor HVOF (SPHVOF) [159] and liquid or gas suspensions (SHVOF) [160] are attracting growing interest due to their enhanced control over particle size, stoichiometry, and the chemical composition of the resulting coatings [161,162].

5.3.2. Heat Treatment of HVOF-Sprayed Carbide Cermet Coatings

Despite the advancements achieved with HVOF-applied WC-based coatings, there remains significant potential for further microstructural refinement and enhancement of mechanical properties. For instance, not all particles are in a molten or semi-molten state upon impact with the substrate, leading to the formation of micropores and small cavities within the coating and at the coating–substrate interface [163,164]. These defects, along with the lack of a true metallurgical bond between the coating and the base material, limit the performance of HVOF-applied coatings. Hybrid systems combining thermal spraying with laser remelting or post-deposition heat treatments have shown promise in enhancing adhesion, reducing porosity, and improving tribological performance [140,165]. Figure 13 illustrates the laser remelting process used for WC-NiCr coatings processed by HVOF and presents corresponding SEM results. A high-power laser beam is directed onto the coated surface, generating a localized molten pool that facilitates improved metallurgical bonding between the substrate and the coating (Figure 13a) [166]. This technique significantly reduced the presence of pores in the WC cermet coatings, as evidenced by the comparative microstructural images in Figure 13b,c.

Laser ablation is also being explored in ongoing research to further enhance erosion resistance and minimize the risks of abrasive wear [167]. Laser ablation is a precise material removal method that operates by irradiating a surface with a high-intensity laser beam. This results in localized vaporization or sublimation of the target material, thereby enabling controlled surface modification and material removal. WC-CoCr sprayed with the High-Pressure High-Velocity Liquid Fuel (HP-HVOLF) technique and subjected to laser ablation demonstrated enhanced erosion resistance, which was attributed to increased microhardness and improved surface hydrophobicity [168].

5.3.3. Deep-Learning-Based Prediction of the HVOF Process and Coating Performances

The integration of computational modeling and machine learning into the design and optimization of cermet coatings is an emerging frontier, enabling tailored solutions for diverse industrial demands [169,170]. Recent research has explored the potential of machine learning (ML) to revolutionize the generation of tribo-mechanical maps [171,172,173,174,175,176,177]. By utilizing extensive datasets that include coating compositions, processing parameters, and wear behaviors, ML algorithms can predict tribo-mechanical performance under untested conditions, thus accelerating the development of next-generation thermally sprayed coatings. The HVOF spray process has been extensively studied using artificial neural network (ANN) models. Liu et al. [177] employed ANN to establish relationships between process parameters and the mechanical and tribological performance of Cr₃C₂-25NiCr coatings produced via HVOF spraying. Their results indicate that, for predicting microhardness and porosity, the most influential parameters, classified from most to least significant, are spray distance, oxygen flow rate, and CH₄ flow rate. In contrast, for wear rate, oxygen flow rate has the greatest influence, followed by spray distance and CH₄ flow rate. However, their study did not consider in-flight particle velocity and temperature—critical intermediate factors that influence coating quality [178]. The authors emphasized that integrating ANN models into the HVOF control system requires more comprehensive data—including feedstock properties, real-time diagnostics, coating characteristics, and performance metrics—to improve model accuracy and enable ANN-assisted control across various spray methods and materials.

More recently, Giu et al. investigated the effects of the in-flight particle characteristics on the microstructural properties of Cr₃C₂-25NiCr coatings [175]. Building on these findings, they applied advanced models to the HVOF spraying process, introducing a hierarchical neural network that integrates physics-informed neural networks (PINNs) and convolutional neural networks (CNNs). This approach enabled the prediction of in-flight particle behaviour and their impact on key coating attributes, including porosity, microhardness, and wear rate. The hybrid PINN-CNN model achieved a 1% prediction error and over 96% variability in the target outputs (R²), outperforming traditional ML models in predicting in-flight particle behavior and coating performance. It reduced overfitting common in ANN models and improved transferability through the PINN component. Overall, the hybrid PINN-CNN model offered accurate predictions and a solid theoretical basis for optimizing high-performance HVOF-sprayed coatings. Additionally, a few ANN models have been applied to predict the cavitation or erosion resistance of HVOF-sprayed binary coatings. In their study, Becker et al. developed an ANN model trained on experimental data for Cr₃C₂-37WC-18M and WC-20Cr₃C₂-7Ni coatings, evaluating the effects of fuel type, flow rate, and stoichiometric ratio. Input parameters included particle velocity, temperature, coating thickness, porosity, microhardness, and fracture toughness [172]. Cavitation and slurry erosion tests at a 90° impact angle were conducted to assess mass loss and wear rates. The ANN incorporated 10 input variables and produced 8 output parameters, demonstrating strong predictive capability. Results confirmed the significant influence of fuel type and stoichiometric ratio on coating performance, offering a reliable tool for optimizing HVOF-sprayed carbide coatings for hydro-turbine applications.

6. Sustainability Considerations in HVOF Coating Processes

6.1. Process Limitations and Possible Solutions

This section critically examines the environmental impact associated with the thermal spraying processes, particularly regarding materials, gases, and energy consumption during the operational phase. HVOF thermal spraying often employs specific gases and metallic feedstocks, including metals and alloys, categorized as critical raw materials or identified as supply risks [179]. Environmental considerations outlined in guidelines such as the IFC Thermal Power Plants framework (2008) [180], address several factors, including solid waste generation, air emissions, wastewater discharge, hazardous material handling, water usage, greenhouse gas (GHG) emissions, and overall energy efficiency. Figure 14 presents a schematic overview of the key contributors to the environmental impact of thermal spraying systems [181].

Thermal spray technologies (HVOF, APS, and FS) in energy and transport sectors reveal significant dissipative losses, mainly due to inefficient powder adhesion and low recycling rates of critical feedstock materials (e.g., Cr, Co, Ni, and W) [182]. The coating process generates the greatest losses due to poor powder adhesion and low recycling rates of critica lmaterials. HVOF is a powder-based thermal spray process capable of emitting airborne particulates, including ultrafine aerosols that are enriched with toxic metallic species such as hexavalent chromium, nickel, and cobalt, which are associated with serious occupational and environmental hazards [183]. In response to these concerns, increasing emphasis has been directed toward resource-efficient practices such as the recovery and recycling of overspray materials, along with the development of cleaner feedstocks to mitigate exposure risks [184]. Previous studies have shown that waste powders from thermal spray operations can be efficiently recycled with minimal processing, especially through spray drying, which agglomerates fine particles into a reusable feedstock. Vardavoulias et al. [185] demonstrated that the recycled metal–ceramic agglomerates are suitable for cost-effective methods like flame spraying; however, the high gas flow and thermal energy in HVOF systems hinders the effective deposition of recycled powders, limiting their applicability. The choice of process gases also plays a significant role in shaping the operation’s environmental performance. HVOF systems typically rely on combustible gases mixed with oxygen [181,186]. Future studies should address energy-efficient designs and optimized combustion in modern systems to limit emissions of greenhouse gases and airborne pollutants, supported by measures such as cleaner energy inputs and improved fuel management strategies [186].

6.2. Environmental Impact Evaluation via Life Cycle Assessment

Life cycle assessment (LCA) is a standardized methodology for quantifying the environmental impacts associated with industrial processes, enabling the assessment and comparison of their environmental performance [187]. Governed by the ISO 14040 standard [188], LCA provides a systematic framework for measuring resource depletion, energy consumption, emissions, and waste generation. In this context, HVOF processes are considered more environmentally friendly than traditional coating methods. They produce less overspray and waste and do not involve hazardous chemicals, thereby reducing environmental impact and improving workplace safety. The adoption of HVOF technology also supports energy-efficiency goals. For example, in jet-engine turbines, high-thermal-resistance coatings enable operation at higher combustion temperatures, improving fuel efficiency and reducing CO₂ emissions [189].

In the field of thermal spray coatings, LCA can be applied to compare alternative coating technologies and guide more sustainable choices. For example, Igartua et al. (2020) conducted a comparative LCA of piston-ring coatings produced by HVOF spraying and traditional hard chromium plating, using a 1 m² coated surface as the functional unit (FU) [190]. The HVOF process demonstrates significantly lower environmental impacts across multiple categories, including resource scarcity, human health, and ecosystem damage. Figure 15 shows that HVOF resulted in substantially lower ecological impacts: a 45% reduction in resource scarcity, a 36% reduction in human health damage, and a 50% reduction in ecosystem damage. These findings support the growing adoption of HVOF as a more sustainable alternative to electroplating techniques that often involve hazardous substances like hexavalent chromium.

Further studies have corroborated these findings. For instance, Montavon et al. [191] conducted an LCA of the HVOF process for depositing hard chromium alternatives on aircraft landing gear components. Their analysis showed that, even when electricity is sourced primarily from renewables, HVOF spraying of WC-Co coatings is more environmentally favorable than electroplated hard chromium in terms of human health and ecosystem impacts, especially when hydrogen fuel is used. Krishnan et al. [192] conducted a comparative study, which revealed that HVOF spraying typically resulted in 5–10 times lower human-health impacts and 30–50 times lower ecosystem impacts compared to electroplated hard chromium coatings, particularly in aerospace applications. However, they observed that HVOF may have similar resource-consumption impacts to electroplating, depending on the regional energy mix. The evaluation using LCA and Techno-Economic Analysis (TEA) was also employed to compare HVOF installation with chromium electrodeposition from environmental and economic perspectives [193]. The results showed that HVOF significantly reduced coating costs by 20.9% per functional unit due to lower labor demands. Environmentally, HVOF also demonstrated a clear advantage in reducing toxicity-related impacts—as illustrated in Figure 16—although outcomes in other impact categories were more variable.

HVOF is also considered more environmentally sustainable than other thermal spray processes. In their study, Rúa Ramirez et al. assessed the environmental and economic performance of WC-Co coatings deposited using Cold Gas Spray (CGS), Atmospheric Plasma Spray (APS), and HVOF technologies [194]. The analysis revealed that all three processes exhibited low environmental impact in terms of CO₂ emissions, with HVOF demonstrating the best economic performance. CGS was limited by the low deformability of WC-Co and high labor demands, while APS was hindered by high electricity consumption. HVOF, on the other hand, offered a more efficient and cost-effective solution for coating deposition, making it the preferred choice for WC-Co coatings. Further work by Rúa Ramirez et al. [195] quantified the inputs (energy and materials) and outputs (emissions and waste) for the LCA of APS, HVOF, and CGS processes using WC-12Co powder. Measurements included electricity, gas, feedstock, alumina (for sandblasting), CO₂, CO, and noise. Among the processes, HVOF stands out for offering a favorable balance of performance and environmental impact, with relatively high deposition efficiency and minimal CO and CO₂ emissions despite higher hydrogen use. APS showed higher electricity consumption, and CGS had lower efficiency. Overall, HVOF presents a promising option for sustainable coating applications.

6.3. Ecological and Health Challenges: Future Directions

Despite significant technological advances, the implementation of thermal spray coating processes still faces significant challenges. Thermal spraying can pose safety risks to operators and lead to inconsistent material properties, particularly when manual intervention and insufficient thermal control are involved. Furthermore, the health hazards associated with cobalt (Co) and chrome (Cr), along with the classification of tungsten (W) and cobalt as critical raw materials, are currently major factors driving the development of alternatives to WC-CoCr and Cr₃C₂ systems [196]. Recently, niobium carbide (NbC)-based hard metals have emerged as promising candidates for thermally sprayed wear-resistant coatings [196,197]. In their study, Bollelli et al. investigated HVOF-sprayed NbC-40 vol.% (Fe-20 wt.%Cr-15 wt.% Mo) coatings as a cost-effective and less hazardous alternative to NbC-NiCr [197]. The coatings were dense (~1.2–1.4% porosity), hard (~1000 HV_0.3), and showed consistent properties regardless of gas flow rate. They exhibited higher wear rates at room temperature compared to conventional NbC-NiCr, Cr₃C₂-NiCr, and WC-CoCr coatings. Their performance improved with temperature, with a significant drop in wear rate above 300 °C due to the formation of a protective tribo-oxide film. This film remained effective up to 600 °C, making the coatings suitable for high-temperature sliding wear applications. Recently, Tegelkamp et al. optimized HVOF parameters for NbC-FeCr coatings using different nozzles and in-flight diagnostics, identifying ideal conditions of 2150 °C and 700m/s [198]. These coatings exhibited 2.1–4.6% porosity, 770–900 HV_0.3 hardness, and FeNb₂O₆ phases. Wear rates (1.2–2.1 × 10⁻⁶ mm³/(N·m)) were comparable to WC-CoCr and Cr₃C₂-NiCr, with best results achieved using a short spray distance and small nozzle shape.

Technical limitations also exist in HVOF spraying, chief among them being the need for precise process control to ensure uniform and reliable coating quality. In this field, operators manually measure coating thickness to verify proper deposition, an activity that exposes them to hazardous conditions such as elevated levels of dust, fumes, noise, and heat [198]. Integration of virtual sensors and advanced thermal modelingis proposed to improve precision, safety, and efficiency in industrial thermal spray processes by reducing operator intervention [199]. Recent research advancements focus on optimizing process parameters to enhance sustainability and increase efficiency. Multi-objective optimization algorithms are being applied to determine optimal parameters for HVOF thermal spraying offering valuable insights into their practical implementation [200].

7. Conclusions

The HVOF process for developing wear-resistant carbide cermet coatings is a proven solution in the automotive sector, offering a viable alternative to hard chrome plating while extending the service life of internal engine components like cylinder liners, piston group assemblies, brake discs, and rail vehicle parts.

Several factors influence the tribological performance of cermet coatings. Among these, coating thickness is a major parameter influencing the wear performance. Feedstock powder characteristics—including nature, size, and morphology—are also considered as a key factor affecting the wear behavior of carbide cermet coatings. Service conditions, such as temperature and loading, play a critical role in determining tribological performance. Coatings develop protective tribo-oxide layers at elevated temperatures below 600 °C, modifying the wear mechanisms. Additionally, finer powders (5–20 µm) have been shown to enhance Vickers hardness, reduce porosity, and improve wear resistance. Thicker coatings have demonstrated improved resistance to cracking and increased coating density. FGCs further reduce the risk of delamination and enhance the wear resistance of carbide cermet coatings.

Future challenges in the field include refining carbide powder grain sizes to the submicron or nanostructured range, incorporating rare earth elements such as CeO₂, La₂O₃, and Er₂O₃ and transition elements like molybdenum. These materials aim to modify the solidification process and microstructure of carbide cermet coatings, thereby enhancing tribological performance and resistance to high-temperature oxidation.

Research is advancing toward integrating hybrid methods to HVOF to enhance carbide-cermet coating performance. Techniques like in situ mixing, spray drying, and the use of gas-phase (SHVOF) or suspension-based precursors (SPHVOF) are being explored as alternatives to conventional ball milling, offering improved structural homogeneity. Post-processing techniques such as laser remelting or laser ablation of HVOF-sprayed coatings have also shown promise in reducing porosity and enhancing tribological properties.

Integrating machine learning into HVOF process optimization and predicting the performance of cermet coatings represents an emerging frontier. These approaches enable the estimation of tribo-mechanical behavior under untested conditions. ANN, hierarchical neural networks, and statistical techniques such as ANOVA are currently used to optimize process parameters and service conditions. Addressing wear and friction challenges remains critical for reliable operation of machine components exposed to high temperatures, particularly in demanding sectors such as aerospace, marine gas turbines, power generation, and the oil and gas industries.

Despite substantial technical progress, the application of thermal spray coating processes continues to face significant challenges. This review critically examined the limitations of the HVOF process, particularly in relation to environmental and health concerns. Material losses due to poor powder adhesion and low recycling rates, along with the generation of ultrafine aerosols enriched with toxic metallic species, represent notable issues. From an environmental standpoint, LCA markedly reduces toxicity-related impacts compared to other thermal spray techniques. To enhance precision, safety, and efficiency in industrial thermal spray applications, the implementation of virtual sensors and advanced thermal modeling is being proposed, aiming to reduce dependence on direct operator intervention.