Thermal Spray High-Entropy Alloy Coatings: A Review

High-entropy alloys (HEAs) are a new generation of materials that exhibit unique characteristics and properties, and are demonstrating potential in the form of thermal spray coatings for demanding environments. The use of HEAs as feedstock for coating processes has advanced due to reports of their exceptional properties in both bulk and coating forms. Emerging reports of thermal sprayed HEA coatings outperforming conventional materials have accelerated further exploration of this field. This early-stage review discusses the outcomes of combining thermal spray and HEAs. Various synthesis routes adopted for HEA feedstock preparation and their properties are discussed, with reference to the requirements of thermal spray processing. The HEA feedstock is then compared and correlated with coating microstructure and phase composition as a function of the thermal spray processing route. Subsequently, the mechanical behavior of thermal spray HEA coatings is summarized in terms of porosity, hardness, and tribological properties, along with their oxidation and electrochemical properties, followed by their potential applications. The thermal spray methods are contrasted against laser cladding and surface alloying techniques for synthesizing thick HEA coatings. Furthermore, HEAs that have displayed excellent properties via alternative processing routes, but have not been explored within the framework of thermal spray, are recommended.


Introduction
The performance of a material in industrial applications is associated with its surface characteristics. The wide availability of surface modification technologies permits an economic substitution of a poor base material with a coating that exhibits more desirable surface characteristics and performance. Surface modification is highly valuable in conditions involving corrosive environments, wear protection, thermal insulation, or severe stress applications. For instance, in thermal barrier coatings (TBCs), yttriastabilized zirconia (YSZ) is used as an overlay protective coating to shield the nickel or cobalt based superalloys from extreme high temperature conditions. Thermal spray coating (TSC) is a versatile and rapidly growing surface modification technology.
Thermal spraying is a line-of-sight process, wherein feedstock material, in the form of powder, wire, rod, or suspension, is fed into a spray torch and heated up to a molten or near-molten state and propelled toward a base material (Ref [1][2][3]. Thermal spray (TS) processes are classified into three prime categories: (1) use of combustion heat sources, for example detonation gun or high-velocity oxygen fuel spray (HVOF); (2) plasma or arc formation using electrical energy, for example atmospheric plasma spray (APS); and (3) low-temperature processes that use energy evolving from gas expansion, for example cold spray (CS). Figure 1 (Ref 4) illustrates the flexibility offered by TS in terms of the wide spectrum of flame temperature-particle velocities available. Plasma spray (atmospheric and vacuum), high-velocity arc spray, HVOF, warm spray, and cold spray have been employed to synthesize thermal sprayed HEA coatings.
Atmospheric plasma spray (APS) is the most versatile thermal spray technique that holds few limitations concerning the materials that can be sprayed. Therefore, APS is well established at the industrial level ( Ref 5). A feedstock (powder or wire) is introduced into the plasma stream where complete or partial melting may occur, eventually depositing as lamellar splats onto the substrate. Numerous process parameters, of which the prime ones are gas flow rate, power input, carrier gas flow rate, stand-off distance, and powder feed-rate, are crucial in determining the coating density, quality, and uniformity. APS is economically attractive since this manufacturing process is performed under ambient conditions, with the primary drawback being in-flight oxidation (IFO) of feedstock particles. APS is best suited for forming ceramic coatings with porosity of less than several percent but can be optimized to reduce oxidation of metal powders. The alternative is to spray with an inert gas shroud, or in a soft vacuum/reducing atmosphere, but the cost of employing a vacuum system, e.g., by low-pressure vacuum spraying, increases by an order of magnitude.
HVOF falls under the ambit of combustion spray processes. A hydrocarbon fuel (C x H y ; typically, kerosene, methane, propane, propylene, acetylene or natural gas) is ignited with an oxidizer (air or oxygen). The temperature and pressure so generated then heat and accelerate feedstock particles towards the substrate. Laval nozzles are used to further accelerate the particles up to speeds of 2000 m/s. Although particle temperatures are lower than those achieved in plasma spray (Fig. 1), higher particle speeds result in lower IFO and better coating densities, with porosity that may be less than 1 percent.
Cold spray (also termed as 'kinetic spray') accelerates feedstock particles by means of a high-pressure gas that passes through a de Laval nozzle (Ref 6, 7). One advantage of CS is that the feedstock is not heated significantly, thus avoiding melting and any temperature-dependent phase transformations or IFO. Figure 1 (Ref 4) illustrates the spread of particle size, temperature, and velocity offered by TS processes. Using this scheme, an appropriate processing window can be identified based on the nature of the feedstock material (metallic/ceramic) and the desired coating microstructure, such as density and phase content. For example, for ceramic coatings with a predetermined porosity requirement, such as hydroxyapatite coatings for orthopedic applications, APS would be preferred. However, for depositing dense metal coatings, such as coating aluminum alloys on Fig. 1 Spectrum of thermal spray processes across particle velocity and flame temperatures attainable (Ref 4) magnesium parts for corrosion protection, cold spray would be the preferred method.
TS technology has seen developments in feedstock materials across metal alloys, composites, intermetallics, ceramics, and cermets, the exact choice depending on the final application. High-quality feedstock, with respect to particle size distribution (referred to as the 'cut') and particle morphology, is vital to produce high-quality coatings (Ref 4,8). The simultaneous surge in harsh industrial conditions and incremental development in conventional materials has pushed materials research toward the development of new alloys, for instance high-entropy alloys (HEAs). The adoption of these new HEA feedstocks will potentially allow novel surface engineering solutions that are tailored for demanding industrial environments.
HEAs are defined as solid solution alloys of five or more elements in equi-or near-equi-atomic ratios ( . Indeed, research on HEAs has demonstrated that they have outperformed conventional materials, not only at room and high temperature, but also at cryogenic temperatures (Ref 12,13).
A current major focus of HEA research has been directed to decoding the science governing the fundamental nature of these multicomponent alloys. The stabilization of solid solution phases at elevated temperatures arises due to a combination of mixing enthalpy, higher configurational entropy, and similar atomic characteristics (atomic radius, valency, crystal structure) (Ref 14) that favors the formation of solid solutions such as FCC or BCC over intermetallic compounds ( Ref 15,16). The combined interactions among all the integral elements, termed as 'cocktail effects' (Ref 17), promote these distinctive attributes of HEAs. This is reflected in the development of bulk HEAs with wear resistance (Ref 18,19), oxidation (Ref 20,21) and corrosion resistance (Ref 22), and mechanical properties (Ref [23][24][25][26] that have outperformed traditional materials. These technical advantages are now being translated to coatings via thermal spray, laser cladding, and sputtering processes. Laser cladding, also termed as laser surface alloying, involves preparing a powder bed of feedstock slurry (generally, blended unalloyed elemental powders) onto the substrate and scanning it with a focused laser beam. In another variant of the process, termed 'direct laser deposition', feedstock powder is propelled coaxially with the laser beam, instead of preplacement as slurry. In both processes, melting of the substrate surface along with the feedstock slurry occurs, followed by rapid solidification and generation of a coating. While laser clad/surface-alloyed coatings are usually defect free and exhibit good metallurgical bonding with the substrate, the drawbacks include a heat-affected zone, extreme residual stresses, and possible deterioration of the HEA coating and substrate properties due to elemental dilution ( Ref 27). When prealloyed HEA powders are used as feedstock for laser cladding, the coating often consists of multiple alloy and intermetallic phases that may require further processing before the desired properties are achieved ( Ref 28).
Another prevalent technique for generating HEA coatings is via sputtering or vapor phase deposition in which the HEA is atomically deposited layer-by-layer ( Ref 29). These processes result in thin films, and are increasingly being used to generate HEA carbide/nitride/boride films that exhibit excellent tribological properties ( Ref 30). Reviews discuss these coating categories (Ref 31); however, a comprehensive review of HEA coatings manufactured by thermal spray methods is not available.
Thermal sprayed HEA coatings offer potential applications where novel HEA compositions can be explored. TS technology is well established at the manufacturing scale, for example, in generating reproducible and prime-reliant coatings for aerospace and power generation industries. In addition, several HEA compositions have been demonstrated to be competitive alternatives to conventional materials where there is a strong need for new protective coatings to address extreme engineering environments. Figure 2 shows the number of research articles published on HEA thermal spray coatings in peer-reviewed journals where the rise in total citations per year is also illustrated. The first report on thermal sprayed HEAs arose in year 2004, when Huang et al. atmospheric-plasmasprayed AlSiTiCrFeCoNiMo 0.5 and AlSiTiCrFeNiMo 0.5, and analyzed the oxidation and wear resistance of these coatings ( Ref 32). A rapid rise in the development of TS HEA coatings can be observed after 2016, with 'The microstructure and strengthening mechanism of thermal spray coating Ni x Co 0.6 Fe 0.2 Cr y Si z AlTi 0.2 high-entropy alloys' by Wang et al. (Ref 33) and 'Plasma-Sprayed High Entropy Alloys: Microstructure and Properties of AlCoCrFeNi and MnCoCrFeNi' by Ang et al. (Ref 34), being the two most cited articles.
This current paper reviews the literature that is specific to thermal sprayed HEA coatings. ''Elemental Selection Statistics for High-Entropy Alloys'' section elucidates the concepts and statistics employed for designing HEAs toward desired applications. ''HEA Feedstock: Synthesis and Properties'' section focuses on the various methods of generating HEA feedstock for thermal spray, and examines the effect of synthesis techniques on the coating properties, followed by ''Microstructural and Phase Composition Aspects'' section, wherein the microstructural and phase analyses of TS processes in conjunction with the feedstock are discussed. A major part of TS HEA research is focused on wear and oxidation resistance of coatings. Thus, ''Properties of TS HEA Coatings'' section concentrates on those properties of HEA coatings as well as on their electrochemical performance. ''Applications'' section addresses the targeted applications as well as suggestions for other potential HEAs not yet processed via thermal spray. ''Future Trends: An Optimistic Future for HEAs'' section summarizes the state of the art of knowledge on TS HEA coatings and makes recommendations concerning the future scope for this growing technology.

Elemental Selection Statistics of High-Entropy Alloys
High-entropy alloys tend to demonstrate complex and unique characteristics due to interactions of multiple elements and their diverse processing routes. The additional exposure to high temperature and then rapid solidification during thermal spray generates HEA characteristics that are challenging to understand. The screening of a specific HEA TS feedstock that will engender desirable properties that target a particular application for thermal spray process is a demanding undertaking. Figure 3 analyzes 30 peer-reviewed articles and depicts the relative occurrence of elements that have been employed in HEA feedstocks for TS processing. The majority of HEAs have a base comprising CrFeCoNi, 1 with the primary addition of Al, Si, and Ti.
The inclusion of Mn, Cu, Nb, and Mo has also been explored. The selection of these alloys for HEA feedstocks has been influenced by their performance in other processing routes such as casting and other deposition techniques.
The final microstructure and, hence, properties of the coatings not only depend on the thermal spray process but also on the synthesis routes and composition of the HEAs. It is notable that identical alloy chemistries, prepared via different feedstock synthesis techniques and thermal spray process parameters, resulted in different coating characteristics. This need not necessarily be a disadvantage, since knowledge concerning the physical metallurgical behavior guides the selection of the HEA elements for an intended application; for example, Al, Cr, Ti, Si for oxidation resistance; Cr for corrosion resistance; B2-forming Al-Ni/ Co/Fe, for wear resistance; and Co/Ni-based FCCs for enhanced plasticity are appropriate alloy constituents for HEAs. Multiple compositions can simultaneously be explored for their phase composition and expected properties using high-throughput experiments and computational techniques as demonstrated by Senkov et al. (Ref 36). The diversity of TS processes provides a tool to manipulate the characteristics of HEAs as coatings and tune them for specific applications.

HEA Feedstock: Synthesis and Properties
Four synthesis routes have been reported to construct HEA feedstocks for TS: (1) blending, (2) arc melting followed by mechanical milling, (3) mechanical alloying, and (4) gas atomization. Each technique imparts unique implications on the powder phase and characteristics and, therefore, on the coating microstructure and properties. The following discussion will focus on the particle size and morphology; extent of alloying and homogeneity; powder yield; and flowability, since these factors influence the TS processing. Figure 4 outlines the particle size distributions with respect to the synthesis techniques as reported in the TS-HEA literature. It is evident that each process has a characteristic output size range, which further depends on the synthesis process parameters. Gas atomization provides the widest cut, that can then be tailored to the thermal spray process by sieving. On the other hand, mechanical alloying results in a much finer cut that exhibits nanocrystallinity.

Blending
Blending involves mixing the powders without promoting any bonding or alloying. The particle size and shape of the blended mix will retain elemental qualities, leading to individual interactions under APS processing and an inhomogeneous coating microstructure. Figure 5 (reproduced from Fig. 4a  Blending is thus an effective method for mixing prealloyed powders to improve their individual properties, but is not a recommended route for the synthesis of primary HEA feedstock.

Arc Melting Followed by Mechanical Milling (AM-MM)
This method first fuses the desired alloy by arc melting. Then, the alloy button is crushed into smaller pieces, which is followed by ball milling into powder. The need for alloying at an elemental level is achieved by processing in the liquid state, while the requirement for the appropriate particle cut is accomplished by pulverization. Complex non-equiatomic compositions can be created by arc melting without incurring significant contamination from ball milling by this route.
Out of the four reports that employ this method, three reported ball milling to less than 44 lm particle size, while one is \ 34 lm, as illustrated in Fig. 4. Information regarding milling parameters adopted, such as rotation speed, duration, milling media, quantity of powder charged, and other essential experimental data, is not available. However, from the authors' experience, it is estimated that up to 90% of the powder weight charged would be recovered in this process, thereby making it a high yield process. The particle shape is irregular since the particles are fragmented pieces of the cast button. Hence, the powder flowability is likely to be influenced adversely; but the spray parameters can be adjusted to take this into account. The coating microstructure that evolves from these powders is relatively homogeneous in terms of the phases observed since alloy formation has already occurred during the casting procedure.
Huang et al. (Ref 32) were the first to generate thermal sprayed HEA coatings, and AM-MM was their method of choice. These coatings, as well as those documented by Hsu et al. (Ref 39,40), exhibited excellent oxidation resistance, which depends largely on both, the alloy composition, and the microstructural homogeneity of the feedstock.

Mechanical Alloying (MA)
Mechanical alloying (MA) is a well-established synthesis technique in HEA literature. Elemental powders are subjected to high-speed rotation and high-energy impacts, causing them to undergo repeated cold welding and fracturing, resulting in 'mixing' at an atomic scale (Ref 41). Over 200 reports, reviewed by Vaidya et al. (Ref 42), indicate that while this route assures alloy formation, the alloy phase formation is subject to multiple process parameters, e.g., the milling medium, milling atmosphere, process control agent (PCA), milling duration, and rotation speed. Each of these prime factors influences the extent of alloying, implying that there is no single distinct recipe for synthesizing an HEA via MA, unlike arc melting. Another drawback concerns the particle size and morphology. The 'wet' MA route, which uses organic solvents such as toluene as PCAs, results in finer particles of irregular shape, \ 30 lm after 10 h of milling ( Ref 34). In contrast, the 'dry' MA route, usually performed under argon atmosphere and at much higher rotation speeds, results in larger particles (20-75 lm) (Ref 43) with a wide  Dry milling of metals and alloys leads to excessive cold welding and, hence, derives larger particles that are flaky in nature. In contrast, wet milling results in finer, sphericallike particles due to the presence of the liquid surrounding the particles, which prevents excessive agglomeration of particles. However, contamination in wet milling is a concern, and dry milling is preferred when it is necessary to avoid contamination from the milling medium. Higher milling speeds allow for higher impact energy input into the material. It must be noted, however, that ball milling and mechanical alloying is a highly stochastic process with multiple controlling parameters ( Ref 41).
The powder characteristics influence powder flowability during its injection in TS, which generally prefers atomized prealloyed powder feedstock (Ref 44). There is also the issue of contamination from organic solvents and/or from the milling medium (e.g., stainless steel, tungsten carbide, zirconia) that may affect feedstock composition. As reported by Praveen et al. (Ref 45), the use of toluene as process control agent in MA of AlCrFeCo resulted in the formation of M 23 C 6 -type carbides on subsequent processing. While it was an unsolicited addition of carbon to the alloy, it improved the sintered alloy's thermal stability and mechanical properties, thereby illustrating both the advantages and deficiencies of this method.
Finally, mechanically alloyed particles are nanocrystalline in nature with a grain size \ 15 nm, which can be leveraged depending on the thermal spray process and intended application. For example, Anupam et al. (Ref 46) cold-sprayed MA HEA powders on Ni-based superalloy substrate and, after 25 h of isothermal oxidation, observed the diffusion of molybdenum from the substrate into the coating, which may be attributed partly to nanocrystalline grain boundary diffusion. On the other hand, TSCs generated for MA feedstock are largely homogeneous, with a possibility of greater IFO promoted by the powder nanocrystallinity, especially if the process is associated with a high temperature as experienced in APS.
A variation of the MA/MM process has been used by Srivastava et al. (Ref 47) to develop HEA feedstock for HVOF processing. Starting with milling elemental powders for 2 h in toluene, followed by drying in Ar, the powders were compacted into green pellets and subjected to pyrolysis in argon at 1100°C for 1 h. Alloying is expected to have occurred at this stage. The pellets were then crushed and sieved. Spray drying in a 6 wt.% polyvinyl alcohol (PVA) suspension was carried out to agglomerate the fine powders. This method, while protracted, avoids the nanocrystallinity and contamination incurred from MA, as well as conferring the desired particle size. However, care must be exercised to confirm alloy formation and homogeneity throughout the process.

Gas Atomization (GA)
Inert gas atomization is the preferred technique for manufacturing feedstock for TS. It has gained popularity in the HEA coating community and is the most used synthesis method. Gas atomization involves forcing the liquid alloy through a nozzle under high pressure within an inert gas environment. The liquid stream fragments into spherical droplets that rapidly solidify ( Ref 48). Faster cooling allows phase separation to occur only at very fine scales, as illustrated in Fig. 7 (Ref 49), and can usually be mitigated during spraying or annealing after coating. Gas atomization is appealing because it results in spherical particles with good flowability and homogeneous alloy formation. The wide size ranges of particles can be sieved to desired size cuts.
In summary: GA is tailor made for synthesizing feedstock for thermal spray processes and is suitable for HEAs. Mechanical alloying presents a strong alternate method, although requiring milling parameter optimization and being time intensive. Arc melting followed by mechanical milling is suitable for laboratory-scale studies, while blending can only be used as a post-alloying technique to further enhance feedstock properties.
It is also imperative to mention that while most of the studies synthesized their own feedstock in a laboratory setup, there are reports of two commercially available . While this points toward a growing future for thermal sprayed HEA coatings, it must be borne in mind that the quality of the feedstock defines the coating quality. Thus, care must be exercised when selecting either laboratory or commercially produced HEAs.
In contrast to usual prealloyed feedstock for thermal spray processes, laser cladding with HEAs is carried out with blended elemental powders mixed into a binder to form a thick slurry, which is preplaced on the substrate and exposed to the laser beam. The slurry melts under the laser beam to form a melt pool that allows mixing and alloying of the blended powders. The melt pool solidifies rapidly at 10 3 -10 6 K/s after passing of the laser beam and supersaturated solid solutions and intermetallic phases are often observed. There are several reports of laser clad HEA coatings using alloy powders synthesized via mechanical alloying (Ref 50) or gas atomization (Ref 51) rather than employing elemental blends. In another variation of laser cladding, termed 'laser surface alloying', the alloy coating is formed by mixing elements of the feedstock and the molten substrate. This transforms the otherwise detrimental problem of elemental dilution from the substrate (Ref 52) into a benefit. Several process parameters, primarily laser power, defocus distance, scanning speed, and track overlap, must be optimized for desirable results. This has been discussed further in the ''Post Processing of TS Coatings'' section.

Microstructural and Phase Composition Aspects
The microstructure of a TSC depends strongly on the type of TS method, the specific TS parameters, and the nature of the feedstock. The TS technique includes a multitude of coating parameters that are designated within 'spray tables,' e.g., plasma current and voltage; primary and secondary gas flow rates for plasma spray; carrier gas flow rate; stand-off distance; and design and dimensions of the nozzle for HVOF, D-gun, and cold spray processes. The HEA feedstock characteristics also govern the coating microstructure, which is strongly related to the feedstock synthesis technique (described in the previous section). Therefore, both the feedstock and TS process must be optimized to achieve a repeatable coating that is free from undesirable phases and adverse microstructural artifacts. This section delineates the effects of the TS technique used with respect to the as-sprayed microstructure of HEAs.
It is important to note that depending on the feedstock processing route and TS route chosen, the alloy may be prone to phase transformations. For example, HEA powders synthesized via mechanical alloying result in metastable and often supersaturated solid solution phases. These powders, when sprayed by relatively high-temperature HVOF or APS processes, will undergo melting and rapid solidification, once again resulting in supersaturated solid solution phases. There is also a possibility of in-flight oxidation (IFO) resulting in a composite alloy-oxide splat microstructure. On the other hand, coatings produced via low-temperature techniques such as cold spray or warm spray will tend to retain the feedstock phases. There is further scope to induce phase changes in the coating microstructure by post-processing, by thermal treatments and/or laser remelting of the coating surface, as is elucidated in ''Post Processing of TS Coatings'' section.
Compositions of HEAs are traditionally based on either the transition element quartet of Co, Cr, Fe, and Ni, or refractory elements such as W, Mo, Ta, and Nb. Additions of Al, Mn, Ti, Si, Cu, V, Hf, and Zr have been included in bulk HEAs and their microstructure and properties reported. This section charts the development of certain alloy families that have transitioned from the bulk form to TS Fig. 7 Typical gas-atomized high-entropy alloy particles with (a) spherical morphology and satellite particles, and (b) dendritic growth discernible at higher magnifications (Ref 49) coatings and that have been optimized for desired applications.

Early Work
The earliest published work on thermal sprayed HEAs is by Huang et al. (Ref 32) and is among the first reports on highentropy alloys as a concept. Their report on air plasmasprayed AlSiTiCrFeCoNiMo 0.5 and AlSiTiCrFeNiMo 0.5 initiated the field of TS-HEA coatings, buoyed by initial but promising results on the improved oxidation and wear resistance of the coatings vis-à-vis traditional materials. The effect of synthesis route on the HEA's microstructures is reflected from the phases observed. While the as-cast alloys consisted of B2 ? FCC1 ? FCC2 phases and a typical dendritic microstructure, only single-phase BCC splats along with several unidentified oxide phases were detected in the coatings. This is attributed to the melting followed by rapid solidification in the APS process, resulting in a supersaturated solid solution. Another aspect introduced by APS processing concerns IFO, which is a major issue when dealing with oxygen-sensitive materials. In this case, Al, Cr, Ti, and Mo are all oxygen active elements and likely constitute the many oxides detected by x-ray diffraction (XRD) and scanning electron microscope (SEM) of the coating. Nevertheless, the coating showed encouraging wear and oxidation resistance in comparison with traditional materials and laid the groundwork for the field of TS-HEA coatings.
It is worthwhile to mention that reducing the extent of IFO of traditional materials during APS by varying process parameters has been researched ( Ref 53,54). A major theme of these articles is that addition of a shrouded environment vastly reduces the IFO of the feedstock. Azarmi et al. (Ref 55) in an experiment designed to delineate the effects of process parameters on the oxide content and porosity of APS Alloy 625 coatings found that the primary gas flow rate, particle size, and stand-off distance most affected the extent of IFO. In the field of APS-HEAs, as will be discussed in ''AlCrFeCoNi Coatings'' section, IFO has been found to depend strongly on the feedstock synthesis route, particle size, distribution, and the primary gas flow rate during spraying. Liang et al. (Ref 56) reported CrFeCoNiCu(B) coatings synthesized by high-velocity arc spraying. The feedstock consisted of blended HEA powders extruded into cored wires and used as consumable electrodes with opposite polarities. An arc is struck between these electrodes, thus melting the wire ends, and compressed air flow is used to atomize and propel the molten droplets toward the substrate where they solidify and form the coating. A dense (\ 5% porosity) coating, consisting of FCC phase splats and a dark phase that maybe oxide or an unidentified interstitial solid solution, was formed that exhibited a good interface with the substrate. The novelty of this work lies in the addition of boron in the alloy, which they credit to prevent IFO because boron acts as a sacrificial oxygen getter. These coatings exhibited good hardness and bond strengths, lending confidence to this TS method for synthesizing HEA coatings. However, there have been no further reports on high-velocity arc-sprayed HEA coatings.

AlCrFeCoNi Coatings
The AlCrFeCoNi family of alloys has been researched extensively in the bulk form sprayed mechanically alloyed AlCrFeCoNi via APS and reported significant IFO that transformed the two-phase MA powder into a multi-phase alloy-oxide composite coating. This was attributed to the fine particle size and nanocrystalline nature of the feedstock, which intensified the alloy powder's interaction with the atmosphere during spraying. The coating exhibited a homogeneous lamellar microstructure with good anisotropic mechanical properties.
In 2017, the same composition was sprayed via 'supersonic air plasma spray' by Lin et al. (Ref 63). Blended elemental powders were used as feedstock that formed the coating in addition to attendant oxides. Complete alloying was not possible, and it was not deemed to be an HEA coating in the as-sprayed form, although lamellar splats were observed. Annealing the coatings at 600 and 900°C allowed interdiffusion and phase transformations resulting in BCC, Ni 3 Al, and Cr-O phases. Post-processing treatments will be discussed at the end of this section. Cheng et al. (Ref 64) systematically studied AlCrFe-CoNi APS coatings using gas-atomized powders as feedstock, analyzing the effect of particle size and spray parameters (e.g., plasma current and argon primary gas flow rate) on the coating phases. They demonstrated the phase adjustable nature of TS-HEA coatings by controlling the above-mentioned parameters. First, the phase evolution in the particles as a function of temperature was analyzed. The starting phases in the as-atomized condition were B2 Ni-Al-rich dendritic and BCC Fe-Cr interdendritic microstructures. The BCC phase transformed to FCC at 600°C and then to r phase at 800°C. Further annealing at 1000°C revealed dissolution of the r phase and reappearance of FCC.
The powder particles were also observed to melt far below the melting temperature of 1362°C of the cast alloys, manifesting as B2 ? BCC phases after heat treatment at 1200°C. Subsequently, the GA powders were sieved into two cuts, small (10-60 lm) and large (60-90 lm) and sprayed under several plasma current and argon gas flow conditions. Increasing the plasma current increases the temperature of the plasma, as does raising the primary Ar gas flow, and thus the particle temperature rises. Smaller particles gave rise to a major BCC with minor FCC ? oxide microstructure at lower plasma powers, which transformed to FCC becoming the major phase at higher plasma power.
Three of these coatings are illustrated in Fig Interestingly, the larger particles, which could retain more heat and remain molten longer than smaller particles, did not transform to FCC. Coatings generated using higher-energy inputs and larger particle sizes exhibited good melting and splat behavior, with negligible IFO, albeit with some interlamellar cracking.
Another point of comparison for AlCrFeCoNi TS coatings is available in the work of Mu et al. (Ref 65), where they reported APS coatings of gas-atomized alloy particles. BCC phase observed in the particles is transformed to BCC ? oxide in the APS coating. Table 1 lists the feedstock and APS parameters employed by Ang, Cheng, and Mu for generating AlCrFeCoNi plasma-sprayed coatings and the corresponding phases observed. This summary table may aid in optimization of APS spray and feedstock selection parameters to develop phase-adjustable coatings for these alloy systems. The primary phases observed for each condition are mentioned in bold font. Mu et al. (Ref 65) further characterized the coating via x-ray photoelectron spectroscopy (XPS), which detected multiple oxide phases on the coating surface, all attributed to IFO during spraying. Transmission electron microscopy (TEM) studies of the as-sprayed coatings also showed nano-oxide particles dispersed in the alloy matrix, which were attributed to the formation of a by-product from the HEA-plasma-air interaction. -type spinels, and unoxidized unmelts with equiatomic composition were detected at different length scales. They also provide a model delineating the effects of particle size and temperature during spraying and the consequential result in the coating. An example is illustrated in Fig. 9(a), where a medium-sized (5-15 lm) mechanically alloyed AlCrFe-CoNi particle is exposed to the highest temperature zone (particle temperatures of 1200-2300°C) of the plasma plume. It is theorized that such particles would melt completely, spheroidize, and oxidize partially in flight. They would further splat on impact, with the possibility of splashing. Figure 9(b) is a representative image of the HEA feedstock used. Figure 9(c) and (d) shows splats corresponding to medium-sized particles in the high-temperature zone and identified in the coating cross section and top view, respectively. This work (Ref 66) focuses on the experience of mechanically alloyed HEA particles, which are much finer, irregular shaped, and nanocrystalline in nature. These concepts can be applied broadly to any TS-HEA process and be used in conjunction with Table 1 to select processing parameters for developing specific coating microstructures. Figure 10 is based on reference 66 and presents a unified model that mechanistically describes the combined influences of temperature and impact on splat behavior. This model is an extension of the work represented in Fig. 9 and, although formulated for HEA feedstocks, can be applied to thermal spray processes in a generic fashion. Therefore, the focus concerns particle size effects as they experience various temperatures regimes within a thermal spray environment. Anupam et al. (Ref 46) also developed cold-sprayed HEA coatings using MA AlCrFeCoNi. The phases present in the powder were translated to the coating without any IFO due to the low particle temperatures experienced (\ 400°C). Particle deformation into splats was observed, although former particle boundaries were still obvious as the new inter-splat boundaries and attributed to the brittle nature of the BCC alloy particles. This work illustrates that cold spray is also a viable technique for spraying temperature-and oxidation-sensitive HEAs, albeit with further process parameter optimization.
A    Fe with Mo to confer enhanced corrosion and slurry erosion resistance. MA powders used as feedstock exhibited nanocrystalline nature with BCC phases and WC contamination. These phases were translated into the coating with some IFO products.

AlTiCrFeCoNi Coatings
The In comparison with the APS coatings, HVOF HEA coatings were observed to be denser with some scattered porosity. The phases transformed from B2 ? BCC in the feedstock to one FCC ? two BCC phases. Some degree of IFO was also observed that is attributed to the higher content of fines in the feedstock.
Chen et al. (Ref 73) also employed HVOF to generate Al 0.6 TiCrFeCoNi coatings using GA feedstock. Dense (\ 2% porosity) coatings exhibiting the same phases as the feedstock were produced, ascribed to the lower process temperature and low degree of IFO. Two BCC phases, one NiAl-rich and the other Fe-Cr-rich, constituted both the powder and the coating, although in slightly different fractions. Peak broadening in XRD was observed for the coating vis-à-vis the powder and is attributed to rapid cooling in HVOF and attendant strains induced during solidification. HVOF is, therefore, demonstrated to be a promising technique for generating low-oxide HEA coatings for oxygen-sensitive compositions.
Tian et al. (Ref 38) extended the scope of TS-HEA coatings by reinforcing the HEA with Ni60 particles to improve their hardness and wear resistance. The blended powders were sprayed via APS and resulted in a non-homogeneous lamellar microstructure with larger splats of Ni60 and finer, more oxidized splats of the HEA. This microstructure may be correlated directly with differences in the feedstock; that is, HEA particles synthesized via mechanical alloying were finer, irregular, and nanocrystalline. Prealloyed Ni60 particles were larger and spherical and thus behaved differently in-flight than the HEA particles. Nevertheless, reinforcement was achieved as evidenced by an improvement in the coating's wear resistance vs. the HEA-APS coating. Thus, the potential of improving physical properties by thermal spray processing to form HEA coatings was established.

AlSiCrFeCoNi Coatings
AlSiCrFeCoNi alloys have also generated some traction primarily due to their high-temperature oxidation resistance. Bhattacharya  constitution was achieved via HVOF. The APS coating was reportedly inhomogeneous, exhibited larger oxide and porosity content, and was rejected in favor of the denser HVOF coatings for subsequent oxidation studies. Interestingly, two 'layers' were observed in the HVOF coating, the lower layer appearing denser than the top one. This morphological effect may be due to a tempering and shot-peening-like treatment experienced by the bottom layers during deposition of the top layers. This study is novel because the authors used thermodynamically calculated phase diagrams to choose the TS-HEA alloy composition. This method reduces the critical compositional variable of TS-HEA coatings property-parameter optimization. Phase separation into a Fe-Co-Cr-Al-rich BCC, a Fe-Corich FCC, and Cr 3 Si was observed in the coating, attributed to the use of blended elemental feedstock. The BCC phase fraction was observed to increase with Si content, and at x [ 1, no FCC or Cr 3 Si was observed, due to the formation of a supersaturated solid solution during APS. Oxide splats were also detected in the coating cross section along with porosity and interlamellar cracks. Thus, while some supersaturation may be achieved in-flight during plasma spray, it is not the recommended method for alloying using blended powders. It is noted, on the other hand, that supersaturation may be achieved in laser cladding since the time at temperature is significantly greater.
The effect of Al content on phase formation and wear behavior was examined by Xiao et al. (Ref 76) in MA-APS Al x SiCrFeCoNi (x = 0.5, 1, 1.5 molar ratios) coatings. BCC ? FCC phases observed in the MA powder were translated into the coating with some IFO. An increase in the Al content did not affect the phases significantly. Heat treatment of the coatings led to evolution of the Cr 3 Ni 5 Si 2 phase, which improved the wear resistance of the coatings.

Non-equiatomic Al-Si-Ti-Cr-Fe-Co-Ni coatings
The non-equiatomic Al-Si-Ti-Cr-Fe-Co-Ni family of alloys are being explored for their oxidation and wear behavior at The BCC ? Cr 3 Si phases in the powder were retained in the coatings with some IFO. The extent of the IFO was more under APS, and both high-temperature processes resulted in the formation of a supersaturated solid solution with lesser degrees of Cr 3 Si precipitation than in the as-cast form. Post-spraying heat treatment was used to optimize coating microstructures and will be discussed separately toward the end of this section.
Learnings from previous studies on these compositions were used by Hsu et al. (Ref 77) to systematically study AlSiTi 0.2 Cr 1.5 Fe 0.2 Co 0.6 Ni deposited by three techniques: APS, HVOF, and warm spray (WS). Warm spray is a modification of the HVOF process, where the gas flow rates are controlled to regulate the combustion rate and therefore the temperatures experienced by the feedstock particles (Ref 78). In terms of particle temperatures experienced, warm spray falls between cold spray and HVOF (Fig. 1), hence the eponym. BCC ? FCC ? Cr 3 Si ? oxide phases were observed in all the coatings. One major difference was that the feedstock was gas-atomized powder, as opposed to the AM ? MM powders used earlier, likely due to better flowability and higher yield offered by GA. Typical lamellar microstructures were observed in all three coatings, with salient differences appearing in the phase fractions and degree of IFO (Fig. 11). For instance, precipitation of the Cr 3 Si phase, which was observed in the as-cast and GA states, was subdued in APS and HVOF due to higher process temperatures that promoted the formation of supersaturated solid solutions.
Process temperatures in warm spray were sufficiently low to avoid phase transformations and allow retention of the BCC ? Cr 3 Si phase constitution of the feedstock, but high enough to allow particle deformation and improved inter-splat bonding. The WS microstructures are reminiscent of cold-sprayed AlCrFeCoNi coatings (Ref 46), which also retained feedstock phases, characterized by the deformation of alloy particles into splats with no IFO. The combined advantage of higher process temperatures and no IFO was critical in WS HEA coatings exhibiting better oxidation resistance than their HVOF and APS counterparts. Warm spray, thus, has also demonstrated excellent capability for spraying oxygen-and temperature-sensitive materials, which opens opportunities for the development of TS-HEA coatings.

CrFeCoNi (Mn/Nb/Mo) Coatings
Other compositions based on CrFeCoNi have also been explored via the TS route. Addition of Mo to the quartet was studied by Li  identified the multiple oxide phases using x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Nano-oxide reinforcement of the APS coating due to IFO was also observed. It has been noted that for the same spray parameters, the deposition efficiency of CrFeCoNiMo was lower than that for AlCr-FeCoNi, due to the higher melting point of the former alloy, resulting in fewer instances of melting and splatting. 95 Nb 5 coatings via vacuum plasma spray (VPS), employing GA FCC HEA powders. VPS is currently the preferred alternative to APS for spraying oxygen-sensitive materials but is also an expensive process where the object to be coated is constrained by the size of the vacuum chamber. No oxide peaks were detected in the VPS HEA coatings, and a dense, homogeneous microstructure was observed. In comparison with the as-cast form, where an FCC matrix ? HCPordered Laves phases were observed, VPS resulted in a supersaturated FCC solid solution with a much lower degree of precipitation. Phase separation into CrFeCoNirich, Cr-Nb-rich, and CoFeNb-rich phases was observed in the coating cross section.
Cantor alloy, CrMnFeCoNi, has also been deposited by Yin et al. (Ref 49) via cold spray using GA feedstock. The FCC particles of dendritic morphology did not undergo any phase transformation, nor was there oxidation of the coating. The coatings were dense, aided by the ductile nature of the alloy, which is advantageous since bonding in CS depends on plastic deformation of feedstock particles. The coating also exhibited significant grain refinement from strains generated from particle deformation on impacting the substrate. Wang et al. (Ref 81) also coated GA CrMnFeCoNi particles via APS and reported FCC phase being conserved in the coating. However, APS coatings exhibited porosity and inter-lamellar cracking. To mitigate this, laser remelting was conducted, as elucidated in the following section. In contrast, Xiao et al. (Ref 82) reported similar microstructures for GA-APS CrMnFe-CoNi, but with a fluffy, porous top surface. This is believed to arise from volatilization of manganese during spraying, which has a boiling point 2061°C. This manganese vapor reacts with oxygen, resulting in the porous manganese oxides detected on the coating surface.
Finally, a Co-free CrMnFeNi coating was developed by Lehtonen et al. (Ref 83) for applications in the nuclear industry. A GA-cold spray route using N 2 as the process gas was used. The major FCC phase in the powder was observed in the coating, while a minor BCC phase is believed to have dissolved during processing. As in the work of Yin et al. (Ref 49), grain refinement along the particle boundaries was observed and attributed to the intense plastic deformation typical of the CS process.

Post-processing of TS Coatings
The microstructure and properties of TS-HEA coatings may be improved further using post-processing treatments. Two popular trends exist: (1) heat treatment and (2) laser remelting of the coating surface.
Annealing and heat treatment of the as-sprayed coating are conducted with two main objectives in mind. The first is to relieve strains incurred during rapid solidification and allow densification to occur. The second is to promote phase transformations such as precipitation from supersaturated alloy phases. Annealing can also be used to promote limited interdiffusion between the coating and substrate to improve bond strength without deteriorating the substrate properties. Annealing can also be used to homogenize coating chemistries that undergo low-temperature TS processes, as in the case of cold-sprayed  800°C in nitrogen to precipitate Cr 3 Ni 5 Si 2 phase from Al x SiCrFeCoNi APS coatings to improve their wear resistance. Laser remelting of a TS coating surface is the other popular post-processing technique. By controlling the laser parameters such as laser energy and scan rate, melting is limited to the top few tens of micrometers of the TS coating, effectively avoiding element dilution from the substrate. The alloy melt pool so formed is followed by rapid solidification, resulting in a dense and nearly defectfree coating. A columnar and dendritic (Ref 61, 75) microstructure with supersaturated solid solutions, governed by the rapid cooling rates, is often observed, as illustrated in Fig. 12. Laser remelting assists in fluxing out oxide splats that are formed during IFO during the APS/ HVOF process, thereby resulting in an oxide-free top layer of HEA coating (Ref 63). The sequential processes of TS and laser remelting may be combined to tailor HEA coatings with optimum mechanical and microstructural features for desired applications.
It is important to note that laser remelting (LR) of the TS coating is an empirical method. The laser parameters must be optimized, especially if surface alloying is intended to develop the correct stoichiometry in the final TS-LR coating. Attendant issues such as heat-affected zone and solidification cracking in the remelted zone also need to be addressed. As illustrated by Wang et al. (Ref 81), different laser powers can result in different degrees of melting and therefore influence substrate dilution and solidification cracking.
Traditional post-processing treatments for thermal spray coatings include vacuum heating and high-energy beam remelting. Alternative treatments such as microwave processing, in which energy is directly transferred to the material through interaction with electromagnetic waves with molecules resulting in volumetric heating (Ref 85), are available. There is also work using hot isostatic pressing (HIPing) to improve coating density and effect phase changes in the coating microstructure ( Ref 86,87). Besides these, traditional diffusion-based surface treatments such as carburizing/nitriding/aluminizing can also be applied to TS-HEA coatings to engineer their properties for targeted applications. The post-processing parameters must also be optimized with regard to the coating and intended application.

Comparison of TS HEA Coatings with LC HEA Coatings
In this section, the microstructure observed for thick HEA coatings synthesized via laser cladding (LC) has been compared against their TS counterparts. Laser cladding, laser metal deposition, and laser surface alloying are monikers for the same process. As mentioned previously, LC usually employs blended powders as feedstock, which melt under the laser beam and the alloy rapidly resolidifies into a characteristic dendritic structure.
Although there are more than 60 reports on laser clad HEA coatings, this section will focus on those studying the same HEA composition for corresponding TS HEA coatings, thereby permitting equivalent comparisons.
The AlCrFeCoNi family of alloys has seen significant attention in the laser cladding community. The literature has reported several phase combinations over a range of laser processing parameters for the same HEA composition. Katakam    Thus, TS HEA coatings exhibit lamellar splats and IFO can be expected along with some inherent cracks and porosity. In contrast, laser clad HEA coatings are characterized by equiaxed or columnar dendritic morphologies and are practically defect free. Argon shielding during laser scanning is a standard practice with laser cladding to avoid oxidation of the melt pool. Both coatings are prone to supersaturated solid solution formation due to higher cooling rates. Additionally, grain refinement is observed for laser clad HEA coatings.

Properties of TS HEA Coatings
The following section focuses on the properties exhibited by TS HEA coatings, many of which have outperformed traditional materials. Subdivisions have been made based on (1) porosity, (2) hardness and strengthening mechanisms, (3) tribology, (4) oxidation resistance, and (5) electrochemical studies.

Porosity Measurement
High-entropy alloy coatings developed by thermal spray processes feature composite and lamellar microstructures resulting from splat bonding and rapid solidification. The lamellar microstructure consists of porosity, micro-cracks, oxides (if any), and unmelts. Ang et al. (Ref 34) indicated that the lamellar microstructure of thermal sprayed HEA coatings confers anisotropic mechanical properties. Porosity is the key artifact found within the lamellar microstructure and, other than the thermal spray process, is a major factor that restricts the mechanical properties of these coatings. The overall performance of the HEA coatings can be highly influenced by controlling the size and amount of the pores as well as their location within the 3-D structure of a coating. Table 2 presents the porosity data for thermal sprayed HEA coatings. The HEA coatings produced by cold spray exhibit noteworthy low porosity compared to APS and HVOF processes. The ability of imparting high kinetic energies to particles, leading to large impact velocities, led to low porosity in the cold-spray-processed coatings. HVOF coatings, although processed at lower particle velocities than cold spray coatings, also revealed lower porosity, whereas APS HEA coatings, processed at lower velocity and high particle temperatures, resulted in higherporosity coatings that exhibited phase transformations. and 429 HV were obtained for the as-sprayed HVOF and APS coatings, respectively. A nearly twofold increase was noted in both coatings after annealing at 800 and 1100°C for 250 h. These values are much higher than those observed for CoNiCrAlY-based coatings, which imply better wear resistance for the HEA coatings.

Hardness Analysis
Another study (Ref 34) explored HEAs as alternatives to bond coats; MA AlCrFeCoNi and CrMnFeCoNi HEAs  . The high hardness in WS coatings was attributed to its dense structure and the degree of Cr 3 Si precipitation compared to the supersaturated solid solutions for the other two spray processes. However, heat treatment at 800°C for 10 h allowed Cr 3 Si precipitation to occur in the APS and HVOF coatings, thereby increasing their hardness over that of the warm-sprayed coating.
The HEAs that contain the quaternary metals of CrFe-CoNi in the bulk state exist as an FCC solid solution and possess a superior combination of corrosion resistance, thermal stability, high ductility, and fracture toughness. Thus, it is considered as a high-performance coating material and has been widely investigated as feedstock for different TS processes. However, a similar HEA system sprayed using APS attained microhardness value of 273 ± 35 HV 0.2 , which increased marginally to 332 ± 43 HV 0.2 after annealing at 800°C for 2 h in an argon atmosphere. Note that large and overlapping statistical scatter of these data is typical of coatings formed by APS. Post-annealing, an increase in cohesive strength among splats and the oxides content were stated as the major factors responsible for the microhardness (Ref 82) increase. Cold-sprayed Co-free equiatomic CrMnFeNi sprayed at different powder feed rates and gas feed pressures exhibited a peak hardness of 304 ± 10 HV 0.3 under a low feed rate (1 rpm) and high gas feed pressure (60 bar). The overall increase in hardness was observed due to work hardening from the high particle deformation characteristics of cold spraying ( Ref 83).
As stated in earlier sections, laser cladding (LC) has been widely used to develop thick HEA coatings. In summary: Among all TS processes used to fabricate HEA coatings, HEAs sprayed with HVOF achieved the highest average hardness: 789 ± 54 HV 0.1 , in the case of atomized Al 0.6 TiCrFeCoNi HEA feedstock, as depicted in Fig. 14. Numerous reports on the mechanical behavior of bulk HEAs attribute their outstanding properties to strains arising from 'lattice distortion,' one of the four core effects thought to govern the behavior of HEAs. However, recent reports have found that the strains arising from the 'mismatched' lattice are not significantly different from those experienced by traditional solid solution alloys ( Ref 35). Thus, it is more appropriate to attribute the observed high hardness and strength values to supersaturated solid solution strengthening, precipitation hardening, and other mechanisms induced by the processing technique. Table 2 summarizes the hardness and the possible strengthening mechanisms operating in TS-HEA coatings. Both properties are observed to depend strongly on the processing route of the feedstock, HEA composition, and TS route. High-temperature TS processes invariably result in supersaturated solid solutions, while intermediate cooling rates and/or process temperatures allow fine second-phase precipitation to occur. Low-temperature TS processes on the other hand rely on substantial plastic deformation of particles, resulting in grain refinement, to improve the mechanical properties of coatings. There is further latitude for tuning these properties by post-coating thermal treatments. Combining these process tools with the extensive scope of applying physical metallurgy concepts to the HEA materials hyperspace, such as tuning the microstructure to allow strain hardening to occur due to load-induced phase transformations, e.g., TWIP/TRIP effects, opens technological potential for the future of TS-HEA coatings.

Tribological Assessment
TS-HEAs have demonstrated microstructural stability and high hardness; thus, they are promising candidates for extreme environment applications. This section discusses the tribological behavior observed across TS-HEA coatings, some of which is summarized in Table 3. On the other hand, at 500°C, the volume wear rate increased to 0.93 ± 0.02 9 10 -4 mm 3 /N-m due to a hardness decrease and operation of a more severe adhesive wear mechanism than at room temperature. Major adhesive  wear with minor abrasive wear were indicated as the prevalent mechanisms from the wear surface morphology. Also, the wear surface was observed to have oxidized slightly at 500°C compared to that at 25°C. Wear volume rate at 700°C was measured as 0.23 ± 0.02 9 10 -4 mm 3 / N-m, which increased further at 900°C. The wear mechanisms at both temperatures were tribo-oxidation wear and abrasion wear. In summary, APS AlTiCrFeCoNi HEA coating exhibited excellent wear resistance at 700°C compared to stainless steel due to the development of tribofilms and the formation of a hard and brittle r-CrFe precipitate phase during high-temperature wear testing. Lobel et al. (Ref 37) sprayed blended, MA-and GAbased equi-atomic AlTiCrFeCoNi feedstock onto steel substrates via APS, and analyzed the wear performance of the coatings using three methods: ball-on-disk test, oscillating wear test, and scratch test. Coatings obtained using atomized feedstock exhibited the highest wear resistance in both the ball-on-disk and oscillating wear tests owing to the homogenous microstructure and higher hardness of GA-APS coatings. In contrast, the coating deposited using MA powder performed better in scratch testing compared to blended and atomized powder. TiCrFeCoNi. The wear behavior of the coating was examined using a pin-on-disk tribometer at room temperature (RT), 300 and 500°C. The measured wear rates were 1.044 9 10 -4 , 2.757 9 10 -4 , and 2.674 9 10 -4 mm 3 /N-m at RT, 300, and 500°C, respectively. Morphological analysis of wear tracks depicted that both abrasion and fatigue wear contributed to coating wear. However, severity of both wear mechanisms increased with an increase in test temperature. Moreover, the difference between wear rates at 300 and 500°C was not significantly high due to the formation of an oxide layer on the wear track at 500°C. XRD analysis revealed that no phase changes occurred after friction testing at different temperatures, and the coating exhibits the BCC phase, i.e., the same as in the as-sprayed state. The fracture toughness of the as-sprayed coating obtained was 8.4 MPa m 1/2 , which further decreased to 5 MPa m 1/2 post-heat treatment at 800°C due to the precipitation of r-CrFe phase, signifying increased coating brittleness due to heat treatment. The volume wear rate of as-sprayed coatings had a decreasing trend with an increase in aluminum content from 0.5 to 1.5 under dry sliding conditions. The wear rates obtained were 5.5 9 10 -5 , 4.3 9 10 -5 , and 3 9 10 -5 mm 3 /N-m for Al 0.5-SiCrFeCoNi, Al 1.0 SiCrFeCoNi, and Al 1.5 SiCrFeCoNi, respectively, which were slightly higher than previously attained by Tian et al. (Ref 74). The wear rate decreases significantly to 6.7 9 10 -6 mm 3 /N-m for the Al 1.0-SiCrFeCoNi coating after heat treatment due to the formation of BCC and Cr 3 Ni 5 Si 2 phases. SEM analysis of the wear surface displayed the formation of grooves, micropits due to spalling of splats, and micro-cracks at the interface of splats, which all suggest abrasive wear as the main wear mechanism. In contrast, the wear rate under water sliding conditions was lower than under dry sliding conditions due to lubricating as well as cooling effects promoted by the water media. However, the addition of aluminum to HEA and heat treatment had little influence on wear rate under these conditions as opposed to dry sliding conditions. coating at 25-400°C due to the lubrication effect of silver. However, at high temperature (600 and 750°C), the silver lubrication effect seems negligible since the wear mechanism changes to adhesion and oxidative wear, leading to a further decrease in the friction coefficient for both coatings. The wear rates obtained for HEA-Ag and HEA coatings at 25°C were 0.8 9 10 -5 and 4 9 10 -5 mm 3 /N-m, and at 750°C were 8.9 9 10 -6 and 3.3 9 10 -5 mm 3 /N-m, respectively, which were much lower than for both SKH51 (C = 0.80-0.88, Cr superior wear resistance of APS coating was due to the lubrication effect created by the formation of an oxide layer during wear. The dominant wear mechanism for the HVOF coating was abrasive wear, and lower degree of oxide inclusions apparently reduced the self-lubrication effect, thereby lowering the wear resistance compared to the APS coating. Xiao et al. (Ref 82) also studied the wear behavior of APS CrMnFeCoNi HEA coatings prepared with varying H 2 flow rates. The wear rate was reported to be halved when the H 2 flow rate was doubled during plasma spraying. The wear rates obtained were 5.3 9 10 -4 and 2.7 9 10 -4 mm 3 /N-m for coatings developed using H 2 flow rate 3 and 6 L/min, respectively. It was explained that an increase in secondary gas flow rate enhances the cohesive strength of splats, thus leading to a reduction in splats spalling during wear testing. The cross-sectional SEM analysis of the as-sprayed coating after wear testing revealed the formation of micro-cracks along the splat interfaces as well as spalling pits just below the surface.  assessed the wear behavior of AlTi x CrFeCoNi-based LC coatings at room and high temperatures. The addition of Ti to the AlCrFeCoNi enhanced the wear resistance at both temperatures. The wear volume rates were 1.3 9 10 -8 and 5.8 9 10 -8 mm 3 /N-m at room temperature and 600°C, respectively, which were significantly lower than the measurements for APS AlTiCrFeCoNi HEA coatings (Ref 70). Oxidative wear was the major wear mechanism for both coatings.
In summary: analysis of the wear results shown in Table 3 and Fig. 16 concludes that APS HEA coatings performed better than both HVOF and CS HEA coatings. However, the commercial opportunity for TS HEA coatings in wear applications needs to be validated further due to the limited open-source literature and lack of hightemperature wear data.  6 Ni HEAs as bond coats in a thermal barrier coating (TBC) system that incorporated a YSZ topcoat. These HEA-based TBC systems were then exposed to oxidation at 1100°C, and the results were compared with a conventional MCrAlY-YSZbased TBC system. It was concluded that the AlSiCr 1.

Electrochemical Properties
Corrosion protection to the advanced engineering materials, especially under highly aggressive environments, such as marine applications and chemical processing industries, is generally furnished by a coating system with superior electrochemical performance. Coatings with high corrosion resistance under different environments act as a barrier to the engineering component, and enhance the component lifetime. Thermal sprayed HEA coatings are not yet widely investigated with respect to their corrosive behavior in comparison with their laser clad and cast counterparts (Ref [105][106][107][108][109]. Two reports have been published concerning of the corrosion behavior of HEA coatings formed by a thermal spray process ( Ref 68,80). Wang et al. (Ref 80) examined the corrosive behavior of vacuum-plasma-sprayed (CrFeCoNi) 95 Nb 5 HEA coatings in a 3.5 wt.% NaCl solution. Polarization curve analysis showed that (CrFeCoNi) 95 Nb 5 HEA coatings possessed a higher corrosion potential (-0.37 V) and lower current density (7.23 9 10 -6 A/cm 2 ) compared to other similar HEA systems, thus representing better corrosion resistance. SEM analysis of coating after electrochemical treatment indicated that selective corrosion occurred, with the formation of tiny dish-shaped pits and large pits on the coating surface. The interdendritic phases rich in Nb and Cr were more susceptible to corrosion compared to the Co-, Ni-, and Fe-rich dendritic phase. Formation of stable oxides such as Cr 2 O 3 and Nb 2 O 5 in the passivation film promoted the corrosion resistance of the coating, as detected by XPS analysis.
The second report by Vallimanalan et al. (Ref 68) analyzed the corrosion behavior of HVOF-sprayed AlCrCo-NiMo HEA coatings and compared it with a conventional NiCrSiB-based HVOF coating. Corrosion rate calculation using a Tafel plot suggested that the HEA coating demonstrated a lower corrosion rate (0.00276 mm/year) compared to NiCrSiB coating (0.018 mm/year), thus concluding higher corrosion resistance for this particular HEA coating.

Applications
A coating provides improved properties in the working environment without compromising the properties of the engineering component, thereby increasing the lifetime of the part and providing economic and efficiency gains over the lifetime of the engineering assembly.
A prime example is thermal barrier coatings (TBCs) on superalloy blades in turbines. Superalloys have evolved through several generations and exhibit excellent creep properties and oxidation resistance under extreme harsh environments. However, oxidation resistance comes at the expense of creep performance. Thus, rather than compromising the superalloy creep resistance by adjusting compositions to benefit thermal stability, a suitable TBC can be designed to mitigate the oxidation and thermal resistance. The outcome is that the turbine blade lasts longer with lower maintenance. The coating is sacrificial in nature and can be stripped and replaced periodically, which is much more economical than replacing the superalloy blade.
The potential of high-entropy alloys lies in the unexplored materials hyperspace, as summarized by Miracle et al. in their review (Ref 110). New HEA compositions are being discovered that exhibit exceptional properties. This knowledge may be used to design HEAs for specific applications, which can be translated into coatings using the versatile thermal spray processes.
TS coatings are primarily developed for (a) wear, (b) oxidation, (c) corrosion, (d) biomedical, and (e) electronic applications, each of which is open for TS-HEAs.

Wear and Abrasion Resistance
The sheer number of industries requiring wear-and abrasion-resistant coatings is staggering. From earth-moving and mining equipment to agricultural machinery such as harvester blades, and from transmission, steering, and suspension components in automobiles to wear plates in bottling and canning industries, there are ample opportunities for developing HEA TS coating solutions for each case-specific scenario.
In addition to traditional industries, there is also scope in the renewable energy sector, for example in wind energy where turbine blades need abrasion resistance from dust and hail, as well as being lightweight yet strong-an ideal case for coatings. Wear resistance HEA coatings have been a focus of the TS HEA community, as is detailed in ''Tribological Assessment'' section. The AlCrFeCoNiX family exhibits good wear resistance at room and elevated temperatures, relying on solid solution strengthening and, in some cases, precipitation hardening and operating via oxidative wear mechanisms. On the other end of the HEA spectrum, refractory HEAs (RHEAs) have excellent wear and abrasion resistance in cast and laser clad forms (Ref 111,112), though they have not been fully exploited by TS manufacturing methods.
There is also scope for generating metal matrix composites as illustrated by Mu et al. (Ref 65), where ceramic particles are incorporated into a metallic binder. Other variants, such as replacing metallic binders with ductile FCC HEAs (i.e., the CrFeCoNi alloy family), and the ceramic with B2 phase or refractory HEAs, may be viable. Precedent exists from Zhu et al. (Ref 113) in the form of Ti(C,N) cermets using FCC AlCrFeCoNi alloy as binders, which exhibited better oxidation resistance than conventional materials. It is also possible to adjust spray parameters to generate highentropy ceramics in situ for wear resistance applications. Extensive literature on thin-film HEA-C/N/B/O compositions exists (Ref 31); therefore, there is similar potential to develop thick coatings via TS processes.

Oxidation Resistance
As detailed in ''Oxidation Behavior'' section, there has been a focus on exploring HEAs as alternatives to MCrAlY bond coat materials (Ref 10). Compositions with varying quantities of Al, Cr, Si, Ti, and Ni have shown promising results. Besides bond coats, oxidation resistance is required at lower temperatures (\ 700°C) in power generation industries, where Fe and Cr would be the primary oxidation-resistant elements. Furthermore, RHEAs, which are known to be limited by poor oxidation resistance at elevated temperatures, can be tuned with the addition of Al, Ti, etc., as evidenced by reports on cast RHEAs ( Ref 114,115). Processing these coatings by vacuum plasma spray (VPS) or controlled atmospheric plasma spray (CAPS) is recommended to avoid IFO while promoting particle melting and splat formation.

Corrosion Resistance
Protection from corrosion is an important and viable area for application of TS HEA coatings. Practically, all areas of industry operating in non-ambient conditions require some form of corrosion resistance. The market for thermal sprayed corrosion-resistant coatings is extremely large: from oil and gas/petroleum industries, chemical and metal processing industries, to marine environments-offshore oil rigs, naval applications in ships and submarines; and from food processing industries to powder generationhydro, thermal, geothermal, and land-based turbines. Each industry has specific stringent working environments for which TS HEA coatings can be tailored. Whereas there is a growing body of work on the corrosion behavior of TS HEA coatings, there is a rich body of literature exploring and proving the promise of corrosion-resistant HEAs in the cast or laser clad form ( Ref 22,91). These compositions can be adopted by appropriate TS methods where it will be necessary to optimize the coating thickness and density for optimum performance.

Biomedical Applications
Thermal spray is also a popular method for developing titanium and hydroxyapatite coatings for orthopedic applications ( Ref 116). TS processing provides the right balance between coating density and defect (pores, cracks) distribution required for osteointegration. APS is the favored method for spraying biomaterials.
Developing biocompatible HEAs for drug delivery and other related applications is a major focus of the HEA research community. have explored the TiFeZrNbTa system for biomedical applications as dental or orthopedic implants. It is imperative to mention that currently the metallic implants are formed or machined, and the TS coating on them is polymer or composite in nature. However, there is scope for biocompatible wear-resistant TS HEA composite coatings. Sputtered HEA carbide coatings have been under investigation by Vladescu et al. (Ref 119) due to their biocompatibility and mechanical properties, and such materials processing data can spearhead applications for in situ HEA ceramics generated by TS.

Electronics
Thermal spray coatings are applied in the electronics industry for EMI/RFI (electromagnetic interference/radio frequency interference) shielding. These coatings prevent noise and interference associated with the operation of electronic equipment. Typically, materials with high electrical conductivity and/or high values of magnetic constant and dielectric constant are preferred for EMF shielding via reflection and absorption, respectively.
Nanocrystalline ferromagnetic alloys containing Fe, Co, and Ni have been suggested as promising candidates for EMF shielding. Yang et al. (Ref 120) were the first to explore the EM wave absorption properties of AlCrFeCoNi (70 wt.% powder dispersed with paraffin matrix) synthesized by mechanical alloying. They report an effective absorption bandwidth of 2.7 GHz, indicating the powders' excellent EM wave absorption properties. Zhang et al. (Ref 121) also studied the AlCrFeCoNi/epoxy system, focusing on the effect of different powder morphologies and sizes on the EM absorption properties. They conclude that finer (0.2-7 lm) wet milled HEA exhibited higher total shielding due to enhancement of absorption, vis à vis larger flaky alloy particles, and are excellent candidates for microwave shielding applications. This nature of HEA particles can be translated to TS coatings by low-temperature cold spray processes. High-entropy alloys present technical and commercial opportunities that address these needs. High-entropy alloys in the form of bulk alloys, thin films, and coatings have already been exploited throughout the recent timeline of discovery, especially in traditional technologies such as casting, laser cladding, and sputtering.
High-entropy alloys are still an emerging class of materials where fundamental science is not keeping pace with their practical engineering application. This deficiency is magnified in the area of using HEAs as functional coatings, wherein traditional thermal spray technology can play a major role. Alternative thermal spray and other processing techniques, such as cold spray and high-speed laser cladding, have gained importance. Herein lie opportunities for scientific discoveries that will enable databases of physical properties that can further be exploited for high functional performance.
The near-future developments of TS-HEAs lie in the sub-topics of (1) feedstock preparation, (2) TS process optimization, (3) mechanical, chemical, and thermal testing of HEA coatings, (4) initiation and normalization of HEA databases, and (5) (Ref 130-133). The process technologies for these sub-categories of potential feedstocks have been described in ''HEA Feedstock: Synthesis and Properties'' section. However, other preparation methods such as aerosol technology and solgel methods are also viable. There has been no TS-related work on high-entropy polymers presumably because the excessive TS thermal environment causes adverse material damage.
TS process optimization: Atmospheric plasma spraying is the current method of choice in forming TS-HEAs since it is economical and relatively simple. Cold spray (Ref 46, 49, 83) has been used with success for HEAs but will be limited to ductile materials. Other TS methods of specific relevance include (1)  . These three classes of TS processing will come on line when HEAs are proven and qualified by APS, since they enable the creation of more precise chemistries and refined microstructures that mimic traditional HEA routes.
The process optimization will be guided by the implementation of advanced diagnostic systems that allow realtime monitoring of the particle flux conditions (Ref 139) during the spray process.
Mechanical, chemical, and thermal testing of HEA coatings: This review compiles and summarizes the known literature on HEAs. It is critical to indicate that there are deficiencies in some reports. For example: (1) testing conditions are not fully documented, and (2) replication of test results has not been universally implemented within individual studies. It is recommended that future studies will follow documented metallographic practice (Ref 140), mechanical testing procedures (Ref 4), and thermal tests (Ref 141-143).
Initiation and normalization of HEA databases and modeling studies: The results from the previous section can be formalized into an intelligent database (Ref 144) that will enable quick determinations of HEA coating solutions. As well, the work by Senkov et al. (Ref 36) is important since it provides a pathway to 'identifying promising compositions for more time-intensive experimental studies.' These conjoint approaches, i.e., database approach with alloy selection, are mutually validating since they accelerate the development of HEA exploration. This approach can be extended to novel materials that are suggested in the 'Feedstock preparation' paragraph above.
Qualification and industrial adoption of HEA coatings: HEAs are on the verge of adoption in industries that address extreme environments that need to address wear and thermal environments ( Ref 145). Market expansion will arise where other functional properties are desirable, e.g., high-entropy alloy-based solar absorber coatings (Ref 146,147). Therefore, the unrealized potential of HEA applications will be more openly revealed as the unique materials properties of HEAs are discovered.
A final comment concerning 'Future Trends' relates to the changing environment concerning the research method as it progresses from Technological Readiness Level 1 through to 9 (TRL1-9). HEA technology, overall, is approaching TRL7-8. TS-HEA technology is at TRL4-5, hence implying their optimistic future ( Ref 148). The mechanism that will support this growth is found on interdisciplinary cooperation since there are many unknowns that need to be uncovered.

Summary and Conclusions
Thermal sprayed HEA coatings exhibit unique and outstanding properties in comparison with traditional materials and have established a positive commercial outlook in surface engineering within a short time span.
This early-stage review on thermal sprayed HEA coatings focuses on HEA as feedstock and its synthesis routes with their properties, coating phase compositions along with microstructural development, and mechanical and oxidation behavior of HEA coatings. In addition, this paper (1) suggests applications targeted for these coatings, and (2) other HEA compositions that have not been reported in the thermal spray domain.
The concluding remarks of this early-stage review paper are: 1. Most HEAs in the surface engineering field are processed through cladding and vapor deposition techniques. However, there is a growing market for thick HEA coatings. APS and HVOF are the major thermal spray techniques used for generating HEA coatings, with fewer reports on WS and CS. 2. HEA feedstock containing Al, Si, and/or Ti with a base of CrFeCoNi has been the major research interest since the focused application area is for performance at high temperatures. 3. Development of HEA feedstocks has unique implications on the TS coating microstructure and properties. Gas atomization (GA) is the leading processing route for HEA feedstocks. Blending, mechanical alloying (MA), arc melting followed by mechanical milling (AM-MM) are the other major HEA feedstock synthesis routes. While gas atomization delivers the widest particle size range, mechanical alloying results in a finer particle size of nanocrystalline character. 4. The microstructural advancement of the APS, HVOF, WS, and CS HEA coatings is described in conjunction with their phase composition development. As reported, thermal sprayed HEA coating microstructures are composed of one or more solid solution phases. In some cases, mainly APS, oxides are also reported. The prime feature of the TSC is a lamellar microstructure with development of complementary phases that are revealed by atomic number contrasts. 5. Mechanical behavior of the thermal sprayed coatings in terms of porosity, hardness and wear, along with oxidation resistance, is discussed. HVOF HEA coatings, with their dense microstructure, achieved the highest hardness. However, in most cases the hardness and wear resistance were superior to conventional materials due to the development of supersaturated multicomponent phases and different strengthening mechanisms. Excellent oxidation resistance of the thermal sprayed HEA coatings has displayed potential in replacing conventional bond coat materials. 6. The major focus of the thermal sprayed HEA coatings has been to develop wear-and oxidation-resistant coatings, especially for high-temperature applications.
In contrast, there are only two reports available on analyzing the corrosion behavior of TS-HEA coatings. Therefore, the corrosion-resistant applications for TS-HEAs have not been fully exploited. Also, fracture toughness, being an important mechanical property, to evaluate the flaw behavior within coating during deformation, has only been reported in one article. Thus, future studies that discover vital properties including not only fracture toughness but also creep, fatigue, and tensile strength are required for the formal establishment of thermal sprayed HEA coatings for different applications. 7. Thermal sprayed HEA coatings have shown immense potential of being next-generation coatings within their relatively brief time span under development. TS-HEAs have demonstrated promise, especially for applications in extreme engineering environments.
There is further scope to study their properties in situ by methods such as higher temperature nanoindentation.
One of the major proponents of surface engineering is its ability to elevate the lifetime of an inferior part using a high-performance surface coating. High-entropy alloys are a recent addition to this field, with the potential to replace several traditional coating materials on the basis of their excellent properties. To this end, selection of elements and the proportion in which to alloy them is of great importance. Although HEAs were initially defined as equiatomic alloys exhibiting single solid solution phases, recent literature has evidenced superior strength-ductility properties derived from multi-phase non-equiatomic compositions. TS-HEAs have followed a similar developmental path, for example, incorporating post-processing treatment to enhance wear properties of the coating.
The most commonly used base elements that confer high operational performance and phase stability consist of Al, Co, Cr, Fe, Mo and Ni, of which Co, Mo, and Ni are considered as strategic materials that are not only regulated by government policies but are expensive raw materials. The advantage of the thermal spray manufacturing route is that only a coating of 50-300 lm thickness is required, thus saving the need for expensive feedstocks. Likewise, experimenting with additional principal or trace elements, for example Ti and W, reduces quantities of these costly elements to achieve performance.
The advantage of TS manufacturing revolves around their ability to have rapid buildup of an engineered surface in comparison with vacuum-based film technology such as physical vapor deposition (PVD). The costlier alternatives of vacuum or low-pressure plasma spray are currently adopted for spraying oxygen-sensitive alloys.
The cost of an APS TS installation is moderate and either factory or field operations have been established globally for local niche markets. TS-HEA research is thus focused on employing APS while controlling the extent of IFO by optimizing composition and coating process parameters, to engineer desirable coatings economically. Additive manufacturing by cold spray is another venture for exploiting the advantages offered by HEAs without being uneconomical. Therefore, the feasibility of producing a HEA coating by TS within a production environment has been proven to be economically viable with many case histories documented (Ref 149) for specific industries that use non-HEA feedstocks.
Thermal sprayed high-entropy alloy coatings are in the initial stage of exploration. This early-stage review highlights and summarizes knowledge and understanding from a diverse array of original sources. The future for TS-HEAs is positive in terms of interesting science and potential commercial outcomes for engineering applications.
Historical Perspective and Dedication This manuscript has been finalized over a critical point in our history as we recover from a global pandemic. This manuscript is testament to scientific cooperation between high-entropy alloy research groups in Australia and India over this challenging period. It is timely and appropriate to recognize Professor S. Ranganathan (Emeritus Professor & Senior Homi Bhabha Fellow, Indian Institute of Science, Bangalore, India) whose article titled 'Alloyed pleasure: Multimetallic cocktails' (Current Science, 2003, 85(10), 1404-1407, 267 citations) has been inspiring. The authors are grateful for his leadership and mentoring in physical metallurgy that has spanned five decades. We dedicate this review to Prof. S. Ranganathan.
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